asured on the radio link during the communication. 1 C/I is the channel-to-interference ratio used in GSM. Channel interference can be either adja- cent channel interference or cochannel interference based on these standards. Regardless of the interference source the point is that the more unreliable the data link (air) the less reliable throughput. Thus, coding is used to protect the data blocks at the expense of overhead ratios being higher. 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 Radio and MS-PCUSN Interfaces Coding Scheme-1 (CS-1) For CS-1, shown in Figure 6-34, 40 bits are used for BCS to increase pro- tection. It has 3 USF bits, 181 header and data bits, and 40 BCS bits, total- ing 224 bits. After adding four tails bits (always set to 0), the half-rate convolutional coding for forward error correction (the same as for TCH and SDCCH in GSM) is applied. USF are decoded as part of the data. The pay- load rate (radio block except BCS and USF) is 9.05 Kbps (181 bits/20 ms) and the data maximum throughput accepted by this coding scheme on one time slot is 8 Kbps. Coding Scheme-2 (CS-2) CS-2 and CS-3 are punctured versions of CS-1. For CS-2, shown in Figure 6-35, 16 bits are used for BCS using a Cyclic Redundancy Check (CRC) code. USF bits are precoded (6 bits in total) to increase robustness. The half- rate convolutional coding (the same as for CS-1) is applied (to the precoded USF + headers [MAC and RLC] + data + BCS) and 132 bits puncture the result in order to get the 456 desired bits. The USF bits (first 12 bits) are not affected by puncturing. USF can be decoded either as a block code or as part of the data. The payload (radio block except USF and BCS) rate is 13.4 Kbps (268 bits/20 ms) and the Figure 6-34 Coding scheme-1 Headers and Data 4 tail bits rate 1/2 convolutional coding 456 bits Downloaded