PDCHs
may also be used by TBFs in Dual Transfer Mode (DTM). The DTM TBFs may either
use PDCHs already allocated for normal GPRS/EGPRS traffic or trigger allocation
of additional PDCHs that then can be used for normal GPRS/EGPRS traffic too.
Some PDCHs can later be returned to the CSD when they are no longer needed, or
when CS traffic or Abis demand requires it.
There
are different types of PDCHs; dedicated PDCHs, semi-dedicated PDCHs and
on-demand PDCHs.
Dedicated PDCHs:
The
number of dedicated PDCHs in a cell is set by the operator. These PDCHs are
allocated at configuration from the CSD to the PSD for GPRS/EGPRS traffic and
can then not be pre-empted by CS traffic. A GSL resource and an Abis resource
is allocated at the time of allocation. The number of dedicated PDCHs can only
be decreased by the operator.
Semi-dedicated PDCHs:
The
number of semi-dedicated PDCHs in a cell is also set by the operator. These
PDCHs are allocated at configuration from the CSD to the PSD for GPRS/EGPRS
traffic and cannot be pre-empted by CS traffic. The number of semi-dedicated
PDCHs can only be decreased by the operator. Semi-dedicated PDCHs will be
activated first when there is need for more PDCHs for PS traffic, that is the
GSL resource is not allocated until needed. The Abis resource is not allocated
until needed either if the channel has a flexible Abis resource, When there is
no longer any traffic need for a semi-dedicated PDCH (no TBF is reserved on
it), it will be put in the PSD idle list (PIL). Eventually the GSL resource and
the Abis resource (if a TCH with flexible Abis is used) will be returned to the
respective pool.
On-demand PDCHs:
On-demand
PDCHs are allocated from the CSD only when there is need for more PDCHs for PS
traffic. At on-demand PDCH allocation also the GSL resource is allocated. The
Abis resource is also allocated if the channel has a flexible Abis
resource, When there is no longer any
traffic need for an on-demand PDCH (no TBF is reserved on it), it will be put
in the PIL. Eventually the GSL resource and the Abis resource (if a TCH with
flexible Abis is used) will be returned to the respective pool and the PDCH
will be deallocated from the PSD and returned to the CSD. On-demand PDCHs can
also be pre-empted by CS traffic.