 (bytes) (Kbps) Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Radio and MS-PCUSN Interfaces Chapter 6 used for the access burst: one with 8 information bits and one with 11 infor- mation bits. For implementation reasons, it is convenient to regard the error detection as part of the physical link layer (GSM RF layer), even though the backward error correction procedure belongs to the RLC layer. For CS-2 and CS-3, the second step consists in precoding the USF for a specific protection of this information. For CS-1 to CS-3, adding four tail bits (at 0) and applying a punctured convolutional encoding of rate 1/2 (for error detection and correction on the receiver side) give the desired coding rate (22.8 Kbps). Indeed, a radio block, resulting in 456 bits after channel coding, is delivered every 20 ms. For CS-4, no convolutional coding is applied for error correction. The fol- lowing list covers the differences to help compare the performance of the different coding schemes: CS-1 gives the largest throughput for low C/I Superscript(1 values (because the other CSs do not protect the data sufficiently, SO the RLC layer requires more RLC data blocks retransmissions). CS-2 and CS-3 show similar performances. CS-4 is the most adapted for high C/I values. By dynamically adapting the coding scheme to the radio channel condi- tions, it is possible to optimize the communication performances. CS-1 is mandatory for the BSS, whereas CS-1 to CS-4 are mandatory for mobile stations. Only CS-1 and CS-2 are used in GRPS Phase II. The CCU, equivalent to the Signal Processing Unit (SPU), is inside the BTS. It is in charge of the channel coding. Initially, the coding scheme is determined on a TDMA-per-TDMA basis at the OMC, SO it will be fixed in time. In GPRS Phase II, the most adapted coding scheme is determined dynamically by the PCU according to the quality me