 PCUSN. Channel Codec Unit (CCU) The role of the CCU is to perform the following functions: The channel-coding functions (such as forward error correction and interleaving on the air interface) Radio channel measurements (quality of receive signal) Radio management The CCU is located in the BTS, in a choice of software-only upgrades or with hardware and software upgrades depending on the coding being per- formed. The initial releases of the CCUs support only a limited number of coding schemes. The four choices are numbered CS-1 through CS-4. These are shown in the graph in Figure 4-3 and described in the following list: CS-1 offers a data rate between 8 Kbps to 64 Kbps. CS-2 offers a data rate between 12 Kbps and 96 Kbps. CS-3 offers a data rate between 14.4 Kbps and 96 + Kbps. CS-4 offers a data rate between 20 Kbps and 115 + Kbps. Figure 4-3 The four coding rates and data speeds expected. 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. Function of GPRS Elements Chapter 4 This all depends on the mobile station (MS) equipment capabilities and the number of time slots accepted (multiclass). Higher data rates are far more sensitive to radio link quality: CS-1 is mandatory for the BSS and is also used for signaling. CS-1, CS-2, CS-3, and CS-4 are mandatory for mobile stations. CS-4 has no forward error correction. Data Link Layer-Layer 2 In GSM, only Layers 1 and 2 are directly related to the air interface where the Link Access Protocol on Dm channel (LAPDm) signaling is used between the mobile station and the BTS. Other interfaces within a GSM network use Link Access Procedure on the D channel (LAPD) and Signaling Systems Number 7 (SS7). The link layer uses LAPDm, a modified version of LAPD used in Integrated Services Digital Network (ISDN). In general terms, the link layer receives services from the physical layer and provides services to the