nformation; in this case they are the ATM cells. As the payload is not exactly divisible by 53 a cell may cross a payload boundary to occupy space in the next container. The advantages of the SDH based approach are discussed below: It can be used to carry either the ATM based payloads or the STM signals using the same deployed transmission systems. This means that if a high capacity fibre based transmission is initially deployed for the transmission of STM payloads, then the transmission can easily be migrated to transfer ATM cells over the same media to support the B-ISDN. Some specific connection can be circuit switched using an SDH channel. For example, a connection carrying constant bit rate video traffic needs a good quality of service at constant bit rates. Thus that signal can be mapped into its own exclusive payload envelope of the STM-1 signal, which can be circuit switched. Using the SDH interfaces, many ATM streams can be combined to provide higher bit rates than 155.52 Mbps. For example, four separate ATM streams with a bit rate of 155.52 Mbps can be combined to build a 622 Mbps signal. This could prove more useful than a single 622 Mbps ATM stream. The ATM Layer The ATM layer is known to be responsible for establishing connections and passing cells through the ATM network. To do this, it uses information in the header of each ATM cell. From the above lines it is transparent that understanding this layer would need the knowledge of the two basic entities of this layer, which are the virtual connections and the ATM cell format. Virtual connections have already been discussed in detail, SO here they will briefly be revisited before moving on to the ATM cell structure in detail. The basic terminology of the virtual channel and path connections as given in the ITU-T Recommen- dation I.311 can be looked at to obtain a formal definition of these terms: Virtual channel (VC). This is a generic term used to describe the unidirectional transport of ATM cells associated by a common unique identifier value. Virtu