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P800/P802
White Paper, January 2003
99
In the above example, terminal A is a phone using one timeslot for a voice call, terminal B is a
P800 using one timeslot for a CSD data call and terminal C is a phone using one timeslot for a
CSD WAP session. In each case, the timeslot is allocated whether data is transmitted or not.
In order to achieve higher data rates, the GSM terminal may be allocated more timeslots within
the transmit and receive channels. Since common applications such as browsing benefit from
having high data receive rates without requiring high transmit rates, HSCSD configurations are
typically asymmetric, meaning that data speed in one direction (receive) is higher than the data
speed in the other (transmit). This also saves battery life.
The slot allocation and hence the speed is set by the GSM network when the call is initiated, and
may be dynamically altered depending on network conditions such as the number of active users
in each cell. In the above example, the P800 (terminal B) has been allocated 2 receive timeslots.
The HSCSD maximum data rates (kbps) using the P800 are shown below.
9.6kbps per
timeslot
14.4kbps per
timeslot
Rx 19.2 28.8
Tx 9.6 14.4
The timeslot usage is not linked to demand from applications – an HSCSD connection will be
maintained even if no data is flowing. The data ‘pipe’ is (as far as practicable) maintained fully
open for you whether data flows or not. HSCSD connections are typically charged by connection
duration in the same way as CSD calls.