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GPRS (RLC/MAC - TBF) - 8

Radio Link Control/Medium Access Control (RLC/MAC) TBF[Under GPRS]
» MS-BSS Interface; RLC/MAC - 04.60 «
» GPRS radio channels - 5.02 «

With the background of earlier articles, let us look at Layer 2 of Um interface in little more detail.

RLC/MAC is sort of GPRS counter part of GSM RR as it does Radio Resource management. It splits LLC PDU in blocks for transmission. A RLC/MAC block require four bursts/time slots.

These block flow is managed by a GPRS concept called, TBF - Temporary Block Flow.

From 04.60 (sec. 5.2.1),

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 comprises 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 (i.e. until there are no more RLC/MAC blocks to be transmitted and, in RLC acknowledged mode, all of the transmitted RLC/MAC blocks have been successfully acknowledged by the receiving entity).

TBF is identified by TFI (Temporary Flow Identity). Another important parameter of (downlink) RLC/MAC header used for access resolution is USF (Uplink Status Flag). USF indicate owner or use of next uplink radio block on the same timeslot (remember that UL is 3 burst delayed as for GSM). For PCCCH, USF=FREE (value 7) would indicate PRACH in next uplink block.

To request TBF, MS need to send PACKET CHANNEL REQUEST for either one phase access or two phase access.

One phase access (also known as Short phase)

In one phase access, network grants certain number of uplink RLC/MAC blocks without any further requests. MS directly goes to PDTCH, wait for its USF and once it is received, sends blocks in corresponding uplink. This type of certain number of block allocation is known as "fixed allocation". One phase access is meant for SM/GMM signaling or may be for small SNDCP PDU (user data) transfer.

Two phase access

In two phase access, MS gets single block of (fixed) allocation which MS need to use to send PACKET RESOURCE REQUEST. Network then grants resources in PACKET UPLINK ASSIGNMENT. MS uses this allocation to send RLC/MAC blocks. Two phase access is meant for SNDCP PDU transfer.

For resource allocation, following three medium access modes are possible:

Dynamic allocation

Once MS has been informed of dynamic allocation via PACKET UPLINK ASSIGNMENT message, MS need to monitor for downlink RLC/MAC blocks on certain PDCH(s) having certain USF value; PDCH information and USF value will be mentioned in PACKET UPLINK ASSIGNMENT message.Once assigned USF is found, MS can send blocks in uplink. Note that after every 4 blocks, MS still need to wait for its USF again and repeat the procedure till all blocks are sent. Note that network need to know the number of blocks that MS need to send - this is done by parameter CV (Countdown Value) in uplink RLC/MAC header. MS will put at its maximum when sending first block and decrease it after every block. Value 0 indicates that MS has sent all blocks. Similarly when network is sending blocks in downlink, it indicates by downlink header parameter, FBI (Final Block Indicator) - set to 1 if the block is last.

Fixed allocation

For fixed allocation, MS need not wait for USF value and directly start transmitting at assigned TS/FN.

Extended Dynamic allocation

If more than one time slot is assigned in PACKET UPLINK ASSIGNMENT message, MS still need to check USF for all assigned time slots in downlink for contention resolution. In extended dynamic allocation, this restriction is removed; MS need to check USF for only first time slot and can assume allocation of RLC/MAC blocks on rest of the time slots.

Flow Control is managed by BSN (Block Sequence Number) field in RLC/MAC header.

In addition to few header parameters described above, couple of important parameters mentioned in RLC/MAC control messages are TLLI and USF granularity.

TLLI (Temporary Logical Link Identity)
» Addressing, Identification - 23.003 «

A signaling procedure (like GMM procedure for GPRS attach, SM procedure for PDP context activation) require number of TBFs. Though each TBF is identified by TFI, subscriber identification over a signaling procedure is done by TLLI. TLLI is derived from P-TMSI or randomly chosen (like in GMM attach where MS do not have P-TMSI).

USF granularity

This field tells MS the number of RLC/MAC blocks that can be transmitted in one go (when its USF value is found on downlink RLC/MAC block).

With this as background, let us look at message sequence diagrams from RLC/MAC point of view.

References: GPRS in Practice by McGuiggan

Copyright © Samir Amberkar 2010§

Radio Physical layer « GPRS Index » RLC/MAC - One-phase access