GPRS uses some GSM broadcast channels for frequency tuning ( Frequency Control Channel [FCCH]) and synchronization ( Synchronization Channel [SCH]). However, for other purposes, specific new packet logical channels are defined, which are carried by a packet-switched channel Packet Data Channel (PDCH). PDCH is the generic name for the physical channel allocated to carry packet logical channels. The packet (logical) channels are used in the uplink (UL) or downlink (DL) direction. Actually, unlike GSM, GPRS does not have a real duplex channel, except for the Packet Timing Advance Control Channel (PTCCH).

The packet channels are classified in three families (the direction of flow is shown as UL, DL, or UL/DL). These are shown as the logical channels in Figure 6-2:

Figure 6-2: The logical channels assigned in GPRS.

• The Packet Common Control Channels (PCCCH) are very similar in makeup to the Common Control Channel (CCCH). When not allocated in a cell, packet transfer can be initiated by the CCCH. The PCCCH is made up of several logical channel functions consisting of

  • Packet Random Access Channel (PRACH UL) is used for random access. The PRACH is an uplink-only function used by the mobile station to initiate an uplink transfer for sending data or signaling information. The access burst used on the PRACH is also used to obtain any timing advance information. The mobile station to transmit the initial packet channel request uses the PRACH. It is...