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6.3 Layered overview of radio interface
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The GPRS radio interface can be modelled as a hierarchy of logical layers with specific functions. An example of such layering is shown in Figure 3. The various layers are briefly described in the following subclauses. The physical layer has been separated into two distinct sub-layers defined by their functions: - Physical RF layer performs the modulation of the physical waveforms based on the sequence of bits received from the Physical Link layer. The Physical RF layer also demodulates received waveforms into a sequence of bits which are transferred to the Physical Link layer for interpretation. - Physical Link layer provides services for information transfer over a physical channel between the MS and the Network. These functions include data unit framing, data coding, and the detection and correction of physical medium transmission errors. The Physical Link layer uses the services of the Physical RF layer. The lower part of the data link layer is defined by following functions: - The RLC/MAC layer provides services for information transfer over the physical layer of the GPRS radio interface. These functions include backward error correction procedures enabled by the selective retransmission of erroneous blocks. The MAC function arbitrates access to the shared medium between a multitude of MSs and the Network. The RLC/MAC layer uses the services of the Physical Link layer. The layer above RLC/MAC (i.e., LLC described in GSM 03.60 [2] and defined in GSM 04.64 [12]) uses the services of the RLC/MAC layer on the Um interface. Um Network SNDCP LLC (Note) RLC MAC Phys. Link Phys. RF SNDCP LLC RLC MAC Phys. Link Phys. RF MS Scope of GSM 03.60 Scope of GSM 03.64 Note: In the network the LLC is split between BSS and SGSN. Figure 3: GPRS MS – Network Reference Model ETSI ETSI TS 101 350 V7.0.0 (1999-07) 18 (GSM 03.64 version 7.0.0 Release 1998)
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6.4 Physical RF Layer
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The GSM Physical RF layer is defined in GSM 05 series recommendations, which specify among other things: - The carrier frequencies characteristics and GSM radio channel structures (GSM 05.02 [4]); - The modulation of the transmitted wave forms and the raw data rates of GSM channels (GSM 05.04 [6]); and - The transmitter and receiver characteristics and performance requirements (GSM 05.05 [7]).
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6.5 Physical Link Layer
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The Physical Link layer operates above the physical RF layer to provide a physical channel between the MS and the Network.
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101 350
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6.5.1 Layer Services
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The purpose of the Physical Link layer is to convey information across the GSM radio interface, including RLC/MAC information. The Physical Link layer supports multiple MSs sharing a single physical channel. The Physical Link layer provides communication between MSs and the Network. The Physical Link layer control functions provide the services necessary to maintain communications capability over the physical radio channel between the Network and MSs. Radio subsystem link control procedures are currently specified in GSM 05.08 [8]. Network controlled handovers are not used in the GPRS service. MS performed cell-reselection is used, see subclause 6.5.6.
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6.5.2 Layer Functions
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The Physical Link layer is responsible for: - Forward Error Correction (FEC) coding, allowing the detection and correction of transmitted code words and the indication of uncorrectable code words. The coding schemes are described in subclause 6.5.5. - Rectangular interleaving of one Radio Block over four bursts in consecutive TDMA frames, as specified in GSM 05.03 [5]. - Procedures for detecting physical link congestion. The Physical Link layer control functions include: - Synchronisation procedures, including means for determining and adjusting the MS Timing Advance to correct for variances in propagation delay , GSM 05.10 [9]; - Monitoring and evaluation procedures for radio link signal quality; - Cell (re-)selection procedures; - Transmitter power control procedures; and - Battery power conservation procedures, e.g. Discontinuous Reception (DRX) procedures.
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101 350
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6.5.3 Service Primitives
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Table 2 lists the service primitives provided by the Physical Link layer to RLC/MAC layer. More detailed description is given in GSM 04.04. ETSI ETSI TS 101 350 V7.0.0 (1999-07) 19 (GSM 03.64 version 7.0.0 Release 1998) Table 2: Service primitives provided by the Physical link layer Name Request indication response confirm Comments PH-DATA X X Used to pass message units containing frames used for RLC/MAC layer respective peer-to-peer communications to and from the physical layer. PH-RANDOM ACCESS X X X Used to request and confirm (in the MS) the sending of a random access frame and to indicate (in the network) the arrival of a random access frame. PH-CONNECT X Used to indicate that the physical connection on the packet data physical channel has been established. PH-READY-TO- SEND X Used by the physical layer to trigger, if applicable, piggy backing, the start of timer for the RLC/MAC layer and the forwarding a data unit to the physical layer PH-EMPTY- FRAME X Used by the RLC/MAC layer to indicate that no frame has to be transmitted after receiving the PH-READY-TO-SEND primitive ETSI ETSI TS 101 350 V7.0.0 (1999-07) 20 (GSM 03.64 version 7.0.0 Release 1998)
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6.5.4 Radio Block Structure
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Different Radio Block structures for data transfer and control message transfer purposes are defined. Radio Block consists of MAC Header, RLC Data Block or RLC/MAC Control Block. It is always carried by four normal bursts. For detailed definition of radio block structure, see GSM 04.60. MAC header RLC header RLC data MAC header RLC/MAC Control Message (part of physical link RLC data block (part of physical link RLC /MAC control block Radio Block Radio Block Figure 4: Radio Block structures MAC header contains control fields which are different for uplink and downlink directions. MAC header is constant length, 8 bits. RLC header contains control fields which are different for uplink and downlink directions. RLC header is variable length. RLC data field contains octets from one or more LLC PDUs. Block Check Sequence (BCS) is used for error detection. RLC/MAC Control message field contains one RLC/MAC control message.
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6.5.5 Channel Coding
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NOTE: The text in this subclause is informative. The normative text is in GSM 05.03 [5]. Where there is a conflict between these descriptions, the normative text has precedence. Four coding schemes, CS-1 to CS-4, are defined for the packet data traffic channels. For all other packet control channels than Packet Random Access Channel (PRACH) and Packet Timing Advance Control Channel on Uplink (PTCCH/U), coding scheme CS-1 is always used. For access bursts on PRACH, two coding schemes are specified. All coding schemes are mandatory for MSs. Only CS-1 is mandatory for the network.
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101 350
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6.5.5.1 Channel coding for PDTCH
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Four different coding schemes, CS-1 to CS-4, are defined for the Radio Blocks carrying RLC data blocks. The block structures of the coding schemes are shown Figure 5 and Figure 6. ETSI ETSI TS 101 350 V7.0.0 (1999-07) 21 (GSM 03.64 version 7.0.0 Release 1998) rate 1/2 convolutional coding puncturing 456 bits USF BCS Radio Block Figure 5: Radio Block structure for CS-1 to CS-3 block code no coding 456 bits USF BCS Radio Block Figure 6: Radio Block structure for CS-4 The first step of the coding procedure is to add a Block Check Sequence (BCS) for error detection. For CS-1 - CS-3, the second step consists of pre-coding USF (except for CS-1), adding four tail bits and a half rate convolutional coding for error correction that is punctured to give the desired coding rate. For CS-4 there is no coding for error correction. The details of the codes are shown in table 3, including: - the length of each field; - the number of coded bits (after adding tail bits and convolutional coding); - the number of punctured bits; - the data rate, including the RLC header and RLC information. Table 3: Coding parameters for the coding schemes Scheme Code rate USF Pre-coded USF Radio Block excl. USF and BCS BCS Tail Coded bits Punctured bits Data rate kb/s CS-1 1/2 3 3 181 40 4 456 0 9.05 CS-2 ≈2/3 3 6 268 16 4 588 132 13.4 CS-3 ≈3/4 3 6 312 16 4 676 220 15.6 CS-4 1 3 12 428 16 - 456 - 21.4 CS-1 is the same coding scheme as specified for SACCH in GSM 05.03 [12]. It consists of a half rate convolutional code for FEC and a 40 bit FIRE code for BCS (and optionally FEC). ETSI ETSI TS 101 350 V7.0.0 (1999-07) 22 (GSM 03.64 version 7.0.0 Release 1998) CS-2 and CS-3 are punctured versions of the same half rate convolutional code as CS-1 for FEC. CS-4 has no FEC. CS-2 to CS-4 use the same 16 bit CRC for BCS. The CRC is calculated over the whole uncoded RLC Data Block including MAC Header. The USF has 8 states, which are represented by a binary 3 bit field in the MAC Header. For CS-1, the whole Radio Block is convolutionally coded and USF needs to be decoded as part of the data. All other coding schemes generate the same 12 bit code for USF. The USF can be decoded either as a block code or as part of the data. In order to simplify the decoding, the stealing bits (defined in GSM 05.03) of the block are used to indicate the actual coding scheme.
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101 350
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6.5.5.2 Channel coding for PACCH, PBCCH, PAGCH, PPCH,PNCH and PTCCH
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The channel coding for the PACCH, PBCCH, PAGCH, PPCH,PNCH and downlink PTCCH is the same as the coding scheme CS-1 presented in subclause 6.5.5.1. The coding scheme used for uplink PTCCH is the same as for PRACH.
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101 350
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6.5.5.3 Channel Coding for the PRACH
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Two types of packet access burst may be transmitted on the PRACH: an 8 information bits access burst or an 11 information bits access burst called the extended packet access burst. The mobile shall support both access bursts. The channel coding for both burst formats is indicated in the following subclauses.
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101 350
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6.5.5.3.1 Coding of the 8 data bit Packet Access Burst
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The channel coding used for the burst carrying the 8 data bit packet access uplink message is identical to the coding of the access burst as defined for random access channel in GSM 05.03.
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a02c9ef5f89363d47206d8defd4c4a56
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101 350
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6.5.5.3.2 Coding of the 11 data bit Packet Access Burst
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The channel coding for 11 bit access burst is the punctured version of the same coding as used for 8 bit access burst.
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a02c9ef5f89363d47206d8defd4c4a56
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101 350
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6.5.6 Cell Re-selection
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NOTE: The text in this subclause is informative. The normative text is in GSM 03.22 and GSM 05.08. Where there is a conflict between these descriptions, the normative text has precedence. In GPRS Packet Idle and Packet Transfer modes, cell re-selection is performed by the MS, except for a class A MS (see GSM 02.60) while in dedicated mode in which case the cell is determined by the network according to the handover procedures. The new cell re-selection criteria C31 and C32 are provided as a complement to the current GSM cell re-selection criteria. This provides a more general tool to make cell planning for GPRS as similar to existing planning in GSM as possible.C31 is a signal strength criterion used to decide whether prioritised cell re-selection shall be used. For cells that fulfil the C31 criterion, the cell with highest priority class shall be selected. If more than one cell has the highest priority, the one of those with the highest C32 value shall be selected. If no cell fulfils the C31 criterion, the one among all cells with the highest C32 value shall be selected. C32 is an improvement of C2. It applies an individual offset and hysteresis value to each pair of cells, as well as the same temporary offsets as for C2. Additional hysteresis values apply for a cell re-selection that requires cell or routing area update. Cell re-selection procedure apply to the MSs attached to GPRS if a PBCCH exists in the serving cell. If the PBCCH is not allocated, then the MS shall perform cell re-selection according to the C2 criteria. ETSI ETSI TS 101 350 V7.0.0 (1999-07) 23 (GSM 03.64 version 7.0.0 Release 1998) In addition, the network may control the cell re-selection as described in subclause 6.5.6.3.
