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GPRS (RLC/MAC - One-phase) - 9

RLC/MAC One-phase access [Under GPRS >> RLC/MAC]
» MS-BSS Interface; RLC/MAC - 04.60 «
» GPRS radio channels - 5.02 «
» Conformance tests - 51.010-1, section 41 «
» GMM procedures - 24.008 «

Let us take GMM attach procedure as an example.

MS BTS

FCH/SCH
Synchronisation with BTS

BCCH
SYS_INFO 1-4
Get information related to channel configuration, GPRS support, location of PBCCH etc.

PBCCH
PACKET SYSTEM INFORMATION 1-8, 13-15
Get information related to GPRS channel configuration etc.

RLC/MAC receives LLC PDU containing GMM Attach message, splits in blocks. MS need to start TBF procedure.

PCCCH
Wait for USF=FREE (7).
USF=FREE in DL PCCCH indicates PRACH on next uplink RLC/MAC block.

PRACH
PACKET CHANNEL REQUEST
GMM PDU so one-phase will be requested. This message contain random reference number for later resolution. MS need to listen to PCCCH for resource allocation.

PCCCH
PACKET UPLINK ASSIGNMENT
MS gets above message with matching reference number. Message contain dynamic allocation - physical channel information (timeslot), USF, TFI, USF granularity, TA1 etc.

PDTCH
Wait for assigned USF
MS should start tranmission of blocks over corresponding uplink once assigned USF appear over downlink RLC/MAC blocks

PDTCH
BLOCK 0 [CV=3, TLLI, assigned TFI]
The network need to respond back with PACKET UPLINK ACK/NACK message after contention resolution. The contention resolution is completed on the network side when the network receives an RLC data block that comprises the TLLI value that identifies MS and TFI value associated with that TBF.

PDTCH
BLOCK 1 [CV=2, TLLI, assigned TFI]

PACCH
PACKET UPLINK ACK
Contain TLLI, (optional) Timing Advance. This indicate that at network side, at least one RLC/MAC block has been correctly received and contention resolution in terms of TLLI and TFI is OK on network side. The contention resolution is OK on MS side when TLLI match.

PDTCH
BLOCK 2 [CV=1, TLLI, assigned TFI]

PDTCH
BLOCK 3 [CV=0, TLLI, assigned TFI]

PACCH
PACKET UPLINK ACK
Final indicator bit will be 1 to indicate final ack (i.e. all RLC/MAC blocks received and no retransmission needed). Header field, RRBP (Relative Reserve Block Period) now specify uplink PACCH radio block reserved for MS to send PACKET CONTROL ACK. Validity of RRBP is based on S/P (Supplementary/Polling) bit.

PACCH
PACKET CONTROL ACK
MS sends this message in uplink radio block mentioned in RRBP field of PACKET CONTROL ACK. This message indicate to network that TBF can be released.

This complete one TBF. Network can reuse this TFI now. For further communication, MS or Network need to initiate TBF procedure again. In current example of GMM attach, MS has sent GMM Attach request, and need to wait for response from Network


1: Note about Timing Advance

GPRS has number of ways to Timing Advance control

One method is to calculate TA based on PACKET CHANNEL REQUEST burst sent over PRACH and inform MS in PACKET RESOURCE (UL/DL) ASSIGNMENT. During block flow, packet UL ack/nack can be used for informing TA. Later Network can update TA by sending PACKET POWER CONTROL/TIMING ADVANCE message.

GPRS also has a concept of Continuous Timing Advance procedure. This uses PTCCH (Packet Timing Control CHannel). A MS (group of MSs) is given a TA index (TAI) and TS number in PTCCH. TA Information Element sent in PACKET CHANNEL REQUEST and PACKET RESOURCE (UL/DL) ASSIGNMENT messages contain either TA value or (TAI and TS). MS then need to send access burst over PTCCH as allocated and receive timing advance value in downlink channel.

We will continue with GMM attach example.

References: GPRS in Practice by McGuiggan

Copyright © Samir Amberkar 2010§ § §

RLC/MAC - TBF « GPRS Index » RLC/MAC - One-phase access continued