obile station is activated for data services, it will be in one of two states, as shown in Figure 6-29. Packet transfer mode Packet idle mode In the packet transfer mode, the mobile station is allocated an RR pro- viding a TBF for a physical point-to-point connection on one or more phys- ical channels. This enables the unidirectional transfer of the LLC frames between the network and the mobile station. In the packet idle mode of operation, the radio resource providing the TBF does not exist. The mobile station monitors the relevant paging subchannels on PCCCH. The transi- tion from packet idle to packet transfer mode can be triggered implicitly Figure 6-29 Packet Idle The states for radio resource allocation. LLC frames to send Waiting for LLC frames Establishment of TBF to send Packet Transfer 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. Radio and MS-PCUSN Interfaces Radio and MS-PCUSN Interfaces whenever a higher layer needs to transfer an LLC frame. This accommo- dates packet data, which is typically characterized by discontinuous traffic with short bursts of high activity interleaved with periods of idle time. BSS GPRS Protocol (BSSGP) Layer The BSSGP provides a connectionless link with unconfirmed data transfer between the BSSGP and the SGSN. The BSSGP shown as the model in Fig- ure 6-30 has a reliable link below it in the form of the network service. The network service is a Frame Relay network. The Base Station Subsystem GPRS Protocol (BSSGP) acts as an interface between the LLC frames and the RLC/MAC blocks in the BSS and as an interface between the RLC/MAC derived information and the LLC frames in the SGSN. Also, a BSS could receive information from the underlying Frame Relay network in multiple ways. Basically, the BSS controls several mobiles and each mobile could get information on different Network Service-Virtual Links (NS-