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101 350
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6.5.6.1 Measurements for Cell Re-selection
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The MS shall measure the received RF signal strength on the BCCH frequencies of the serving cell and the neighbour cells as indicated in the BA-GPRS list, and calculate the received level average (RLA) for each frequency, as specified in GSM 05.08. In addition the MS shall verify the BSIC of the cells. Only channels with the same BSIC as broadcast together with BA-GPRS on PBCCH shall be considered for re-selection. When the number of downlink PDCHs assigned to certain types of multislot MS (see GSM 05.02, annex B) does not allow them to perform measurements within the TDMA frame, the network shall provide measurement windows to ensure that the MS can perform a required number of measurements. The network shall provide periods of inactivity during a fixed allocation to allow the MS to make adjacent cell power measurements and BSIC detection.
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6.5.6.2 Broadcast Information
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The PBCCH broadcasts GPRS specific cell re-selection parameters for serving and neighbour cells, including the BA (GPRS) list. A BA (GPRS) identifies the neighbour cells, including BSIC, that shall be considered for GPRS cell (re- selection (not necessary the same as for GSM in Idle or circuit switched mode)).
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6.5.6.3 Optional measurement reports and network controlled cell re-selection
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It shall be possible for the network to order the mobile stations to send measurement reports to the network and to suspend its normal cell re-selection, and instead to accept decisions from the network. This applies to both Packet idle mode and Packet transfer mode. The degree to which the mobile station shall resign its radio network control shall be variable, and be ordered in detail by the parameter NETWORK_CONTROL_ORDER [15]. Two sets of parameters are broadcast on PBCCH and are valid in Packet transfer and Packet idle modes respectively. NETWORK_CONTROL_ORDER can also be sent individually to an MS on PACCH, in which case it overrides the broadcast parameter. Additionally, the network may request extended measurement reports from the MS and the reporting shall be maintained in packet idle mode. The reports may include interference measurements (see subclause 6.5.8.3.2). Measurement reports shall be sent individually from each MS as RLC transmissions. Situations may appear where the network controlled cell re-selection procedures (NC1 or NC2 modes of operation) should not be used: - When a class A mobile station is simultaneously involved in a circuit switched service and in a GPRS transfer. In this case, handover for the circuit switched service has precedence over GPRS network controlled cell re- selection. - When an MS is performing Anonymous Access, cell re-selection implying a change of Routeing Area results in the MS returning to the GPRS MM IDLE state, see GSM 03.60 [3]. Therefore, there might be cases where the network controlled cell re-selection would result in the Anonymous Access failing. In that case, the MS shall stop sending measurement reports and ignore cell change orders.
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6.5.7 Timing Advance
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NOTE: The text in this subclause is informative. The normative text is in GSM 04.60 and GSM 05.10. Where there is a conflict between these descriptions, the normative text has precedence. The timing advance procedure is used to derive the correct value for timing advance that the MS has to use for the uplink transmission of radio blocks. The timing advance procedure comprises two parts: - initial timing advance estimation; - continuous timing advance update. ETSI ETSI TS 101 350 V7.0.0 (1999-07) 24 (GSM 03.64 version 7.0.0 Release 1998)
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6.5.7.1 Initial timing advance estimation
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The initial timing advance estimation is based on the single access burst carrying the Packet Channel Request. The Packet Uplink Assignment or Packet Downlink Assignment then carries the estimated timing advance value to the MS. This value shall be used by the MS for the uplink transmissions until the continuous timing advance update provides a new value (see subclause 6.5.7.2.). Two special cases exist: - when Packet Queuing Notification is used the initial estimated timing advance may become too old to be sent in the Packet Downlink (/Uplink) Assignment - when Packet Downlink (/Uplink) Assignment is to be sent without prior paging (i.e., in the Ready state), no valid timing advance value may be available. Then the network has three options: - Packet Polling Request can then be used to trigger the transmission of Packet Control Acknowledgement. This message can be formatted as four access burst from which the timing advance can be estimated. - Packet Downlink (/Uplink) Assignment can be sent without timing advance information. In that case it is indicated to the MS that it can only start the uplink transmission after the timing advance is obtained by the continuous timing advance update procedure. - The poll bit in the Packet Downlink (/Uplink) Assignment message can be set to trigger the transmission of Packet Control Acknowledgement. This can be used if System information indicates that acknowledgement is access bursts. For the case where timing advance information is not provided in the assignment message, the mobile is not allowed to send normal bursts on the uplink until it receives a valid timing advance either in Packet Timing Advance/Power Control message or through the continuous timing advance procedure.
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6.5.7.2 Continuous timing advance update
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MS in Packet transfer mode shall use the continuous timing advance update procedure. The continuous timing advance update procedure is carried on the PTCCH allocated to the MS. For uplink packet transfer, within the Packet Uplink Assignment, the MS is assigned Timing Advance Index (TAI) and the PTCCH. For downlink packet transfer, within the Packet Downlink Assignment, the MS is assigned Timing Advance Index (TAI) and the PTCCH. The TAI specifies the PTCCH sub-channel used by the MS. On the uplink, the MS shall send in the assigned PTCCH access burst, which is used by the network to derive the timing advance. The network analyses the received access burst and determines new timing advance values for all MSs performing the continuous timing advance update procedure on that PDCH. The new timing advance values shall be sent via a downlink signalling message (TA-message) on PTCCH/D. Network can send timing advance information also in Packet Timing Advance/Power Control and Packet Uplink Ack/Nack messages on PACCH. ETSI ETSI TS 101 350 V7.0.0 (1999-07) 25 (GSM 03.64 version 7.0.0 Release 1998)
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6.5.7.2.1 Mapping on the multiframe structure
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Figure 7 shows the mapping of the uplink access bursts and downlink TA-messages on groups of eight 52-multiframes: - the TAI value shows the position where a slot is reserved for a MS to send an access burst (e.g. T1 means 52- multiframe number n and idle slot number 2). TAI value defines the used PTCCH sub-channel. - every second PDCH multiframe starts a downlink TA-message. 52-multiframe number n: uplink TAI=0 TAI=1 B0 B1 B2 0 B3 B4 B5 1 B6 B7 B8 2 B9 B10 B11 3 downlink TA_message 1 TA message 1 52-multiframe number n + 1: uplink TAI=2 TAI=3 B0 B1 B2 4 B3 B4 B5 5 B6 B7 B8 6 B9 B10 B11 7 downlink TA message 1 TA message 1 52-multiframe number n + 2: uplink TAI=4 TAI=5 B0 B1 B2 8 B3 B4 B5 9 B6 B7 B8 10 B9 B10 B11 11 downlink TA message 2 TA message 2 52-multiframe number n + 3: uplink TAI=6 TAI=7 B0 B1 B2 12 B3 B4 B5 13 B6 B7 B8 14 B9 B10 B11 15 downlink TA message 2 TA message 2 52-multiframe number n + 4: uplink TAI=8 TAI=9 B0 B1 B2 16 B3 B4 B5 17 B6 B7 B8 18 B9 B10 B11 19 downlink TA message 3 TA message 3 52-multiframe number n + 5: uplink TAI=10 TAI=11 B0 B1 B2 20 B3 B4 B5 21 B6 B7 B8 22 B9 B10 B11 23 downlink TA message 3 TA message 3 52-multiframe number n + 6: uplink TAI=12 TAI=13 B0 B1 B2 24 B3 B4 B5 25 B6 B7 B8 26 B9 B10 B11 27 downlink TA message 4 TA message 4 ETSI ETSI TS 101 350 V7.0.0 (1999-07) 26 (GSM 03.64 version 7.0.0 Release 1998) 52-multiframe number n + 7: uplink TAI=14 TAI=15 B0 B1 B2 28 B3 B4 B5 29 B6 B7 B8 30 B9 B10 B11 31 downlink TA message 4 TA message 4 B0 - B11 = Radio blocks Idle frames are numbered from 1 to 31 [odd numbers] PTCCH frames are numbered from 0 to 30 [even numbers] Figure 7: Mapping of the uplink access bursts and downlink timing advance signalling messages The BTS shall update the timing advance values in the next TA-message following the access burst. To illustrate this, an MS that transmits an access burst in frames numbered 0, 2, 4, or 6 receives its updated timing advance value in TA message 2. This MS can also find this updated timing advance value in subsequent TA messages 3, 4, and 1, but only has to read these if TA message 2 was not received correctly. An MS entering the Transfer state shall ignore the TA-messages until the MS has sent its first access burst. This is to avoid the use of timing advance values, derived from access bursts sent by the MS that previously used the same TAI.
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6.5.8 Power control procedure
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Power control shall be supported in order to improve the spectrum efficiency and to reduce the power consumption in the MS. For the uplink, the MS shall follow a flexible power control algorithm, which the network can optimise through a set of parameters. It can be used for both open loop and closed loop power control. For the downlink, the power control is performed in the BTS. Therefore, there is no need to specify the actual algorithms, but information about the downlink performance is needed. Therefore the MSs have to transfer Channel Quality Reports to the BTS.Power control is not applicable to point-to-multipoint multicast services. For the detailed specification of power control see GSM 05.08.
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101 350
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6.5.8.1 MS output power
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The MS shall calculate the RF output power value, PCH, to be used on each individual uplink PDCH assigned to the MS: PCH = min((Γ0 - ΓCH - α * (C + 48), PMAX) where ΓCH is an MS and channel specific power control parameter. It is sent to the MS in any resource assigning message. Further, the network can, at any time during a packet transfer, send new ΓCH values to the MS on the downlink PACCH. Γ0 is a frequency band dependent constant. α∈[0,1] is a system parameter. Its default value is broadcast on the PBCCH. Further, MS and channel specific values can be sent to the MS together with ΓCH. C is the received signal level at the MS. PMAX is the maximum allowed output power in the cell. All power values are expressed in dBm. PCH is not used to determine the output power when accessing the cell on PRACH or RACH , in which case PMAX shall be used. ETSI ETSI TS 101 350 V7.0.0 (1999-07) 27 (GSM 03.64 version 7.0.0 Release 1998)
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6.5.8.2 BTS output power
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The BTS shall use constant power on those PDCH radio blocks which contain PBCCH or which may contain PPCH. This power may be lower than the output power used on BCCH. The difference shall be broadcast on PBCCH. On the other PDCH radio blocks, downlink power control may be used. Thus, a procedure may be implemented in the network to control the power of the downlink transmission based on the Channel Quality Reports. The network shall ensure that the output power is sufficient for the MS for which the RLC block is intended as well as the MS(s) for which the USF is intended, and that for each MS in packet transfer mode, at least one downlink RLC block per multiframe is transmitted with an output power that is sufficient for that MS, on a block monitored by that MS.
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6.5.8.3 Measurements at MS side
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A procedure shall be implemented in the MS to monitor periodically the downlink Rx signal level and quality from its serving cell.
