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GPRS (RLC/MAC - Two-phase cont.) - 12

RLC/MAC Two-phase access continued [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 network does downlink TBF.

MS BTS

GMM is in ready state, which mean MS's cell is known. GMM does regular cell update during ready state. In ready state times out, only RA update is performed which mean network need to page MS if required to be contacted.

PCCCH
Listen to paging and assignment messages

Network wants to transfer user data to MS, so it sends downlink assigment on PAGCH (PCCCH).

PAGCH
PACKET DOWNLINK ASSIGNMENT
Contain TLLI, S/P bit=1, Control Ack type=short access bursts, initial TA, TFI, RRBP indicating uplink block to send access bursts. As Control ack type is short access bursts, MS need to send PACKET CONTROL ACK as four access bursts Though MS's cell is known, exact TA may not be known, this process is to know corect TA. Alternatively, network may send PACKET POLLING REQUEST with type of ack as short access bursts to know TA.

PACCH
Four Access bursts
PACKET CONTROL ACK as four access bursts. Network calculates TA and informs back in PACKET POWER CONTROL/TIMING ADVANCE message Alternatively, continuous timing advance procedure can be used.

PACCH
PACKET POWER CONTROL/TIMING ADVANCE
MS listen to PACCH for its TFI. RRBP field to send control ack.

PACCH
PACKET CONTROL ACK

PDTCH
Wait for blocks

Network will now transfer blocks one by one which MS need to acknowledge by PACKET DOWNLINK ACK/NACK. RRBP field is used to provide uplink allocation for the same.

PDTCH
BLOCKS

PDTCH
PACKET DOWNLINK ACK/NACK

Network releases TBF once Ack received for all sent blocks


References: GPRS in Practice by McGuiggan

Copyright © Samir Amberkar 2010§ §

RLC/MAC - Two-phase access « GPRS Index » RLC/MAC - ARQ