endors at the opposite side of the interface are compatible. Speech is PCM coded (see Chapter 3) at this interface. Another important interface is the Abis-interface between the BTS and BSC. At this interface speech is GSM coded, which requires less transmission capacity than the PCM coding. The Abis-interface is not completely standardized and, as a consequence, both BTSs and BSCs have to be purchased from the same manufacturer. The Ater-interface is not standardized either but it is used for terrestrial connections between the BSC and MSC. Speech is GSM coded at the Ater-interface and the transmission Mobile Communications capacity needed at the Ater-interface is one-fourth of the capacity of the A-interface. 5.5.2 Physical Channels The multiple-access scheme used in GSM utilizes two access methods, FDMA and TDMA. Up to eight users may share one of the 200-kHz fre- quency channels, which is divided into eight time slots. 5.5.2.1 FDMA and TDMA A basic concept of GSM transmission on a radio path is that the unit of transmission is a series of about 100 modulated bits. This is called a burst and it is sent in time and frequency windows called a slot as shown in Figure 5.10. The central frequencies of the slots are positioned every 200 kHz (FDMA) within the system frequency band and they occur every 0.577 ms (TDMA). All time slots of different frequencies in a given cell are controlled by the syn- chronization broadcast from the BTS transmitted in the common control channel of that cell. 5.2.2.2 Separation of Transmission Directions in Time and in Frequency For bidirectional user channels, the two directions are related by the fixed separation of frequencies and time instant. The fixed frequency gap between transmission directions is called the duplex distance and it is 45 MHz (in the A transmission burst occupies a window in time and frequency called a slot. There are eight time slots on each carrier frequency. Eight simultaneous calls may use the same frequency. Emission of a mobile station Frequency takes place 