g at BO and according to the multiframe block order. The other PRACH blocks (four PRACH time slots) are indicated through an USF that is annotated as being free in the MAC header of the corresponding DL radio block. The nonreserved blocks may carry either PAGCH or PDTCH/PACCH/PTCCH (unlike the GSM system where a CCCH never carries traffic). Packet Traffic Channels The Packet Traffic Channels are the only ones used in GPRS Phase I. Fig- ure 6-11 provides an example of the Packet Traffic Channel configurations. PDTCH: Packet Data Traffic Channel PACCH: Packet Associated Control Channel PTCCH: Packet Timing Advance Control Channel The main difference between the PDTCH and the TCH logical channels in GSM consists of two points: PDTCH is allocated either for downlink or for uplink, but not for both (simplex channel). 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 6-11 Packet Traffic Channels. PDTCH: Packet Switched Traffic Channel (UL) PACCH: associated control channel (signaling: RLC/MAC control block) (UL and DL) PBCCH: TA update channel (UL and DL) PDTCH: Packet Switched Traffic Channel (DL) The control signaling (PACCH) associated with the assigned PDTCH is described only by its PDCH, but has no fixed position on this PDCH (radio blocks). As a conclusion, the mobile station may transmit a PACCH whenever UL, even by preempting the block position of a PDTCH, and the mobile station has to continuously monitor the DL of the DL control time slot indicated in the Packet Resource or Immediate Assignment. Up to eight PDTCHs may be allocated to one subscriber (on the same TDMA) and up to eight mobile stations may share the same PDCH. Cur- rently, the actual numbers are no more than three total time slots (two downlinks and one uplink for most manufacturers). Some manufacturers' mobile