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GPRS (Radio Physical Layer) - 7

Radio Physical Layer [Under GPRS]
» GPRS radio channels - 5.01/2/3 «

GPRS adds few channels like PBCCH (Packet BCCH), PCCCH (Packet Common Control CHannel) - PRACH (Packet RACH), PAGCH (Packet AGCH), PPCH (Packet PCH), PDTCH (Packet Data TCH) etc. to GSM channel configuration. The channels have functions similar to their GSM counterparts. For example, PRACH is used to request access from network similar to RACH. For GPRS, this access request is known as "Packet Channel request".

GPRS support is advertised through BCCH. Mandatory channel for GPRS capability is PDTCH; other channels may not be present. If other channels are not present (indicated via absence of PBCCH), corresponding GSM channels are used for the same functionality for example RACH is used to packet channel request.

Note that PAGCH and PPCH (both downlink) are on same physical channels allotted for PCCCH-DL; logical division comes from the messages they carry.

Another channel which is not mentioned above is PACCH (Packet Associated Control CHannel). PACCH is associated with PDTCH. In channel configuration, they occupy the same position. Distinction between PACCH and PDTCH is made by "payload type" field in MAC header.

All packet channels including PBCCH (if present) are placed in TCH timeslots in 52-multiframe (26+26) configuration.

Channel coding

GPRS has defined 4 coding schemes: CS1, CS2, CS3, and CS4. Coding is applied to Radio block carrying RLC data blocks. Similar to GSM (as described in GSM article 8), GPRS apply Fire code (depending on coding scheme) and then Convolution code. Interleaving is done on coded data.

CS4 (Coding system 4) puts least number of bits for error detection and correction and so suitable in good radio conditions. Error correction capability increases and net data rate decreases from CS4 to CS1.

Coding system Service (actual) data Effective data rate
CS1 181 9.05 kbps
CS2 268 13.4 kbps
CS3 312 15.6 kbps
CS4 428 21.4 kbps

RLC/MAC control blocks use CS1.

Timing Advance

In GSM, due to presence of dedicated connection, communication of timing advance is possible to be done using L1 header over SACCH messages during call. In GPRS, some other methods need to be used for calculating and conveying timing advance value to MS.

One method is to calculate TA based on access request burst and inform MS in assignment. Later Network can update TA by sending signaling message specific to TA control or mentioning TA in few of the later acknowledgement messages.

GPRS also has a concept of Continuous Timing Advance procedure. This uses PTCCH (Packet Timing Control CHannel). MS need to send access burst over PTCCH and receive timing advance value in downlink channel.

Note about Multislot capability of MS

MS may or may not be able to support uplink and downlink operation at the same time. Depending on the type of operation MS support, MS can be of Type 1 - Not required to transmit and receive at the same time or Type 2 - To be able to transmit and receive at the same time.

In addition, depending on maximum number of timeslots MS can support on uplink and downlink, MS will belong to one of the 29 Multislot classes defined in standard. For example, MS of multislot class 9 should support maximum 3 DL slots/2 UL slots and it is of Type 1, MS of multislot class 13 should support maximum 3 DL slots/3 UL slots and it is of Type 2.

MS informs about its Multislot capability to network in Packet Channel Request (RLC/MAC message) Attach/RA update requests (GMM messages). Note that MS Multislot class may differ for GSM-HSCSD and GPRS operation.

In next article, we will look at RLC/MAC in bit more detail.

References: GPRS in Practice by McGuiggan

Copyright © Samir Amberkar 2010 § § § §

Signaling flow « GPRS Index » RLC/MAC - TBF