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101 350
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6.5.8.3.1 Deriving the C value
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This subclause comprises information about how the MS shall derive the C value in the power control equation. The MS shall periodically measure the received signal strength. In packet idle mode, the MS shall measure the signal strength of the PCCCH or, if PCCCH is not existing, the BCCH. In packet transfer mode, the MS shall measure the signal strength on BCCH. The same measurements as for cell re- selection are used (see 6.5.6.1.). Alternatively, if indicated by a broadcast parameter, the MS shall measure the signal strength on one of the PDCHs where the MS receives PACCH. This method is suitable in the case where BCCH is in another frequency band than the used PDCHs. It requires that constant output power is used on all downlink PDCH blocks. The MS shall measure the signal strength of each radio block monitored by the MS. The C value is achieved by filtering the signal strength with a running average filter. The filtering shall normally be continuous between the packet modes. The different filter parameters for the packet modes are broadcast on PBCCH or, if PBCCH does not exist, on BCCH. The variance of the received signal level within each block shall also be calculated. The filtered value SIGN_VAR shall be included in the channel quality report. An MS transferring a packet in the uplink with fixed assignment is not required to make signal strength measurements and shall thus update PCH only when it receives new ΓCH values.
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6.5.8.3.2 Derivation of Channel Quality Report
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The channel quality is measured as the interference signal level during the idle frames of the multiframe, when the serving cell is not transmitting. In packet transfer mode, the MS shall measure the interference signal strength of all eight channels (slots) on the same carrier as the assigned PDCHs. In packet idle mode, the MS shall measure the interference signal strength on certain channels which are indicated on the PBCCH or, if PBCCH does not exist, on BCCH. If no channels are indicated the MS shall not perform these measurements. Some of the idle frames and PTCCH frames shall be used for this, while the others are required for BSIC identification and the timing advance procedure, see subclause 6.5.9. The MS may not be capable of measuring all eight channels when allocated some configurations of channels. The MS shall measure as many channels as its allocation allows considering its multislot capability. The slots that the MS measures on can be either idle or used by SACCH or PTCCH, depending on the channel type (TCH or PDCH).. The MS shall therefore, for each slot, take the minimum signal strength of one idle frame and one PTCCH frame. Thus the SACCH frames are avoided (except for a TCH/H with two MSs) and only the interference is measured. ETSI ETSI TS 101 350 V7.0.0 (1999-07) 28 (GSM 03.64 version 7.0.0 Release 1998) The interference, γCH , is achieved by filtering the measured interference in a running average filter. The filtering shall be continuous between the packet modes for channels measured in both modes. The different filter parameters for the packet modes are broadcast on PBCCH or, if PBCCH does not exist, on BCCH. In packet transfer mode the MS shall transfer the 8 γCH values and the RXQUAL, SIGN_VAR and C values (see subclause 6.5.8.3.1) to the network in the Channel Quality Report included in the PACKET DOWNLINK ACK/NACK message.
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6.5.8.4 Measurements at BSS side
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A procedure shall be implemented in the BSS to monitor the uplink Rx signal level and quality on each uplink PDCH, active as well as inactive. The BSS shall also measure the Rx signal level and the quality of a specific MS packet transfer.
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6.5.9 Scheduling the MS activities during the PTCCH and idle frames
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The MS shall use the PTCCH and idle frames of the PDCH multiframe for the following tasks: - BSIC identification for cell re-selection (6.5.6.1) - Continuous timing advance procedures (6.5.7.2) - Interference measurements for power control (6.5.8.3.2) It is not necessary to exactly specify the scheduling of these tasks. The PTCCH frames used for timing advance signalling is stated in 6.5.7.2.1. During the frames when the MS receives TA-messages it can also make interference measurements. During the frames when the MS transmits access bursts it may also be possible to make measurements on some channels. The MS shall schedule the BSIC identification as efficiently as possible, using the remaining PTCCH frames and the idle frames and also considering the requirements for interference measurements. When the MS is synchronised to a BTS, it knows the timing of the SCH. Therefore, only a few certain frames are required for BSIC identification. In those frames it may also be possible to make measurements on some channels. When the MS shall synchronise to a new BTS, it has to prioritise that task. It may then use half of the PTCCH and idle frames, i.e. the same amount as available for circuit switched connections. The remaining PTCCH and idle frames shall be used for interference measurements.
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6.5.10 Discontinuous Reception (DRX)
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NOTE: The text in this subclause is informative. The normative text is in GSM 05.02. Where there is a conflict between these descriptions, the normative text has precedence. DRX (sleep mode) shall be supported when the MS is in Packet Idle mode. DRX is independent from MM states Ready and Standby. Negotiation of DRX parameters is per MS. An MS may choose to use DRX or not together with some operating parameters. The following parameters are established: - DRX/non-DRX indicator It indicates whether the MS uses DRX or not. - DRX period A conditional parameter for MSs using DRX to determine the right paging group. The DRX period is defined by the parameter SPLIT_PG_CYCLE. - Non-DRX timer A conditional parameter for MSs using DRX to determine the time period within which the non-DRX mode is kept after leaving the Transfer state. The support for this feature is optional on the network side and the information about the maximum supported value for the timer in the cell is broadcast on PBCCH. ETSI ETSI TS 101 350 V7.0.0 (1999-07) 29 (GSM 03.64 version 7.0.0 Release 1998) An MS in DRX mode is only required to monitor the radio blocks defined by its paging group as defined in GSM 05.02. Paging group definition based on SPLIT_PG_CYCLE is optional on CCCH for both BTS and MS. If not supported, the definition based on BS_PA_MFRMS shall be used. The parameters used to define the paging group for GPRS are shown in the Table 4, together with the corresponding GSM parameters. BS_PCC_CHANS is the number of PDCHs containing PCCCH. An MS in non-DRX mode is required to monitor all the radio blocks where PCCCH may be mapped on the PDCH defined by its paging group. When page for circuit-switched services is conveyed on PPCH, it follows the same scheduling principles as the page for packet data. The same is valid for scheduling of resource assignments for downlink packet transfers for MSs in Ready State (i.e. where no paging is performed). The MS may need to monitor also PNCH in the case of PTM-M services. NOTE: Paging reorganisation may be supported in the same way as for circuit switched GSM. Table 4: Parameters for DRX operation Parameter GPRS Corresponding GSM parameters PCCCH CCCH CCCH DRX period SPLIT_PG_CYCLE BS_PA_MFRMS *) SPLIT_PG_CYCLE **) BS_PA_MFRMS Blocks not available for PPCH per multiframe BS_PAG_BLKS_RES + BS_PBCCH_BLKS BS_AG_BLKS_RES BS_AG_BLKS_RES Number of physical channels containing paging BS_PCC_CHANS BS_CC_CHANS BS_CC_CHANS *) Only when DRX period split is not supported. **) Only when DRX period split is supported.
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101 350
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6.6 Medium Access Control and Radio Link Control Layer
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The Medium Access Control (MAC) and Radio Link Control (RLC) layer operates above the Physical Link layer in the reference architecture. MAC/RLC layer messages and signalling procedures are defined in GSM 04.60 and GSM 04.08.
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101 350
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6.6.1 Layer Services
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The MAC function defines the procedures that enable multiple MSs to share a common transmission medium, which may consist of several physical channels. The MAC function provides arbitration between multiple MSs attempting to transmit simultaneously and provides collision avoidance, detection and recovery procedures. The operations of the MAC function may allow a single MS to use several physical channels in parallel. The RLC function defines the procedures for a bitmap selective retransmission of unsuccessfully delivered RLC Data Blocks. The RCL/MAC function provides two modes of operation: - unacknowledged operation; and - acknowledged operation ETSI ETSI TS 101 350 V7.0.0 (1999-07) 30 (GSM 03.64 version 7.0.0 Release 1998)
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6.6.2 Layer Functions
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The GPRS MAC function is responsible for: - Providing efficient multiplexing of data and control signalling on both uplink and downlink, the control of which resides on the Network side. On the downlink, multiplexing is controlled by a scheduling mechanism. On the uplink, multiplexing is controlled by medium allocation to individual users (e.g., in response to service request). - For mobile originated channel access, contention resolution between channel access attempts, including collision detection and recovery. - For mobile terminated channel access, scheduling of access attempts, including queuing of packet accesses. - Priority handling. The GPRS RLC function is responsible for: - Interface primitives allowing the transfer of Logical Link Control layer PDUs (LLC-PDU) between the LLC layer and the MAC function. - Segmentation and re-assembly of LLC-PDUs into RLC Data Blocks. - Backward Error Correction (BEC) procedures enabling the selective retransmission of uncorrectable code words. NOTE: The Block Check Sequence for error detection is provided by the Physical Link Layer.
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6.6.3 Service Primitives
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Table 5 lists the service primitives provided by the RLC/MAC layer to the upper layers: Table 5: Service primitives provided by the RLC/MAC layer to the upper layers Name request indication response confirm comments RLC/MAC-DATA x x used for the transfer of upper layer PDUs. Acknowledged mode of operation in RLC is used. The upper layer shall be able to request high transmission quality via a primitive parameter. RLC/MAC- UNITDATA x x used for the transfer of upper layer PDUs. Unacknowledged mode of operation in RLC is used. RLC/MAC-STATUS x used to indicate that an error has occurred on the radio interface. The cause for the failure is indicated.
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6.6.4 Model of Operation
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Each PDCH is a shared medium between multiple MSs and the Network. Direct communication is possible only between an MS and the network. The GPRS radio interface consists of asymmetric and independent uplink and downlink channels. The downlink carries transmissions from the network to multiple MSs and does not require contention arbitration. The uplink is shared among multiple MSs and requires contention control procedures. The allocation of radio resources by the PLMN and the use of these resources by the MSs can be broken down into two parts: - The PLMN allocates radio resources for the GPRS (uplink and downlink) in a symmetric manner. ETSI ETSI TS 101 350 V7.0.0 (1999-07) 31 (GSM 03.64 version 7.0.0 Release 1998) - The allocated uplink and downlink radio resources for point-to-point, point-to-multipoint multicast or group call service types are used independently of each other. Dependent allocation of uplink and downlink shall be possible, in order to allow simple MSs to transfer data simultaneously in both directions. Allocation of several PDTCHs for one MS is possible. The access to the GPRS uplink uses a Slotted-Aloha based reservation protocol. The Network Protocol Data Units (N-PDU) are segmented into the Subnetwork Protocol Data Units (SN-PDU) by the Subnetwork Dependent Convergence (SNDC) protocol and SN-PDUs are encapsulated into one or several LLC frames. See GSM 03.60 [3] for information on SNDC and LLC. The details on SNDC can be found in GSM 04.65 [9] and the details on LLC can be found in GSM 04.64 [8]. LLC frames are segmented into RLC Data Blocks. At the RLC/MAC layer, a selective ARQ protocol (including block numbering) between the MS and the Network provides retransmission of erroneous RLC Data Blocks. When a complete LLC frame is successfully transferred across the RLC layer, it is forwarded to the LLC layer. Normal burst Normal burst Normal burst Normal burst BH FH LLC layer RLC/MAC layer Physical layer Information field FCS Info field BH BCS RLC blocks LLC frame Primary block Following block Info field BCS Info field BH BCS FH = Frame Header FCS = BH = BCS = Block Check Sequence Frame Check Sequence Block Header Figure 8: Transmission and reception data flow
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101 350
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6.6.4.1 Uplink State Flag
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The Uplink State Flag (USF) is used on PDCH to allow multiplexing of Radio blocks from a number of MSs. USF is used in dynamic and extended dynamic medium access modes. USF is used only in downlink direction. The USF comprises 3 bits at the beginning of each Radio Block that is sent on the downlink. It enables the coding of 8 different USF states which are used to multiplex the uplink traffic. On PCCCH, one USF value is used to denote PRACH. The other USF values are used to reserve the uplink for different MSs. On PDCHs not carrying PCCCH, the eight USF values are used to reserve the uplink for different MSs. One USF value shall be used to prevent collision on uplink channel, when MS without USF is using uplink channel. The USF points either to the next uplink Radio Block or the sequence of 4 uplink Radio Blocks starting with the next uplink Radio Block.
