Channel encoding

Channel encoding is based on a convolutional code at different code rates and GMSK modulation defined for GSM. The following table summarises the options:

Coding
scheme Speed
(kbit/s)
CS-1 8.0
CS-2 12.0
CS-3 14.4
CS-4 20.0

The least robust, but fastest, coding scheme (CS-4) is available near a base transceiver station (BTS), while the most robust coding scheme (CS-1) is used when the mobile station (MS) is further away from a BTS.

Using the CS-4 it is possible to achieve a user speed of 20.0 kbit/s per time slot. However, using this scheme the cell coverage is 25% of normal. CS-1 can achieve a user speed of only 8.0 kbit/s per time slot, but has 98% of normal coverage. Newer network equipment can adapt the transfer speed automatically depending on the mobile location.

In addition to GPRS, there are two other GSM technologies which deliver data services: circuit-switched data (CSD) and high-speed circuit-switched data (HSCSD). In contrast to the shared nature of GPRS, these instead establish a dedicated circuit (usually billed per minute). Some applications such as video calling may prefer HSCSD, especially when there is a continuous flow of data between the endpoints.

The following table summarises some possible configurations of GPRS and circuit switched data services.

Technology Download (kbit/s) Upload (kbit/s) TDMA Timeslots allocated
CSD 9.6 9.6 1+1
HSCSD 28.8 14.4 2+1
HSCSD 43.2 14.4 3+1
GPRS 80.0 20.0 (Class 8 & 10 and CS-4) 4+1
GPRS 60.0 40.0 (Class 10 and CS-4) 3+2
EGPRS (EDGE) 236.8 59.2 (Class 8, 10 and MCS-9) 4+1
EGPRS (EDGE) 177.6 118.4 (Class 10 and MCS-9) 3+2