 network layer. The services to Layer 3 are provided via a Ser- vice Access Point (SAP), where each point is given a separate identifier called the Service Access Point Identifier (SAPI). One or more endpoints can be associated with a SAP, which are identified by a Data Link Connection Identifier (DLCI). Peer-to-peer protocols exist between the mobile station and the BTS and the virtual connection between the two is called the Data Link Connection. LAPD Data Link Layer At the Layer 2 protocol, we use the 184 bits of user information. This is input from the application or higher-level protocols in the stack. The 23 octets are received and channel coded for the link using a convolutional cod- ing technique, which results in the generation of 57 octets. Now the 57 octets (456 bits) are truncated into the transmission unit of twice the 57 bits plus overhead. Four sets of transmission are sent in the time slots in the four Time Division Multiple Access (TDMA) time slots. This Layer 2 proto- col prepares the data for the link SO that it can be properly transmitted and protected. 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 Function of GPRS Elements Impact on BSC: A New LAPD The Agprs interface is composed of a dedicated Agprs PCM (more than one can be used in the future) 64-Kbps channel. It carries One signaling 64-Kbps LAPD channel (TS-24 if used on a T1 or TS-16 or 31 if used on an E1), which conveys three different SAPIs: Agprs OML for operation and maintenance links between the BSC and Agprs GSL for GPRS radio signaling links between the BSC and PCU Agprs RSL for radio signaling links between the BSC and PCU Figure 4-4 shows the functional view of the LAPD link. The Link Access Procedure for Data channel is a function of ISDN that is modified for GPRS. Figure 4-4 The new LAPD AGPRS Frame for GPRS. BSC 2G Relay Appl