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6.6.4.2 Temporary Block Flow
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A Temporary Block Flow (TBF) is a physical connection used by the two RR entities to support the unidirectional transfer of LLC PDUs on packet data physical channels. The TBF is allocated radio resource on one or more PDCHs and comprise a number of RLC/MAC blocks carrying one or more LLC PDUs. A TBF is temporary and is maintained only for the duration of the data transfer. ETSI ETSI TS 101 350 V7.0.0 (1999-07) 32 (GSM 03.64 version 7.0.0 Release 1998)
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6.6.4.3 Temporary Flow Identity
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Each TBF is assigned a Temporary Flow Identity (TFI) by the network. The assigned TFI is unique among concurrent TBFs in each directions and is used instead of the MS identity in the RLC/MAC layer. The same TFI value may be used concurrently for TBFs in opposite directions. The TFI is assigned in a resource assignment message that precedes the transfer of LLC frames belonging to one TBF to/from the MS. The same TFI is included in every RLC header belonging to a particular TBF as well as in the control messages associated to the LLC frame transfer (e.g. acknowledgements) in order to address the peer RLC entities.
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6.6.4.4 Medium Access modes
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Three medium access modes are supported: - Dynamic allocation; - Extended Dynamic allocation; and - Fixed allocation The Dynamic allocation medium access mode or Fixed allocation medium access mode shall be supported by all networks that support GPRS. The support of Extended Dynamic allocation is optional. The Dynamic allocation and Fixed allocation modes shall be supported in all mobile stations.
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101 350
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6.6.4.5 Acknowledged mode for RLC/MAC operation
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The transfer of RLC Data Blocks in the acknowledged RLC/MAC mode is controlled by a selective ARQ mechanism coupled with the numbering of the RLC Data Blocks within one Temporary Block Flow. The sending side (the MS or the network) transmits blocks within a window and the receiving side sends Packet Uplink Ack/Nack or Packet Downlink Ack/Nack message when needed. Every such message acknowledges all correctly received RLC Data Blocks up to an indicated block sequence number (BSN), thus “moving” the beginning of the sending window on the sending side. Additionally, the bitmap that starts at the same RLC Data Block is used to selectively request erroneously received RLC Data Blocks for retransmission. The sending side then retransmits the erroneous RLC Data Blocks, eventually resulting in further sliding the sending window. The Packet Ack/Nack message does not include any change in the current assignment (and thus does not have to be acknowledged when sent on downlink). A missing Packet Ack/Nack is not critical and a new one can be issued whenever. In Packet Downlink Ack/Nack message, the MS may optionally initiate an uplink TBF. In Packet Uplink Ack/Nack message , the network can assign uplink resources for mobile station using a fixed allocation. When receiving uplink data from a MS the network shall, based on erroneous blocks received from MS, allocate additional resources for retransmission. The acknowledgement procedure of the LLC layer is not combined with the acknowledgement procedure on the underlying RLC/MAC layer.
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6.6.4.6 Unacknowledged mode for RLC/MAC operation
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The transfer of RLC Data Blocks in the unacknowledged RLC/MAC mode is controlled by the numbering of the RLC Data Blocks within one Temporary Block Flow and does not include any retransmissions. The receiving side extracts user data from the received RLC Data Blocks and attempts to preserve the user information length by replacing missing RLC Data Blocks by dummy information bits. The same mechanism and message format for sending temporary acknowledgement messages is used as for acknowledged mode in order to convey the necessary control signalling (e.g. monitoring of channel quality for downlink channel or timing advance correction for uplink transfers). The fields for denoting the erroneous RLC blocks may be used as an additional measure for channel quality (i.e. parameter for link adaptation). The sending side (the MS or the network) transmits a number of radio blocks and then polls the receiving side to send an acknowledgement message. The Packet Uplink Ack/Nack and Packet Downlink Ack/Nack message does not include any change in the current assignment. A missing acknowledgement message is not critical and a new one can be obtained whenever. In Packet Downlink Ack/Nack message, the MS may optionally initiate an uplink TBF. In Packet Uplink Ack/Nack message , the network can assign uplink resources for mobile station using a fixed allocation. ETSI ETSI TS 101 350 V7.0.0 (1999-07) 33 (GSM 03.64 version 7.0.0 Release 1998)
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6.6.4.7 Mobile Originated Packet Transfer
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6.6.4.7.1 Uplink Access
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Network Packet Channel Request Packet Uplink Assignment Packet Resource Request Packet Uplink Assignment PRACH (or RACH ) PAGCH (or AGCH ) PACCH PACCH (Optional) (Optional) Figure 9: Access and allocation for the one or two phase packet access, uplink packet transfer An MS initiates a packet transfer by making a Packet Channel Request on PRACH or RACH. The network responds on PAGCH or AGCH respectively. It is possible to use one or two phase packet access method (see Figure 9). In the one phase access, the Packet Channel Request is responded by the network with the Packet Uplink Assignment reserving the resources on PDCH(s) for uplink transfer of a number of Radio blocks. The reservation is done accordingly to the information about the requested resources that is comprised in the Packet Channel Request. On RACH, there is only two cause values available for denoting GPRS, which can be used to request limited resources or two phase access. On PRACH, the Packet Channel Request may contain more adequate information about the requested resources and, consequently, uplink resources on one or several PDCHs can be assigned by using the Packet Uplink Assignment message. In the two phase access, the Packet Channel Request is responded with the Packet Uplink Assignment which reserves the uplink resources for transmitting the Packet Resource Request. A two phase access can be initiated by the network or a mobile station. The network can order the MS to send Packet Resource Request message by setting parameter in Packet Uplink Assignment message. Mobile station can require two phase access in Packet Channel Request message. In this case, the network may order MS to send Packet Resource Request or continue with a one phase access procedure. The Packet Resource Request message carries the complete description of the requested resources for the uplink transfer. The MS can indicate the medium access method, it prefers to be used during the TBF. The network responds with the Packet Uplink Assignment reserving resources for the uplink transfer and defining the actual parameters for data transfer (e.g. medium access mode). If there is no response to the Packet Channel Request within predefined time period, the MS makes a retry after a random backoff time. On PRACH there is used a 2-step approach including a long-term and a short-term estimation of the persistence (see Figure 10). The optimal persistence of the mobile stations is calculated at the network side. ETSI ETSI TS 101 350 V7.0.0 (1999-07) 34 (GSM 03.64 version 7.0.0 Release 1998) access control decoding of parameters persistence control short-term estimation access analysis long-term estimation persistence instruction set P contention levels ρ(i) access (priority i) network side mobile station Figure 10: Basic principle of random access traffic control The actual persistence values depend on: - the priority i of the packet to be transmitted; - the amount of traffic within higher priority classes; - the amount of traffic within the own priority class. Optionally, the existing backoff algorithm on RACH can be used on PRACH. On RACH, the existing backoff algorithm shall be used. Occasionally, more Packet Channel Requests can be received than can be served. To handle this, a Packet Queuing Notification is transmitted to the sender of the Packet Channel Request. The notification includes information that the Packet Channel Request message is correctly received and Packet Uplink Assignment may be transmitted later. If the Timing Advance information becomes inaccurate for an MS, the network can send Packet Polling Request to trigger the MS to send four random access bursts. This can be used to estimate the new Timing Advance before issuing the Packet Uplink Assignment.
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101 350
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6.6.4.7.2 Dynamic/Extended Dynamic allocation
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101 350
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6.6.4.7.2.1 Uplink Packet Transfer
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The Packet Uplink Assignment message includes the list of PDCHs and the corresponding USF value per PDCH. A unique TFI is allocated and is thereafter included in each RLC Data and Control Block related to that Temporary Block Flow. The MS monitors the USFs on the allocated PDCHs and transmits Radio blocks on those which currently bear the USF value reserved for the usage of the MS. If the resource assigned by the network in the case of extended dynamic allocation does not allow the multislot MS (see GSM 05.02, annex B) to monitor the USF on all the assigned PDCHs, the following rules shall apply: - Whenever the MS receives its USF on one downlink PDCH (e.g. on timeslot 0 while timeslots 0, 2 and 3 were assigned), it shall consider the corresponding uplink block and all subsequent ones from the list of assigned PDCHs as allocated (e.g. on 0, 2 and 3). Hence, if the network allocates a block to this MS on an assigned PDCH, it shall also allocate blocks to this MS on all subsequent PDCHs in the list. For each allocated block, the network shall set the USF to the value reserved for the usage of that MS. These rules apply on a block period basis. ETSI ETSI TS 101 350 V7.0.0 (1999-07) 35 (GSM 03.64 version 7.0.0 Release 1998) - During block periods where it is transmitting, the MS shall monitor the USF on each PDCH in the list of assigned PDCHs, up to and including the first PDCH currently used for transmission. This rule applies on a block period basis. For example, if timeslots 0, 2 and 3 have been assigned and blocks are currently allocated on timeslots 2 and 3, then during this block period the MS monitors USF on timeslots 0 and 2. If the reserved value of USF is found on timeslot 0, then the next allocated blocks shall be on timeslots 0, 2 and 3. If the reserved value of USF is found on timeslot 2, then the next allocated blocks shall be on timeslots 2 and 3. And so on for the subsequent block periods. Because each Radio Block includes an identifier (TFI), all received Radio blocks are correctly associated with a particular LLC frame and a particular MS, thus making the protocol highly robust. By altering the state of USF, different PDCHs can be "opened" and "closed" dynamically for certain MSs thus providing a flexible reservation mechanism. Additionally, packets with higher priority and pending control messages can temporarily interrupt a data transmission from one MS. The channel reservation algorithm can also be implemented on assignment basis. This allows individual MSs to transmit a predetermined amount of time without interruptions. The MS may be allowed to use the uplink resources as long as there is queued data on the RLC/MAC layer to be sent from the MS. It can comprise a number of LLC frames. In that sense the radio resources are assigned on the initially “unlimited” time basis. Alternatively, the uplink assignment for each assignment may be limited to a number of radio blocks (e.g. in order to offer more fair access to the medium at higher loads). The selective ARQ operation for the acknowledged RLC/MAC mode is described in Subclause 6.6.4.5. The unacknowledged RLC/MAC mode operation is described in Subclause 6.6.4.6. Figure 11 shows an example of message sequence for the (multislot) uplink data transfer with one resource reallocation and possible RLC Data Block re-transmissions. Packet Uplink Ack/Nack Data Block (last) Access and Assignment MS Network PDTCH PACCH PDTCH Packet Uplink Assignment Packet Control Acknowledgement PACCH PACCH Data Block PDTCH Data Block PDTCH Data Block (last in send window) PDTCH Data Block PDTCH Data Block PDTCH Data Block PDTCH Data Block PDTCH Packet Uplink Ack/Nack (final) PACCH Figure 11: An example of dynamic allocation uplink data transfer ETSI ETSI TS 101 350 V7.0.0 (1999-07) 36 (GSM 03.64 version 7.0.0 Release 1998)
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6.6.4.7.2.2 Release of the Resources
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The release of the resources is normally initiated from the MS by counting down the last couple of blocks. For the normal release of resources for RLC connection carrying a mobile originated packet transfer, the mechanism based on acknowledged final Packet Uplink Ack/Nack combined with timers is used. After the MS has sent its last RLC Data Block (indicated by the countdown field), the acknowledgement is expected from the network side. By sending the last block, the MS may no longer use the same assignment unless a negative acknowledgement arrives. It also means that the network side may reallocate the same USF(s) to some other user as soon as all the RLC Data Blocks belonging to that Temporary Block Flow are correctly received; that regardless of the possible later errors in the acknowledgements. The next step, in the case of all RLC Data Blocks being correctly received, is that the network sends Packet Uplink Ack/Nack which is to be immediately acknowledged by the MS in the reserved uplink block period. It must be possible for the network not to use the mechanism of acknowledgement for Packet Ack/Nack in which case the release of the resources procedure relies only on timers. The TFI can be reused for another assignment either upon the reception of the acknowledgement for Packet Ack/Nack or after expiry of the guard timer. Further, the premature release or change of assignment for one MS can be initiated by the network. In the case of release, the MS is ordered to interrupt the Temporary Block Flow. The MS shall then reorganise the uplink buffer and issue a new Packet Channel Request to continue the uplink transfer with the RLC Data Blocks containing untransferred (i.e. on the RLC/MAC layer unacknowledged) LLC frames. In the case of the change in assignment, the Packet Uplink Assignment or Packet Timeslot Reconfigure message is issued.
