s Point 3 protocols. SNDCP SNDCP SNDCP SNDCP SNDCP SNDCP SNDCP SNDCP Access Point Identifier 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 Chapter 8 LLC Identifiers A logical link is unique to a particular mobile station. A Data Link Connec- tion Identifier (DLCI) identifies this logical link. A DLCI is composed of the SAPI at the LLC and the TLLI. Figure 8-3 shows the LLC identifier. SAPIs are points at which the LLC provides access to the SNDCP layer (that is, the Network Service Access Point Identifiers [NSAPIs]). The SAPIs that the LLC provides to the SNDCP layer are essentially the four QoS lev- els provided for data communication for different levels of reliability. More than one NSAPI may be using a particular SAPI depending upon the type of reliability required by that service. Several NSAPIs may request the same type of QoS level and hence, an SAPI at the LLC can support multi- ple NSAPIs from the SNDCP layer. The NSAPIs access the SAPIs and the SAPIs are grouped together with the TLLI that is specific for a particular mobile station. In wireless packet data communication, the physical link does not need to be maintained for the entire duration of the call. The mobile may be in the idle state. It tran- sitions back to the active state when it has to send a packet or when it is going to receive packets. The logical link connection is maintained even when the lower layers no longer exist. This means that radio resources are not used until needed, keeping the goal of the always-there and always-on data transmission without consuming the operators' resources. LLC Layer Structure In the functional model of the LLC layer, different functions are given to dif- ferent entities. Functions provided by each Logical Link Entity (LLE) are shown in Figure 8-4 and are as follows: Unacknowledged and acknowledged i