o-SGSN Interface (Gb) Chapter 9 Logical Link Control (LLC) relay toward RLC/MAC These are the layers that are used for relaying packets through the PCU-SGSN points. For example, users' packets take these access points. Frame Relay Networks Frame Relay was selected by the European Telecommunications Standards Institute (ETSI) committees because of its robustness, its speed that oper- ates at up to 2.0 Mbps, and the size of the frame set at up to 1,610 bytes in the payload. These coincide with the demands for GPRS. A Frame Relay network is a packet data network made of Frame Relay switches that are connected to each other, as shown in Figure 9-2. A user (user is a generic name, it could be a Local Area Network [LAN], for example) can gain access to the network by using a Frame Relay Access Device (FRAD). Its job is to build Frame Relay frames. The interface between FRAD and the Frame Relay network is well defined; this is the User-to-Network Interface (UNI). The FRAD A FRAD builds the frames that are used on the network, as shown in Fig- ure 9-3. It is an interworking device between the user and the Frame Relay Figure 9-2 A Frame Relay User A network. User B User C FR Switch 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. PCUSN-to-SGSN Interface (Gb) PCUSN-to-SGSN Interface (Gb) Figure 9-3 01010011 The FRAD. 01010011 User A FR Switch network. It can be a router connected to a LAN, for example, or end-user equipment. The Protocol Data Unit (PDU) The basic frame structure for other synchronous protocols is the same as most high-level data link control (HDLC) frame formats. In this case, the HDLC frame header consists of an address and control information. For Frame Relay, the header is changed, as shown in Figure 9-4, and uses the 2 bytes (octets) to define the following pieces: Data Link Connection Identifier (DLCI) This identifier uses 10-bit 