PRACH (or RACH). In short access or one-phase access, this message contains all the information needed for the channel establishment: Number of requested blocks (in short access case only) Radio priorities (11-bit message only) The BSS acknowledges the request by sending a Packet Immediate Assignment message carried on the PAGCH (or AGCH) and containing the description of the physical channels (PDCHs) reserved for the mobile sta- tion. Two types of access are available: one-phase and two-phase access. These are illustrated in Figure 6-13. 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 6-13 One-phase and two- UPLINK ACCESS PCUSN phase access. Network Packet PRACH (or RACH) Packet Immediate PAGCH (or AGCH) Stops here if 1 phase Packet or short access Request Mandatory for PACCH 2 phase access Assignment PACCH Packet The one-phase access is somewhat insecure and requires an efficient res- olution mechanism; it will be introduced later. Only the two-phase access is accepted by the network in GPRS Phase I (even if the mobile station may require one-phase access). The two-phase access can be initiated by the mobile station or by the net- work. In this case, the mobile station receives a single block on the PAGCH (or AGCH) and responds with a Packet Resource Request message on the PACCH (RLC/MAC control block sent on the allocated UL block) containing the complete description of the requested resources for the uplink transfer. The Packet Resource Request response has two ways of allocating specific blocks on PDCHs: either static or dynamic allocation. Static allocation sends a bitmap to the mobile station that indicates the reserved blocks for uplink transmission in time. Dynamic allocation assigns an USF to each mobile station, SO that they will be able to recognize when they are 