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6.6.4.7.3 Fixed Allocation
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Fixed allocation uses the Packet Uplink Assignment message to communicate a detailed fixed uplink resource allocation to the MS. The fixed allocation consists of a start frame, slot assignment, and block assignment bitmap representing the assigned blocks per timeslot. The MS waits until the start frame indicated and then transmits radio blocks on those blocks indicated in the block assignment bitmap. The fixed allocation does not include the USF and the MS is free to transmit on the uplink without monitoring the downlink for the USF. Unused USF value is used to prevent other mobiles to transmit. If the current allocation is not sufficient, the MS may request additional resources in one of the assigned uplink blocks. A unique TFI is allocated and is thereafter included in each RLC data and control block related to that Temporary Block Flow. Because each Radio Block includes an identifier (TFI), all received Radio blocks are correctly associated with a particular LLC frame and a particular MS. The number of blocks an MS requests in the initial and subsequent allocation request messages shall only account for the number of data and control blocks it intends to send. The MS shall not request additional blocks for the retransmission of erroneous blocks. The network can repeat the allocation of radio resources by setting the parameter in the Packet Uplink Assignment or the Packet Uplink Ack/Nack message. The selective ARQ operation for the acknowledged RLC/MAC mode is described in Subclause 6.6.4.5. The unacknowledged RLC/MAC mode operation is described in Subclause 6.6.4.6. Figure 11 shows an example of message sequence for the (multislot) uplink data transfer with one resource reallocation and possible RLC Data Block re-transmissions.
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101 350
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6.6.4.7.4 Contention Resolution
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Contention resolution is an important part of RLC/MAC protocol operation, especially because one channel allocation can be used to transfer a number of LLC frames. Contention resolution applies for both dynamic and fixed allocation medium access modes. There are two basic access possibilities, one phase and two phase access as defined in Subclause 6.6.4.7.1. The two phase access is inherently immune for possibility that two MSs can perceive the same channel allocation as their own. Namely the second access phase, the Packet Resource Request, uniquely identifies the MS by its TLLI. The same TLLI is included in the Packet Uplink Assignment/Packet Downlink Assignment and no mistake is possible. The one phase access is somewhat insecure and an efficient contention resolution mechanism has to be introduced. ETSI ETSI TS 101 350 V7.0.0 (1999-07) 37 (GSM 03.64 version 7.0.0 Release 1998) The first part of the solution is the identification of the MS. The identification of transmitting MS on the RLC/MAC level is necessary not only for contention resolution but also to be able to establish RLC protocol entity for that Temporary Block Flow on the network side. Additionally, the TLLI is necessary to be able to match simultaneous uplink and downlink packet transfers by taking into consideration multislot capability of that MS. In order to uniquely identify the MS when sending on uplink, the RLC Header for all the RLC Data Blocks on uplink is extended to include the TLLI until the contention resolution is completed on the MS side. The second part of the solution is the notification from the network side about who owns the allocation. That is solved by the inclusion of the TLLI in the Packet Uplink Ack/Nack/Packet Downlink Ack/Nack. This message shall be sent in an early stage, even before the receive window for RLC/MAC protocol operation is full. By doing so, the contention is resolved after the first occurrence of Packet Ack/Nack. The possibility of RLC Data Blocks being captured from “wrong” MS, thus destroying the LLC frame, shall be covered for by retransmissions on the LLC layer.
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6.6.4.8 Mobile Terminated Packet Transfer
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6.6.4.8.1 Packet Paging
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The network initiates packet transfer to an MS that is in Standby state by sending a Packet Paging Request on the downlink PPCH or PCH. The MS responds to the Packet Paging Request by initiating a procedure for page response. The RLC/MAC Packet Paging Response message contains TLLI, as well as a complete LLC frame including also TLLI (see Figure 12). The message sequence described in Figure 12 below is conveyed either on PCCCH or on CCCH. After the Packet Paging Response, the mobility management state of the MS is Ready. Network Packet Channel Request Packet Downlink Assignment Assignment Packet Paging Response (LLC frame) PRACH or RACH PAGCH (or AGCH ) PACCH PPCH or PCH Packet Paging Request ( ( ) ) Figure 12: Paging message sequence for Paging, downlink packet transfer
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101 350
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6.6.4.8.2 Downlink Packet Transfer
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Transmission of a packet to an MS in the Ready state is initiated by the network using the Packet Downlink Assignment message. In case there is PCCCH allocated in the cell, the Packet Downlink Assignment is transmitted on PAGCH. In case there is no PCCCH allocated in the cell, the Immediate Assignment is transmitted on AGCH. The Packet Downlink Assignment message includes the list of PDCH(s) that will be used for downlink transfer. The Timing Advance and Power Control information is also included, if available. Otherwise, the MS may be requested to respond with an Packet Control Acknowledgement (see also Subclause 6.5.7 on timing advance procedures). The MS multislot capability needs to be considered. The network sends the Radio blocks belonging to one Temporary Block Flow on downlink on the assigned downlink channels. Multiplexing the Radio blocks destined for different MSs on the same PDCH downlink is enabled with an identifier, e.g. TFI, included in each Radio Block. The interruption of data transmission to one MS is possible. The acknowledged (i.e. selective ARQ operation) and unacknowledged RLC/MAC mode operation is described in Subclauses 6.6.4.5 and 6.6.4.6. The sending of Packet Downlink Ack/Nack is obtained by the occasional network initiated polling of the MS. The MS sends the Packet Downlink Ack/Nack message in a reserved Radio Block which is allocated together with polling. Unassigned USF value is used in the downlink Radio Block which corresponds to the reserved uplink Radio blocks. Further, if the MS wants to send some additional signalling or uplink data, it may be indicated in the Packet Downlink Ack/Nack message. ETSI ETSI TS 101 350 V7.0.0 (1999-07) 38 (GSM 03.64 version 7.0.0 Release 1998) Figure 13 shows an example of message sequence for (multislot) downlink data transfer with one resource reallocation and possible RLC Data Block re-transmissions. Packet Downlink Ack/Nack Access and Assignment MS Network PDTCH PACCH Packet Downlink Assignment Packet Control Acknowledgement PACCH PACCH PACCH Packet Downlink Ack/Nack (final) Data Block PDTCH Data Block PDTCH Data Block (polling) PDTCH Data Block PDTCH Data Block PDTCH Data Block PDTCH Data Block PDTCH Data Block PDTCH Data Block (last, polling) PACCH Figure 13: An example of downlink data transfer
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101 350
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6.6.4.8.3 Release of the Resources
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The release of the resources is initiated by the network by terminating the downlink transfer and polling the MS for a final Packet Downlink Ack/Nack. It is possible for the network to change the current downlink assignment by using the Packet Downlink Assignment or Packet Timeslot Reconfigure message, which then has to be acknowledged by the MS in an immediate reserved block period on the uplink. The handling of TFI is steered with the same timer that runs on both the MS and the network side after the last RLC Data Block is sent to the MS. When it expires, the current assignment becomes invalid for the MS and TFI can be reused by the network. Further, upon the reception of the final Packet Downlink Ack/Nack from the MS, another timer is started on network side. When it expires, the current assignment becomes invalid for the MS and TFI can be reused by the network. ETSI ETSI TS 101 350 V7.0.0 (1999-07) 39 (GSM 03.64 version 7.0.0 Release 1998)
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6.6.4.9 Simultaneous Uplink and Downlink Packet Transfer
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During the ongoing uplink Temporary Block Flow, the MS continuously monitors one downlink PDCH for possible occurrences of Packet Downlink Assignment or Packet Timeslot Reconfigure messages on PACCH (see Figure 11). The MS is therefore reachable for downlink packet transfers that can then be conveyed simultaneously on the PDCH(s) that respect the MS multislot capability. If the MS wants to send packets to the network during the ongoing downlink Temporary Block Flow, it can be indicated in the acknowledgement that is sent from the MS. By doing so, no explicit Packet Channel Requests have to be sent to the network. Further, the network already has the knowledge of which PDCH(s) that particular MS is currently using so that the uplink resources can be assigned on the PDCH(s) that respect the MS multislot capability. This method may introduce an extra delay when initiating the uplink packet transfer but only for the first LLC frame in a sequence.
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6.7 Abnormal cases in GPRS MS Ready State
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The RLC/MAC error causes and procedures to handle these can be found in GSM 04.08, 04.60 and 05.08.
