Guide to GSM - What's the fuss about ?

Speech Coding

The transmission of speech is, at the moment, the most important service of a mobile cellular system. Speech in GSM is digitally coded at a rate of 13 kbps, so-called full-rate speech coding. This is quite efficient compared with the standard ISDN rate of 64 kbps. One of the most important Phase 2 additions was the introduction of a half-rate speech code operating at around 7 kbps, effectively doubling the capacity of a network.

This 13 kbps digital stream (260 bits every 20 ms) has forward error correction added by a convolution encoder. The gross bit rate after channel coding is 22.8 kbps (or 456 bits every 20 ms). These 456 bits are divided into 8 57-bit blocks, and the result is interleaved amongst eight successive time slot bursts for protection against burst transmission errors.

The digital TDMA nature of the signal allows several processes intended to improve transmission quality, increase the mobile's battery life, and improve spectrum efficiency. These include discontinuous transmission, frequency hopping and discontinuous reception when monitoring the paging channel. Another feature used by GSM is power control, which attempts to minimize the radio transmission power of the mobiles and the BTS, and thus minimize the amount of co-channel interference generated.


Ciphering is used to protect signaling and user data. A ciphering key is computed using the algorithm A8 stored on the SIM card, the subscriber key and a random number delivered by the network (this random number is the same as the one used for the authentication procedure). Secondly, a 114-bit sequence is produced using the ciphering key, an algorithm called A5 and the burst numbers. This bit sequence is then
XORed with the two 57 bit blocks of data included in a normal burst. In order to decipher correctly, the receiver has to use the same algorithm A5 for the deciphering procedure



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