rate adapter unit (RAU) is needed to adapt SM data service to service pro- vided by the external network. For example, if the GSM user has 14.4-Kbps data access to ISDN, RAU inserts its data into the 64-Kbps data stream of an Introduction to Telecommunications Network Engineering ISDN B-channel in a specified way SO that the other end knows where the user data can be found. The functions of the TC and RAU are often com- bined into a single piece of equipment called a transcoder and rate adapter unit (TRAU). 5.5.1.9 Echo Canceler (EC) The EC is needed at the interface between a GSM network and the PSTN. The efficient speech coding of GSM introduces such a long delay that echoes reflected by a hybrid circuit in the subscriber interface of the fixed network (see Chapter 2) of the fixed service would be disturbing. The echo canceler eliminates this echo. 5.5.1.10 Short Message Service Center (SMSC) GSM provides a paging service that is called short message service. The point-to-point SMS provides a mean of sending messages of a limited size to and from MSs. An SMSC acts as a store-and-forward center for these short messages. A short message transmitted by a subscriber is first forwarded through the network to the SMSC of his or her home network operator. The SMSC stores it, extracts the destination telephone number from the message, and forwards the message to its destination. The service center is not stan- dardized as a part of a PLMN, but the GSM network has to support the transfer of short messages between SMSCs and the MSs. 5.5.1.11 Operation and Maintenance Center (OMC) The OMC is a network management system for the remote O&M of a GSM network. The alarms of GSM network elements and traffic measurement reports are collected there. The O&M system handles features related to sys- tem security, faults, and network configuration updates. 5.5.1.12 Interfaces Inside GSM Network The interface between the MSC and BSC is called the A-interface as shown in Figure 5.9. It is standardized and BSSs and MSCs from different v