to SGSN Interface Chapter 8 Network Layer Figure 8-12 Functions of SNDCP. Header N-PDU SNDCP Layer Control Compression Compression Segmentation SNDCP Header Segmented N-PDU SN-DATA PDU/ SN-UNIDATA PDU LLC Layer LLC Header SN-Data PDU/ SN-UNIDATA PDU LLC Frame precedence class, then one common compressor may be used for these network layers. SNDCP Layer-NSAPIs The SNDCP layer takes the packet data information that comes from the network layer, known as the N-PDUs and adds a header that contains the NSAPI information onto it. Figure 8-13 shows the multiple layer protocols. These newly formed packets are called subnetwork packet data units (SPDUs). These are used to differentiate one network layer application from another. This identifier is unique for a particular network layer appli- cation for the duration of that data call. 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. Mobile Station to SGSN Interface Mobile Station to SGSN Interface Figure 8-13 E-Mail E-Business SNDCP NSAPIs. Application Layer PDP context Signaling PDP context PDP context tcic.com AOL.com Transpac SNDCP N-SAPI RLC or BSSGP A variety of network layers are supported (such as IP and X.25) above SNDCP. The network layer packet data protocols share the same SNDCP that performs multiplexing of data coming from the different sources to be sent across the LLC. To identify the different network layers (IP and X.25), the service needs to use the NSAPI. Several NSAPIs may be associated with one SAPI (they may use the same QoS profile). Several applications can use the same PDP context (for example, Netscape and e-mail). SNDCP Compression and Segmentation Separate data compression entities are used for acknowledged (SN DATA) and unacknowledged (SN UNIT DATA) data transfer. SNDCP uses two types of compression, as seen in Figure 8-14: Data header compression (TCP/IP header compression