to enable the network to judge when handover is required. The GPRS slot may also be used by
the base station to judge the time delay, using a logical channel known as the Packet Timing
Advance Control Channel (PTCCT).
Channel allocation
Although GPRS uses only one physical channel (PDCH) for the sending of data, it employs
several logical channels that are mapped into this to enable the GPRS data and facilities to
be managed. As the data in GPRS are handled as packet data rather than circuit switched data,
the way in which this is organized is very different to that on a standard GSM link. Packets
of data are assigned a space within the system acc ording to the current needs, and routed
accordingly.
The MAC layer is central to this, and there are