3 3. Design optimised around large numbers of small cells, each party bits added, generated by a cyclic redundancy code. The carrying less traffic than a GSM 900 cell. purpose of the party bits is to detect errors. The 182 Class 1 bits and the 3 check bits are then reordered into The limited number of GSM 900 technical specifications that had even bits at the start, check bits in the middle, and odd bits at the to be changed for 1800MHz, were released by ETSI in the DCS1800 end. 4 zero tail bits are then added to give (182 + 3 + 4) = 189 bits. specification in January 1991. The 189 bits are encoded using a convolutional code. This pro- duces an output of 378 bits. 48.9.2 Base station structure The 78 Class 2 bits are added giving 378 + 78 = 456. This gives a gross coded bit rate of 22.8kbit/s. The Base Station Subsystem (BSS) is the part of the PCN network As errors in mobile radio systems are predominantly due to which connects the Mobile Station (MS) to the switch (MSC) where fading, it is possible in practice to have fades lasting a whole TDMA burst. The 456 coded bits are reordered and interleaved over 8 calls are routed to the network. Essentially, the BSS acts as a transparent bearer between the two, so that most of the enhanced TDMA frames. During the interleaving process, bits are stolen for services offered by PCN are provided by the MSC and not the BSS. signalling purposes, i.e. the FACCH data. However the 1800MHz PCN BSS is probably the most advanced Encryption is a very important feature of a mobile telecommuni- cellular bearer available, and allows greater quality and quantity of cations service as it enables the confidentiality of conversations to traffic to be carried to the MSC than any alternative network could be maintained. The PCN system uses inherent encryption by the achieve. authentication key Ki which is stored with the IMSI on the PCN The BSS consists of two network elements: the Base Station SIM card. This information is also stored in the HLR. Controller (BSC), and the Base Transceiver 