o used to request the retransmission of a missing or undelivered block. The unacknowledged mode of operation does not guar- antee the arrival of the transmitted RLC blocks. This mode is important to applications that require a constant delay. The RLC layer increases the reliability of the air interface by providing BEC, which enables selective transmission. The RLC data block consists of the RLC header, RLC data field, and spare bits. Each RLC data block may be encoded using any of the available coding schemes (CS-1, CS-2, CS-3, or CS-4) and will affect the degree of seg- mentation and reassembly. If the contents of an LLC-PDU do not fill an entire RLC block, the beginning of the next LLC-PDU will be used to fill up the remaining positions. However, if the LLC-PDU was the last in the cur- rent transmission block, the RLC data block will be filled with spare bits (padding). The structure of the RLC data blocks is dependent upon the transmission direction (uplink or downlink). Mobile-Originated Access Message Sequence Several messages are sent between the mobile station and the network in order to establish a connection path. Figure 6-28 is an example of the mobile-originated sequence messages. 1. The mobile station that is not currently transmitting packets on the uplink receives an LLC frame from the upper layers. It sends a Packet 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 Figure 6-28 Radio The mobile station- resource originated message assigned for LLC frame sequence. MS and TBF ready for transfer Packet Channel Request Packet Uplink Assignment RLC/MAC block (USF) RLC/MAC data block (TLLI) Packet Uplink Ack/Nak (TLLI) Mobile Station Channel Request on the PRACH to request resources from the BS. For GPRS, this message is either 8 or 11 bits long. This small size provides fast