Packet Data Traffic Channels (PDTCHs)

The PDTCH is the
channel that is used for the transfer of actual user data over the air interface.
All PDTCHs are unidirectional—either uplink or downlink. This corresponds
to the asymmetric capabilities of GPRS. One PDTCH occupies a timeslot and
a given MS with multislot capabilities may use multiple PDTCHs at a given
instant. Furthermore, a given MS may use a different number of PDTCHs in
the downlink versus the uplink. In fact, an MS could be assigned a number
of PDTCHs in one direction and zero PDTCHs in the other.
If an MS is assigned a PDTCH in the uplink, it must still listen to the
corresponding timeslot in the downlink, even if that timeslot has not been
assigned to the MS as a downlink PDTCH. Specifically, it must listen for
any PACCH transmissions in the downlink. The reason is the bidirectional
nature of the PACCH, which in the downlink is used to carry signaling from
the network to the MS, such as acknowledgements.