t and variable data rates, data, voice and still and moving pictures, transmission and multimedia applications combining several service components. The idea behind the development of this technology was to provide a platform for development of all future technologies. Because of the variable nature of B-ISDN, ATM was selected as a switch- ing and multiplexing technique as it is capable of giving the desired quality of service for different applications. Information transfer takes place in small packets, also known as cells. One ATM cell is 53 bytes in length and is divided into two parts: header and payload. The header is 5 bytes long and the payload 48 bytes. These cells are multiplexed to form virtual channels, which are subsequently multiplexed to form virtual paths, as shown in Figure 8.5. Transmission media carry these virtual paths, which may vary from one to numerous (depending upon the capacity of the transmission media). Virtual Channel Virtual Path Transmission path ATM Cell Figure 8.5 Multiplexing of ATM cell 8.3.1 Cell Structure The cell structure of the ATM depends upon the type of interface. There are two types of interface. The one between the ATM terminal and ATM switch is known as the user-to- network interface (UNI), and that between two ATM switches is known as a network-to- network interface (NNI). Both types of cell have a 5-byte header and 48 bytes of payload, as shown in Figure 8.6. The difference lies in the structure of the header. The various components of header are described below. 8 7 1 6 5 4 3 1 2 1 5 Byte Header 48 Byte Payload 48 octet 48 octet 53 Byte Cell Payload Payload Figure 8.6 ATM cell structure 3G TRANSMISSION NETWORK PLANNING AND OPTIMISATION Header Error Control (HEC) HEC is for error detection and 1-bit error correction of the 5-byte header. It is 8 bits in length and its sequence is calculated and set in the HEC field at the transmission side while the detected errors are corrected at the receiving side. Identifiers for Virtual Path (VPI) and Virtual Channel (VCI) As n