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6.8 PTM-M Data Transfer
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NOTE: The stage 3 specification for PTM-M data transfer is left for phase 2 of GPRS specification. PTM-M data, in the form of individual LLC frames, is mapped into RLC/MAC-PTM_DATA primitive and distributed from SGSN to the BSS representing the cells that are defined by a geographical area parameter. To the cells concerned, the BSS for each PTM-M LLC frame: - Optionally, sends a “PTM-M new message” indicator on all individual paging channels on PCCCH if allocated, otherwise on CCCH. The indication refers to a PTM-M notification channel PNCH on PCCCH or NCH on CCCH, where a notification for the new PTM-M message can be received. If the indicator option is not supported, or if an MS can not receive the indicator when expected, e.g. because the corresponding block in the multiframe structure is used for other purposes than paging, the MS must read the notification channel. - Sends a PTM-M notification on PNCH or NCH. The notification has the form of a Packet Resource Assignment for the PTM-M LLC frame. The notification includes a group identity IMGI, a unique LLC frame identifier (in the form of an N-PDU number together with a segment offset, see GSM 04.65) and an allocation of a TFI to be used in all RLC blocks of the LLC frame. - Transmits the PTM-M LLC frame on the assigned downlink resources. Transfer of PTM-M data is carried out without any ARQ on the RLC/MAC and LLC layers. Instead, each LLC frame is retransmitted a specified number of times. For each retransmission, the above procedure is performed. The PTM-M notification (resource assignment) includes the unique LLC frame identifier as in the first transmission but a new allocation of TFI. An MS accumulates correctly received RLC blocks from each transmission to assemble an LLC frame. The dimensioning of PNCH shall be scaleable depending on capacity requirements. An NCH may, if capacity allows, be used as a shared notification channel for PTM-M and Advanced Speech Call Items (ASCI). An MS only interested in PTM-M needs to listen only to PNCH/NCH. ETSI ETSI TS 101 350 V7.0.0 (1999-07) 40 (GSM 03.64 version 7.0.0 Release 1998) Annex A (informative): Bibliography 1) ITU-T I.130, Method for the Characterization of Telecommunication Services Supported by an ISDN 2) ITU-T Q.65, Stage 2 of the Method for Characterization of the Services Supported by an ISDN 3) DIS 8886, OSI Data Link Service Definition 4) DIS 10022, OSI Physical Service Definition 5) ISO 10039, Medium Access Control Service Definition 6) ISO 4335, HDLC Procedures 7) ISO 7478, Multilink Procedures 8) ISO 7498, OSI Basic Reference Model and Layer Service Conventions ETSI ETSI TS 101 350 V7.0.0 (1999-07) 41 (GSM 03.64 version 7.0.0 Release 1998) Annex B (informative): Document change history SPEC SMG# CR PHA VERS NEW_VER SUBJECT 03.64 s22 NEW 2+ 2.1.1 5.0.0 GSM 03.64 GPRS Stage 2 Radio 03.64 s23 A022 R97 5.0.0 5.1.0 Unacknowledged mode of RLC/MAC operation 03.64 s23 A023 R97 5.0.0 5.1.0 Improved RLC Service Primitives 03.64 s23 A024 R97 5.0.0 5.1.0 Enhancements to dynamic allocation 03.64 s23 A025 R97 5.0.0 5.1.0 Clarifications to DRX 03.64 s23 A026 R97 5.0.0 5.1.0 Optimisation for network control cell reselection 03.64 s23 A027 R97 5.0.0 5.1.0 Abnormal Cases in GPRS MS Ready State: Leaky Bucket Procedure 03.64 s23 A029 R97 5.0.0 5.1.0 Multiframe structure (details) (revision of SMG2 GPRS 301/97) 03.64 s23 A030 R97 5.0.0 5.1.0 Abnormal Cases in GPRS MS Ready State 03.64 s23 A031 R97 5.0.0 5.1.0 Cell Re-Selection in GPRS 03.64 s23 A032 R97 5.0.0 5.1.0 Definition of PACCH 03.64 s23 A033 R97 5.0.0 5.1.0 Clarifications on Timing advance procedure 03.64 s23 A035 R97 5.0.0 5.1.0 Bit order for USF coding in GPRS 03.64 s23 A036 R97 5.0.0 5.1.0 PTM-M 03.64 s23 A037 R97 5.0.0 5.1.0 Contention resolution 03.64 s23 A039 R97 5.0.0 5.1.0 Deleting parameter XHYST 03.64 s24 A031 R97 5.1.0 5.2.0 Clarification on the use of hysteresis for cell re-selection 03.64 s25 A043 R97 5.2.0 6.0.0 Clarification of the use of TAI 03.64 s25 A049 R97 5.2.0 6.0.0 USF granularity for dynamic allocation 03.64 s26 R97 6.0.0 6.0.1 Editorial changes for Publication 03.64 s27 A050 R97 6.0.1 6.1.0 Changes on all chapters to align the spec. with other GPRS specifications. 03.64 s27 A044 R97 6.0.1 6.1.0 Clarification on PACCH allocation for fixed assignment 03.64 s28 A052 R97 6.1.0 6.2.0 PBCCH scheduling and editorial corrections 03.64 s28 A051 R97 6.1.0 6.2.0 Interference measurements on network control 03.64 s28 A053 R97 6.1.0 6.2.0 51-multiframe PBCCH 03.64 s29 A055 R97 6.2.0 6.3.0 Miscellaneous corrections 03.64 s29 A056 R97 6.2.0 6.3.0 Clarification of polling response 03.64 s29 A057 R97 6.2.0 6.3.0 Correction to 1 phase access contention resolution 03.64 s29 R98 6.3.0 7.0.0 R98 upgrade only ETSI ETSI TS 101 350 V7.0.0 (1999-07) 42 (GSM 03.64 version 7.0.0 Release 1998) History Document history V7.0.0 July 1999 Publication ISBN 2-7437-3365-9 Dépôt légal : Juillet 1999
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1 Scope
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The present document specifies the Network Service used on the Base Station System (BSS) to Serving GPRS Support Node (SGSN) interface (Gb interface). The protocol stack on the Gb interface is defined in the stage 2 Technical Specification GSM 03.60 [3]. The Network Service entity provides network services to the BSSGP entity specified in GSM 08.18 [5]. The layer 1 of the Gb interface is specified in GSM 08.14 [4]. In the present document, the communication between adjacent layers and the services provided by the layers are distributed by use of abstract service primitives. But only externally observable behaviour resulting from the description is normatively prescribed by the present document. The service primitive model used in the present document is based on the concepts developed in CCITT Recommendation X.200 [13].
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2 References
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The following documents contain provisions which, through reference in this text, constitute provisions of the present document. • References are either specific (identified by date of publication, edition number, version number, etc.) or non-specific. • For a specific reference, subsequent revisions do not apply. • For a non-specific reference, the latest version applies. • A non-specific reference to an ETS shall also be taken to refer to later versions published as an EN with the same number. • For this Release 1998 document, references to GSM documents are for Release 1998 versions (version 7.x.y). [1] GSM 01.04: "Digital cellular telecommunications system (Phase 2+); Abbreviations and acronyms". [2] GSM 02.60: "Digital cellular telecommunications system (Phase 2+); General Packet Radio Service (GPRS); Service description; Stage 1". [3] GSM 03.60: "Digital cellular telecommunications system (Phase 2+); Stage 2 Service Description of the General Packet Radio Service (GPRS)". [4] GSM 08.14: "Digital cellular telecommunications system (Phase 2+); General Packet Radio Service (GPRS); Base Station System (BSS) - Serving GPRS Support Node (SGSN) interface; Gb interface Layer 1". [5] GSM 08.18: " Digital cellular telecommunications system (Phase 2+); General Packet Radio Service (GPRS); Base Station System (BSS) - Serving GPRS Support Node (SGSN) interface; BSS GPRS Protocol (BSSGP)". [6] FRF 1.1 (January 19, 1996): "The Frame Relay Forum User-to-Network Implementation Agreement (UNI)". [7] ITU-T I.233.1 (10/95): "ISDN Frame Relaying Bearer Service". [8] ITU-T Q.921 (10/95): "ISDN user-network interface-Data link layer specification". [9] ITU-T Q.922 (02/92): "ISDN data link layer specification for frame mode bearer services". [10] ITU-T Q.931 (10/95): " ISDN user-network interface layer 3 specification for Basic Call Control". ETSI ETSI TS 101 299 V7.1.0 (1999-07) 7 (GSM 08.16 version 7.1.0 Release 1998) [11] ITU-T revised Q.933 (10/95): "Digital Subscriber Signalling System No. 1 (DSS 1) - Signalling specification for frame mode basic call control". [12] ITU-T I.370 (10/95): "Congestion management for the ISDN Frame Relaying Bearer Service". [13] CCITT X.200 (White Book): "Reference model of open systems interconnection for CCITT applications". [14] ANSI T1.602 – ISDN – Data Link Layer Signalling Specification for Application at the User- Network Interface. [15] ANSI T1.606 - Frame Relay Bearer Service - Architecture Framework and Service description – 1990 (R 1996). [16] ANSI T1.617 - DSS1 Signaling Specification for Frame Relay Bearer Service 1991 (R1997). [17] ANSI T1.618 - DSS1 Core Aspects of Frame Relay Protocol for Use with Frame Relay Bearer Service 1991 (R1997).
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3 Definitions, symbols and abbreviations
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3.1 Definitions
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For the purposes of the present document, the following terms and definitions apply. For additional applicable definitions refer to GSM 02.60 [2]. BSSGP Virtual Connection (BVC): An end-to-end virtual communication path between remote Network Service user entities. BSSGP Virtual Connection Identifier (BVCI): The identifier of a BVC, having end-to-end significance across the Gb interface. Network Service Entity Identifier (NSEI): The identifier of an NS Entity having end-to-end significance across the Gb interface. Network Service Virtual Connection (NS-VC): An end-to-end virtual communication path between Network Service peer entities. Network Service Virtual Connection Identifier (NS-VCI): The identifier of an NS-VC having end-to-end significance across the Gb interface. Network Service Virtual Link (NS-VL): A virtual communication path between the BSS or the SGSN and the intermediate network, or between the BSS and the SGSN in case of direct point-to-point configuration. Network Service Virtual Link Identifier (NS-VLI): The identifier of an NS-VL, having local significance at the BSS or SGSN. Network Service Virtual Connection Group: Groups all NS-VCs together which provide communication between the same peer NS entities. This grouping has local significance at the BSS or SGSN. Blocked / unblocked: When an NS-VC can not be used for NS user traffic, it is blocked. When an NS-VC can be used for NS user traffic, it is unblocked. Dead / alive: When an NS-VC is able to provide communication between remote NS entities, it is alive. When it is not able, it is dead. These states are supervised by means of a test procedure, as further described in this Technical Specification.
