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P800/P802
White Paper, January 2003
98
wireless RS232 cable. The modem makes a PSTN or ISDN call to the Point Of Presence
telephone number of the required resource, for example your Internet Service Provider or
company modem pool for intranet access. Since the modem in the GSM network operates to
common standards, no special configuration is required at the ISP or company remote access
servers. The limitation on transmission speed (9.6kbps or 14.4kbps) is due to the lower
bandwidth of the wireless connection from the P800 to the modem.
As well as making data calls itself (built-in E-Mail, browsing and other applications), the P800 may
be connected to a PC using USB cable, infrared or Bluetooth. In this case it acts like a modem.
Either way, a standard CSD call Is limited to 9.6kbps or 14.4kbps.
High Speed Circuit Switched Data (HSCSD)
HSCSD operates in a similar manner to CSD, but allocates more radio capacity between the
P800 and base station in order to increase the overall speed of the connection. Some explanation
of the GSM system will help explain the characteristics of HSCSD.
A GSM mobile phone making a voice call will digitise the speech into a very efficient compressed
data stream. One radio channel has enough capacity to carry 8 of these data streams, so the
channel is divided into 8 repeating timeslots. The phone is allocated a radio channel and timeslot
and will send the data stream in bursts over that channel. Other phones in the cell may make use
of the other timeslots in the channel. A separate receive channel and timeslot is allocated and
works in the same way.
When making a CSD call, the data is similarly sent in bursts over one of the 8 timeslots in the
transmit channel. The basic data rate of this transmission is 9.6kbps, but some networks are
upgraded to a data rate of 14.4kbps.