The following section will explain how the PDCH is allocated. Traffic Channels and packet data channels basically create a common pool of resources, utilising the existing resources in an efficient way. See Figure 26
The PDCHs are allocated to the PCU, the PCU is responsible for assigning channels to different GPRS MSs. The PDCHs can be allocated in different ways:
Dedicated PDCHs are allocated and released by operator command.
On-demand PDCHs, serving as temporary dynamic GPRS recourses, are allocated and released depending on GPRS traffic demand.
The channels that are allocated for GPRS (PDCH) are allocated in sets of maximum four consecutive time slots, such a set is called a PEST (Shown in Figure 27), a PEST can consist of both dedicated and on demand PDCH. All PESTS are on the same frequency or hop the same frequency hopping set. An MS can only be assigned PDCHs from one PEST, at present this limits the maximum number of assigned time slots to four, there is no additional limit on the number of PDCHs that can be allocated in a cell, except the number of available TCHs.