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3.2 Symbols
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Refer to GSM 03.60 [3]. ETSI ETSI TS 101 299 V7.1.0 (1999-07) 8 (GSM 08.16 version 7.1.0 Release 1998)
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3.3 Abbreviations
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For the purposes of the present document, the following abbreviations apply. Additional applicable abbreviations can be found in GSM 01.04 [1]. When there is conflict between the present document and GSM 01.04 [1], the following list takes precedence. BECN Backward Explicit Congestion Notification BSSGP Base Station System GPRS Protocol BVC BSSGP Virtual Connection BVCI BSSGP Virtual Connection Identifier CLLM Consolidated Link Layer Management DE Discard Eligibility FECN Forward Explicit Congestion Notification FR Frame Relay FRF Frame Relay Forum LLC Logical Link Control LSP Link Selector Parameter MAC Medium Access Control NS Network Service NSEI Network Service Entity Identifier NS-SAP Network Service Service Access Point NS-VC Network Service Virtual Connection NS-VCI Network Service Virtual Connection Identifier NS-VL Network Service Virtual Link NS-VLI Network Service Virtual Link Identifier PDU Protocol Data Unit PTP Point-To-Point PTM Point-To-Multipoint PVC Permanent Virtual Connection RLC Radio Link Control SGSN Serving GPRS Support Node UNI User-to-Network Interface
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4 Network Service general description
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4.1 Overview
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The position of the Network Service within the protocol stack of the Gb interface is shown in Figure 1/GSM 08.16. Gb BSS LLC BSSGP L1 SGSN NS L1 MAC BSSGP RLC RELAY NS NOTE: BSSGP, L1, LLC, MAC, RELAY, RLC are outside the scope of this Technical Specification, refer to TS GSM 03.60 [3] for further details. Figure 1/GSM 08.16: Position of the NS within the Gb interface protocol stack ETSI ETSI TS 101 299 V7.1.0 (1999-07) 9 (GSM 08.16 version 7.1.0 Release 1998) The Network Service performs the transport of NS SDUs between the SGSN and BSS. The services provided to the NS user shall be: - Network Service SDU transfer. The Network Service entity shall provide network service primitives allowing for transmission and reception of upper layer protocol data units between the BSS and SGSN. The NS SDUs are transferred in order by the Network Service, but under exceptional circumstances order may not be maintained. - Network congestion indication. Congestion recovery control actions may be performed by the Sub-Network Service (e.g. Frame Relay). Congestion reporting mechanisms available in the Sub-Network Service implementation shall be used by the Network Service to report congestion. - Status indication. Status indication shall be used to inform the NS user of the NS affecting events e.g. change in the available transmission capabilities. The Network Service entity is composed of an entity dependent on the intermediate transmission network used on the Gb interface, the Sub-Network Service, and of a control entity independent from that network, the Network Service Control. There is a hierarchical relationship between both entities. This is reflected in Figure 2/GSM 08.16. The detailed communication mechanisms between both entities is an internal matter for the Network Service and is not further standardized. Sub-Network Service / Sub-Network Service protocol Network Service Control / Network Service Control protocol Network Service Figure 2/GSM 08.16: Internal architecture of the Network Service The Sub-Network Service entity provides a communication service to Network Service Control peer entities. The Network Service Control peer entities use the Sub-Network Service for communication with each other. The peer-to- peer communication accross the Gb interface between remote Network Service Control entities is performed over Network Service Virtual Connections (NS-VCs). An NS-VC is a virtual communication path between Network Service Control peer entities. The Network Service entity provides a communication service to NS user peer entities: the peer-to-peer communication between remote NS user entities is performed over BSSGP Virtual Connections (BVCs). A BVC is a virtual communication path between Network Service user peer entities. A Network Service Entity communicates with only one peer Network Service Entity. Addressing across the Gb interface is further detailed in the rest of this Technical Specification. The Network Service Control entity is responsible for the following functions: - NS SDU transmission: The NS SDUs shall be transmitted on the NS-VCs. The NS SDUs are encapsulated into Network Service Control PDUs which in turn are encapsulated into Sub-Network Service PDUs. - Load sharing: The load sharing function distributes the NS SDU traffic amongst the available (i.e. unblocked) NS-VCs of a group. ETSI ETSI TS 101 299 V7.1.0 (1999-07) 10 (GSM 08.16 version 7.1.0 Release 1998) - NS-VC management: A blocking procedure is used by an NS entity to inform an NS peer entity when an NS-VC becomes unavailable for NS user traffic. An unblocking procedure is used for the reverse operation. A reset procedure is used between peer NS entities in order to set an NS-VC to a determined state, after events resulting in possibly inconsistent states of the NS-VC at both sides of the Gb interface. A test procedure is used to check that an NS-VC is operating properly between peer NS entities.
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4.2 Addressing
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The purpose of this clause is to describe the addressing principles on the Gb interface in a generic way, i.e. irrespective of the exact configuration of the Gb interface and of the exact nature of the intermediate transmission network, when present. Therefore, this clause provides an abstract description of the addressing principles. These principles are then applied to real networks in clause "Sub-Network Service protocol". In this clause, addressing is considered in the general case where an SGSN is connected to several BSSs via an intermediate transmission network. Point-to-point physical connections may also be used, addressing in this special case can be derived from the general case.
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4.2.1 Network Service Virtual Link (NS-VL)
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An SGSN and a BSS may use different physical links for connecting to each other (e.g. because of intermediate equipment or transmission network). Each physical link is locally (i.e. at each side of the Gb interface) identified by means of a physical link identifier. The exact structure of the physical link identifier is implementation dependent. Each physical link supports one or more Network Service Virtual Links (NS-VLs). Each NS-VL is supported by one physical link. The exact nature of the NS-VL depends on the intermediate network used on the Gb interface. In the general case of an intermediate transmission network, the NS-VL is used to access the intermediate network. Communication means internal to the intermediate network are outside the scope of this Technical Specification. The NS-VLs may alternatively be used end-to-end between the BSS and SGSN, in case of a point-to-point configuration on the Gb interface. Each NS-VL is identified by means of a Network Service Virtual Link Identifier (NS-VLI). The significance (i.e. local or end-to-end) and the exact structure of the NS-VLI depends on the configuration of the Gb interface and on the intermediate network used. For example, in the case of a Frame Relay network, the physical link is the FR bearer channel, the NS-VL is the local link (at UNI) of the FR permanent virtual connection (PVC) and the NS-VLI is the association of the FR DLCI and bearer channel identifier.
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4.2.2 Network Service Virtual Connection (NS-VC)
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In order to provide end-to-end communication between the BSS and SGSN irrespective of the exact configuration of the Gb interface, the concept of Network Service Virtual Connection (NS-VC) is used.The NS-VCs are end-to-end virtual connections between the BSS and SGSN. At each side of the Gb interface there is a one-to-one correspondence between NS-VCs and NS-VLs For example, in the case of a Frame Relay network, the NS-VC is the FR permanent virtual connection (PVC). Figure 3/GSM08 16 shows the relationship between NS-VCs and NS-VLs. ETSI ETSI TS 101 299 V7.1.0 (1999-07) 11 (GSM 08.16 version 7.1.0 Release 1998) BSS SGSN NS-VL at the BSS side NS-VL at the SGSN side end-to-end NS-VC intermediate transmission network Figure 3/GSM 08.16: Relationship between NS-VCs and NS-VLs Each NS-VC is identified by means of a Network Service Virtual Connection Identifier (NS-VCI) having end-to-end significance across the Gb interface. An NS-VCI uniquely identifies an NS-VC within an SGSN. The establishment of an NS-VC includes the establishment of physical links, see GSM 08.14 [4], and of NS-VLs. NS-VCs and NS-VLs are permanently established by means of administrative procedures, NS-VCIs are allocated by administrative means as well. The mapping of NS-VCIs on NS-VLIs and on physical link identifiers is held in non- volatile memory.
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4.2.3 Network Service Virtual Connection Group
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The Network Service Virtual Connection Group groups together all NS-VCs providing communication between the same peer NS entities. One NS-VC group is configured between two peer NS entities. This grouping is performed by administrative means. At each side of the Gb interface, there is a one-to-one correspondence between a group of NS- VCs and an NSEI. The NSEI has an end-to-end significance across the Gb interface.
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4.2.4 BSSGP Virtual Connection (BVC)
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The Network Service provides communication paths between remote NS user entities. These communication paths are called BSSGP Virtual Connections (BVCs). Each BVC is used to transport NS SDUs between NS users. A Network Service Entity provides one or more BVCs between peer NS user entities. Each BVC is supported by one group of NS-VCs. Each group of NS-VCs supports one or more BVCs. The NS entity maps between BVC and the related NS-VC group. Each BVC is identified by means of a BSSGP Virtual Connection Identifier (BVCI) having an end-to-end significance across the Gb interface. The BVCI together with the NSEI uniquely identifies a BVC within an SGSN. The BVCI and NSEI are used on the Network Service-Service Access Point (NS-SAP) for layer-to-layer communication.
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4.3 Sub-Network Service functions
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The Sub-Network Service functions of the Network Service shall provide access to the intermediate network (or to the peer entity in case of direct point-to-point configuration) by means of NS-VLs and shall provide NS-VCs between NS peer entities. On each NS-VC, data are transferred in order by the Sub-Network Service. When the Sub-Network Service entity detects that an NS-VC becomes unavailable (e.g. upon failure detection), or when the NS-VC becomes available again (e.g. after failure recovery), the Network Service Control entity shall be informed. Failures may occur due to protocol errors, intermediate transmission network failure, equipment or link failure or other reasons. ETSI ETSI TS 101 299 V7.1.0 (1999-07) 12 (GSM 08.16 version 7.1.0 Release 1998)
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4.4 Load sharing function
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The load sharing function distributes the NS SDU traffic among the unblocked NS-VCs of the same group on the Gb interface. The use of load sharing also provides to the upper layer seamless service upon failure by re-organizing the NS SDU traffic between the unblocked NS-VCs of the same group. The re-organization may disturb the order of transferred NS SDUs. The load sharing function should be implemented in both the BSS and the SGSN. Load sharing applies only to NS SDUs, not to NS signalling such as NS-VC management PDUs.
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4.4.1 Requirements on load sharing function
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All NS SDUs to be transmitted over the Gb interface are passed to the load sharing function along with the Link Selector Parameter (LSP) , the BVCI and the NSEI. LSP and BVCI are used by the NS entity to select amongst the unblocked NS-VCs within the group, addressed by means of the NSEI, where to send the NS SDU. The mapping between LSP and NS-VC is based on an implementation dependent function that meets the following requirements: - For each BVC and NSEI, the NS entity selects the NS-VC out of the group based on the LSP. This is an internal matter for the NS entity and it is not subject to further standardization. - For each BVC and NSEI, NS SDUs with the same Link Selector Parameter shall be sent on the same NS-VC. Thus, the load sharing function guarantees that, for each BVC, the order of all NS SDUs marked with the same LSP value is preserved. In the event of a link failure and subsequent re-organization of the NS SDU traffic between the unblocked NS-VCs, the receiver may get out of order NS SDUs. Further actions implemented to prevent this error are outside the scope of this Technical Specification. - Load sharing functions at the BSS and the SGSN are independent. Therefore, uplink and downlink NS SDUs for a subscriber may be transferred over different NS-VCs. - A change in NS-VCs available for NS user traffic (i.e. one or more NS-VCs become blocked or unblocked) shall result in a re-organization of the NS SDU traffic amongst the unblocked NS-VCs of the same group. - For a BVC, when there is no unblocked NS-VC of the group left between a BSS and a SGSN, the corresponding traffic is discarded by the NS at the sending side. The Link Selector Parameter is locally used at the BSS and at the SGSN and shall not be transmitted across the Gb interface.
