des a net bit rate channel (SDCCH) and the associated control channel (ACCH). of 13kbit/s. It is a block based coder where the input samples are 47/12 Cellular radio systems Table 47.4 GSM logical channels Traffic Channels (TCH) Control Channels (CCH) Speech Broadcast CCH (BCCH) Common CCH Standalone Associated CCH (CCCH) Dedicated CCH (ACCH) (SDCCH) Full rate TCH/F TCH/F9.6 Frequency Correction Paging Channel Fast (FACCH) TCH/F4.8 (FCCH) (PCH) TCH/F2.4 Synchronisation Random Access Slow (SACCH) (SCH) (RACH) Half rate TCH/H TCH/H4.8 Access Grant TCH/H2.4 (AGCH) analysed in blocks with a 20ms duration. Work is also being carried and the combined use of coding and interleaving provides good out to specify a half rate speech coder which will effectively double protection of channel data from the fading, dispersion and inter- the system capacity of GSM. ference effects on the radio path. With the addition of frequency Before being assembled into the timeslots and frames, the digital hopping and diversity techniques, the GSM air interface is particu- speech and signalling data is encoded and interleaved. The speech larly robust. coder output is divided up into three classes of bits and the most One of the penalties to be paid for this is the overall transmission sensitive bits are encoded by adding parity check bits followed by a delay. The speech coder contributes about 25ms and the channel convolutional coder. Signalling data is encoded using a FIRE code. coding and interleaving a further 37ms. The rest of the transmission A process of interleaving is then used to spread the data blocks over delay budget allows for analogue to digital conversions, 16kbit/s a number of bursts. transmission and switching in various parts of the network. The For speech, an interleaving degree of 8 is used, i.e the speech overall one way transmission delay thus amounts to around 90ms. block is spread over 8 bursts, whilst an interleaving degree of 4 is Such a delay means that echo control is necessary even on short used for signalling. 