ion Subsystem is composed of two parts: the Base Transceiver Station (BTS) and the Base Station Controller (BSC). These communicate across the specified Abis interface, enabling (as in the rest of the system) operation between components that are made by different suppliers. The radio components of a BSS may consist of four to seven or nine cells. A BSS may have one or more BS. The BSS uses the Abis interface between the BTS and the BSC. A separate high-speed line (T1 or E1) is then connected from the BSS to the Mobile Central Office, as shown in the architecture in Figure 1-13. Figure 1-13 The Base Station Subsystem. To MSC 2 Mbps Combination of BTS plus BSC Uses Abis Interface 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. Introduction to GSM Chapter 1 The Transcoder and Adaptation Unit (TRAU) Depending on the costs of transmission facilities from a cellular operator, it may be cost efficient to have the transcoder either at the BTS, BSC, or MSC. If the transcoder is located at the MSC, it is functionally still a part of the BSS. This creates maximum flexibility of the overall network operation. The transcoder takes the 13-Kbps speech or data (at 300, 600, 1,200 bps) multi- plexes four of them and places them on a standard 64-Kbps digital PCM channel. First, the 13-Kbps voice is brought up to a 16-Kbps level by insert- ing additional synchronizing data to make up the difference of the lower data rate. Then, four 16-Kbps channels are multiplexed onto a DSO (64-Kbps) channel. Locating the TRAU If the transcoder/rate adapter is outside the BTS, the Abis interface can only operate at 16 Kbps within the BSS. The TRAU output data rate is 64-Kbps standard digital channel capacity. Next, 30 64-Kbps channels are multiplexed onto a 2.048-Mbps E1 service if the CEPT standards are used. The E1 can carry up to 120 traffic and control signals (16-120).