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4.5 NS-VC management function
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The NS-VC management function is responsible for the blocking / unblocking, reset and test of NS-VCs.
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4.5.1 Blocking / unblocking of an NS-VC
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When a condition making an NS-VC unavailable for NS user traffic is locally detected at the BSS or at the SGSN, the NS-VC shall be marked as blocked by the local NS entity and the remote NS peer entity shall be informed by means of a blocking procedure. The remote NS entity shall then mark the NS-VC as blocked and shall consider it as unavailable for NS user traffic. A BSS or SGSN may block an NS-VC because of: - Operation and Maintenance intervention at the Gb interface making the NS-VC unavailable for NS user traffic; - equipment failure at a BSS or an SGSN entity; - equipment or link failure on a BSS or an SGSN site; - failure in the transit network; or - other causes. ETSI ETSI TS 101 299 V7.1.0 (1999-07) 13 (GSM 08.16 version 7.1.0 Release 1998) When the NS-VC becomes available again for NS user traffic, the NS entity which initiated the blocking procedure may inform the remote NS peer entity by means of an unblocking procedure. The remote NS entity shall then mark the NS- VC as unblocked and shall consider it as available for NS user traffic. The blocking / unblocking procedures are further detailed in the rest of this Technical Specification.
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4.5.2 Reset of an NS-VC
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This procedure is used to reset one NS-VC to a determined state between remote entities. This procedure is performed: - when a new NS-VC is set-up; - after a processor re-start; - after a failure recovery when the state of an NS-VC must be set to blocked and alive; or - at any local event restoring an existing NS-VC in the dead state or in an undetermined state. When a reset procedure is initiated, data in transfer may be lost.
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4.5.3 Test of an NS-VC
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The test procedure is used to check that end-to-end communication exists between peer NS entities on a given NS-VC. When end-to-end communication exists, the NS-VC is in the "alive" state, otherwise it is in the "dead" state. A dead NS- VC can not be in the "unblocked" state.
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5 Elements for layer-to-layer communication
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This chapter presents the Network Service in a generic way, no assumptions are made regarding the real protocols providing the network services.
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5.1 Service primitive model
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The service primitive model shown in Figure 4/GSM 08.16 is applicable to both BSS and SGSN. Network Service Network Service user NS-SAP GSM 08.18 GSM 08.16 Figure 4/GSM 08.16: Network Service primitive model The network services are provided at the Network Service-Service Access Point (NS-SAP). ETSI ETSI TS 101 299 V7.1.0 (1999-07) 14 (GSM 08.16 version 7.1.0 Release 1998)
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5.2 Service primitives and parameters
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The Network Service primitives are summarized in table 1/GSM 08.16. The general syntax of the Network Service primitives is: NS - Generic name - Type (Parameters) Table 1/GSM 08.16: Network Service primitives Generic name Type Parameters Request Indication Response Confirm UNITDATA X X - BVCI and NSEI - NS SDU - Link Selector Parameter (in Request only) CONGESTION X - BVCI and NSEI - congestion cause STATUS X - BVCI and NSEI - NS affecting cause - transfer capability
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101 299
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5.2.1 Primitives
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101 299
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5.2.1.1 NS-UNITDATA-Request
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This primitive is used by the NS user entity to send a NS SDU to its peer entity via a BVC. The NS entity sends the NS SDU in unacknowledged mode. The Link Selector Parameter is used to identify the NS SDUs which must be sent in order relatively to each other. The NSEI is used by the NS entity to select the group of NS-VCs corresponding to the addressed remote entity.
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101 299
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5.2.1.2 NS-UNITDATA-Indication
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This primitive is used by the NS entity to provide the NS user entity with a NS SDU received on a virtual connection. The NS SDU are received in unacknowledged mode. BVCI together with NSEI indicate which BVC the NS SDU was received on.
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101 299
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5.2.1.3 NS-CONGESTION-Indication
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The NS entity shall be able to detect when a congestion situation starts and ends on an NS-VC. This primitive is used by the NS entity to report to the NS user entity that congestion is detected or that the congestion situation has disappeared. The BVCI and NSEI of the affected BVC and the congestion cause are reported to the NS user entity. Typically, the cause indicates the direction of transmission affected by the congestion.
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101 299
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5.2.1.4 NS-STATUS-Indication
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There may be situations where an NS-VC becomes unavailable for NS user traffic. When this occurs, the NS user is informed of the degradation of the transfer capacity by means of this primitive including the "transfer capability" parameter. When an NS-VC previously unavailable for NS user traffic becomes available again, the NS user entity is also informed by means of this service primitive, indicating the current transfer capability. This primitive may be used in response to an NS-UNITDATA-Request primitive which the NS is unable to serve because of e.g. NS-VC failure. This primitive may also be used upon recovery after a failure affecting the NS. ETSI ETSI TS 101 299 V7.1.0 (1999-07) 15 (GSM 08.16 version 7.1.0 Release 1998)
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101 299
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5.2.2 Parameters
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101 299
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5.2.2.1 NS SDU
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The NS SDUs are specified in GSM 08.18 [5]. They shall never be inspected by the Network Service entity.
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101 299
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5.2.2.2 Link Selector Parameter
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The Link Selector Parameter is included in the NS-UNITDATA-Request primitive for load sharing purposes as described in clause "Requirements on load sharing function".
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101 299
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5.2.2.3 BVCI I and NSEI
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BVCI and NSEI parameters are included in the service primitives to identify the BVC for which the service is provided. These parameters are used by the NS entity to multiplex the NS SDUs over the NS-VCs.
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101 299
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5.2.2.4 Congestion cause
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The congestion cause shall indicate the affected direction of transmission and may be set to the following values: a) congestion detected, backward b) end of congestion, backward c) congestion detected, forward d) end of congestion, forward
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101 299
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5.2.2.5 Transfer capability
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This parameter is used to report to the NS user entity the current transfer capacity available for a BVC, in terms of bandwidth. This parameter may be set to any value from "0" (zero) to the maximum bandwidth provided by the complete set of NS-VCs associated to the BVC.
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101 299
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5.2.2.6 NS affecting cause
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This parameter is used to indicate to the NS user entity the reason which caused the NS-STATUS-Indication primitive to be used. The cause values are: a) NS-VC failure: a failure is affecting one or more NS-VCs, the NS is still available. b) NS-VC recovery: one or more NS-VCs which were in failure are available again. c) NS failure: the NS can not provide data transfer services to the NS user. d) NS recovery: the NS can provide data transfer services again. ETSI ETSI TS 101 299 V7.1.0 (1999-07) 16 (GSM 08.16 version 7.1.0 Release 1998)
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101 299
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6 Sub-Network Service protocol
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101 299
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6.1 Frame Relay support of the Sub-Network Service protocol
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6.1.1 Overview
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Frame Relay shall be the network used on the Gb interface. The Gb interface may consist of direct point-to-point connections between the BSS and the SGSN, or an intermediate Frame Relay network may be placed between both ends of the Gb interface. Other intermediate equipments may be traversed. Several configurations are possible, the detail of which is outside the scope of this Technical Specification. For the purposes of this Technical Specification the following two types of configurations have to be considered: - Point-to-point physical connections. - Intermediate Frame Relay network. In case of an intermediate Frame Relay network, both BSS and SGSN shall be treated as the user side of the user-to- network interface. The network-to network interface is outside the scope of this Technical Specification. Only Frame Relay permanent virtual connections (PVCs) shall be used on the Gb interface. Therefore ITU-T Q.922 [9]Annex A or T1.618 [17] for PCS1900 and ITU-T Q.933 [11] Annex A or T1.617 [16] for PCS1900, permanent virtual connection procedures, shall be supported. Switched virtual connection procedures in ITU-T Q.922 [9] or T1.618 [17] for PCS1900 and ITU-T Q.933 [11] or T1.617 [16] for PCS1900 shall not be supported. ITU-T Q.921 [8] or T1.602 [15] and ITU-T Q.931 [10] procedures shall not be applicable. The Frame Relay user-to-network interface (UNI) shall be implemented on the Gb interface according to the FRF 1.1 [6] agreement, unless otherwise stated in this Technical Specification. Selected options or deviations from FRF 1.1 [6] are specified in the rest of this Frame Relay chapter. Where discrepancies arise between this Technical Specification and the above mentioned recommendations, this Technical Specification takes precedence. The rest of this Frame Relay clause applies only to PVC usage. The Gb interface addressing principles shall be applied as follows: - The physical link is the Frame Relay bearer channel. - The NS-VL is the local link in one end (at UNI) of the Frame Relay PVC. - The NS-VLI is the Frame Relay DLCI together with the bearer channel identifier. - The NS-VC is the Frame Relay PVC. - The Sub-Network Service entity is the Frame Relay entity.
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6.1.2 Network configuration
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The Gb interface is a User-to-Network (UNI) interface, as defined in FRF 1.1 [6]. Two configurations are possible, either a direct link configuration or PVC(s) via a Frame Relay network. Annex A shows an example of each type of configuration. In case of point-to-point connections, the BSS shall be considered as the user side of the user-to-network interface, the SGSN shall be considered as the network side, see figure 5/GSM 08.16. SGSN (network) BSS (user) Gb Figure 5/GSM 08.16: Direct link configuration ETSI ETSI TS 101 299 V7.1.0 (1999-07) 17 (GSM 08.16 version 7.1.0 Release 1998) In case of an intermediate Frame Relay network, both BSS and SGSN shall be treated as the user side of the user-to- network interface, see figure 6/GSM 08.16. The network-to network interface is outside the scope of this Technical Specification. BSS (user) SGSN (user) Frame Relay network Gb Gb Figure 6/GSM 08.16: PVC via a Frame Relay Network
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101 299
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6.1.3 Services expected from layer 1
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In the context of Frame Relay, the physical link is referred to as the bearer channel. The Frame Relay protocol shall be run across permanently reserved bearer channels on the Gb interface, see GSM 08.14 [4].
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101 299
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6.1.4 Options selected from FRF 1.1
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6.1.4.1 Support of DL-CONTROL sub-layer
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No end-to-end DL-CONTROL sub-layer shall be implemented on the Gb interface.
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6.1.4.2 Frame length
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The default maximum information field size of 1600 octets shall be supported on the Gb interface. Maximum information field sizes greater than 1600 octets may be agreed to between Frame Relay network operators and users at subscription time.
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6.1.4.3 Congestion control procedures
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Congestion control is defined in FRF 1.1 [6] and consists of congestion avoidance and congestion recovery mechanisms. Congestion control on the Gb interface consists of congestion avoidance based on the DE bit and on explicit notifications via the FECN and BECN bits within the address field of the Frame Relay frame. Congestion avoidance based on the CLLM message (see ITU-T Q.922 [9] clause A.7 or T1.618 [17] for PCS1900 and FRF 1.1 [6] clause 2.2.5) is not required at the BSS and SGSN sides. No congestion control mechanism based on implicit congestion detection (see ITU-T Q.922 [9] clause A.6.1) or T1.618 [17] for PCS1900 is required at the BSS and SGSN sides.
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6.1.4.3.1 DE bit usage
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The BSS and the SGSN shall always set the DE bit to 0 (zero).
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