e. from the mobile to the network such as details of current and 2. Network management action. neighbouring cell signal strengths. 3. For maintenance purposes. Fast Associated Control Channel (FACCH). This channel car- ries the same signalling data as the SDCCH. A FACCH is 48.8 Radio channel coding assigned when a SDCCH has not been assigned and obtains access to the physical resource by 'stealing' frames from the The PCN air interface utilises TDMA techniques to conserve band- traffic channel with which it has been assigned. width. ATDMA frame consists of eight timeslots of 577ns duration. Within the PCN system, each RF channel provides eight physical 48.8.3 Speech coding channels. These physical channels can be classified into either traffic channels or control channels. The PCN system uses the regular excited linear predictive coder (PRE-LPC) with long term pitch prediction. The code is block 48.8.1 Traffic channels based, with blocks of 20ms, and the net bit rate is 13kbit/s. Thus one frame consists of 260kbit/s. These channels are intended to carry speech or data and two forms Following subjective tests, 182 of the 260 bits have been found to are defined: be more sensitive to bit errors and these have been called 'Class 1' PCN base station design 48/9 Speech input Speech coding Channel coding Interleaving Encryption Modulation Speech Output Speech Channel De-interleaving Decoding De-cryption Decoding Modulation Figure 48.7 PCN speech coding and decoding process: (a) coding; (b) decoding bits whilst the remaining 78 bits, being robust to error, are called the unique requirements of smaller cells. The main differences 'Class 2' bits. between GSM 900 and PCN 1800 base stations are: The PCN speech coding and decoding processes are indicated in Figure 48.7. The speech coder produces the 260 bits which make up 1. RF channels at 1800MHz instead of 900MHz. one 20ms frame. The bits are delivered to the channel coder in 2. Smaller physical equipment size. descending order of importance. The first 50 Class 1 bits have 