 access. The Packet Channel Request message may specify the throughput, RLC mode, ACK/NAK, and the priority of the message. 2. If the BS has resources available for the mobile station, the network reserves a radio resource (or time slot) for the mobile station and assigns a TFI. The network communicates the assignment to the mobile station with the Packet Uplink Assignment message. This message also contains the mode of operation (fixed allocation or dynamic allocation). In the fixed allocation mode, the assignment message will contain a list of assigned slots for the mobile station to use. 3. In the dynamic allocation mode, the mobile station reads the USF from the incoming RLC/MAC data blocks. The USF specifies the physical channel on which the mobile station is to transfer data. 4. The mobile station starts transmitting data using the RLC/MAC data block. 5. For acknowledged service, the BS sends an acknowledgment over the PACCH on the downlink. RLC is a sliding window algorithm, SO if a valid ACK is not received, the mobile station selectively retransmits the unacknowledged packets. Such a mechanism enables the mobile station to quickly go back and forth between idle mode and packet data transfer mode; hence, radio 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 Chapter 6 resources are utilized only when data is available to transmit. If the BS does not have resources available to process the mobile station request, it may choose to queue the request until resources are available. In this case, the BS sends a Packet Queue Notification message back to the mobile sta- tion on the PCCCH. The Radio Resource (RR) State Model In the beginning of Chapter 5, "Main GPRS Procedures," the models of how and when the resources are allocated were discussed. However, to reiterate the point of grabbing a resource, when the m