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GPRS (RLC/MAC - Two-phase) - 11

RLC/MAC Two-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 «

Now let us look at how MS transmits user data i.e. SNDCP PDU using TBF. This would be two-phase access.

MS BTS

RLC/MAC receives LLC PDU containing SNDCP PDUs. 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
For SNDCP PDU, two-phase access 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 fixed allocation of single block, physical channel (timeslot) information, TFI. MS need to use to send resource request.

PACCH
PACKET RESOURCE REQUEST
Contain TLLI for contention resolution on network side. MS now listen to same PACCH for assignment in assigned TFI.

PACCH
PACKET UPLINK ASSIGNMENT
Message contain dynamic allocation - TLLI, physical channel information (timeslot), USF, new TFI, USF granularity, TA etc. Contention resolution on MS side is complete when TLLI is same as sent in PACKET RESOURCE REQUEST.

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=N, assigned TFI]

PDTCH
BLOCK 1 [CV=N-1, assigned TFI]

PDTCH
BLOCK 2 [CV=N-2, assigned TFI]

PDTCH
BLOCK 3 [CV=N-3, assigned TFI]

PDTCH
Wait for assigned USF
Assuming USF granilarity is 4, MS wait for USF again after sending USF granularity number of blocks.

PACCH
PACKET UPLINK ACK/NACK
The network acknowledges packet transfers by sending PACKET UPLINK ACK/NACK during gaps in the uplink allocation. Contain acknowledgment information for received blocks, (optional) Timing Advance etc.

This procedure continues till all blocks are sent. Depending on uplink ack, retransmission may be needed (if RLC in ack mode). Uplink ack is done even in unack mode except that retransmissions will *not* be done by sending side.

PACCH
PACKET UPLINK ACK
Final Ack 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.


References: GPRS in Practice by McGuiggan

Copyright © Samir Amberkar 2010§ §

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