Station (BTS) as in Figure 48.8. The BTS and BSC communicate with each other over 48.8.4 Echo control an internal 'A-bis' interface which is represented physically by either a 64kbit/s data link or (more commonly) a 2Mbit/s data link. The speech channel coding introduces a one way delay in the order The connection to the MSC is via the 'A' interface, which will of 90ms. In the case of on-net calls, as separate go and return normally be a 2Mbit/s datalink. channels are used, the delay does not cause any particular problems. In practice, a number of BTS's will be connected to each BSC. However, in the case of calls between the PCN and PSTN, calls will The exact number will be limited by a number of factors: mature on analogue telephones. This means that a 2 to 4 wire hybrid will be encountered, which will introduce echo. This echo, with a 1. The number of BTS ports available on the BSC. delay of 90ms, could cause annoyance to customers. Therefore, it is 2. The amount of traffic generated by the BTS's (the maximum necessary to include an echo canceller at the point of interconnec- that a BSC can handle is typically around 500 Erlangs). tion with the PSTN to cancel the echo. 3. The number of transceivers (TRX) used at each BTS. Typically, a BSC cannot handle more than 100 TRX altogether. 4. The physical distance from the BTS's to the BSC. If microwave 48.9 PCN base station design links are used, it can prove difficult to find line of sight paths to more than 20 BTS's, or even less in hilly rural areas. 48.9.1 Development from GSM 900 To the rest of the network, the BSS looks like a single entity. The PCN base stations are logically similar to those used for GSM 900 breakdown of functions between the two units is allocated in the systems, with a number of physical enhancements to tailor them to following manner: 48/10 Personal communications networks 270.833 2Mb/s 2Mb/s 2Mb/s CCITT CCITT CCITT "A-bis" INTERFACE INTERFACE INTERFACE INTERFACE 22.8kb/s 16kb/s Speech 16kb/s Speech 64kb/s Speech 64kb/s Speech RPE-LTP RPE-L