 model instead of trying to imitate the shape of the signal. An example of the so- Introduction to Telecommunications Network Engineering called hybrid methods that use both of the two main principles discussed ear- lier is the voice coding of a cellular network, as briefly reviewed next. 3.7.5 Speech Coding of GSM In cellular networks an efficient coding scheme is needed in order to make maximum use of radio frequencies. The lower our data rate, the narrower the frequency band we need for each call and the more simultaneous calls a given frequency band supports, as we will see in Chapter 4. As an example of these efficient coding schemes we now briefly review the principle that is used in the GSM. During efforts to standardize the speech-coding algorithm for GSM, the goal was to achieve a 16-Kbps data stream with the same speech quality as ordinary PCM. Waveform coding, such as PCM or ADPCM, did not give sufficient quality at this low data rate. Voice coding methods did give a low enough data rate but not good enough quality. In a voice coder or vocoder, the signal is modeled and the codes of the sound elements are sent. In the decoder the speech is reproduced. A combination of these two basic principles was selected. The maxi- mum processing delay was restricted to less than or equal to 65 ms, which requires the use of echo cancellers in the network. The original data rate became 13 Kbps, which was further reduced to 7 Kbps in 1995 with a more efficient coding algorithm. The selected efficient speech coding is always used at the radio path where efficient utilization of transmission channels is more important than in the wireline network. We will see in Chapter 5 that to increase the radio interface capacity we need to make cells smaller and build more base stations, which is very expensive. For switching and interconnection to a fixed tele- communications network, GSM coding is changed into ordinary PCM. GSM's operating principle is as follows. The voice signal is first divided into 20-ms slices. Each slice of