Asynchronous Transfer Mode (ATM) is the CCITT standard for broadband ISDN. It supports integrated voice, data, and video communications both for services available and for future services not yet defined  . In ATM the information to be transmitted in divided into short 53 byte packets or cells, which have a 5 byte header. The reason for such a short cell length is that ATM must deliver real time service at low bit rates and thus it minimizes packetization delay. ATM networks are connection oriented with virtual channels and virtual paths. The virtual channel carries one connection while a virtual path may carry a group of virtual channels. This ensures that cell sequence is maintained throughout the network. The virtual channel is identified by the Virtual Channel Identifier, (VCI), and the virtual path is identified by the Virtual Path Identifier, (VPI). Both the VCI and VPI may change within the network and they are stored in the header of the cell. There is a Payload Type, (PT), field in the header which indicated whether the cell is user data or connection management information and also to indicate congestion in the network. There is also a Cell Loss Priority, (CLP), bit which is set high to indicate that the cell is low priority and set low to indicate high priority. There is a Generic Flow Control, (GFC), field which is for further study, but is essentially used for controlling the source to network connection. The whole header is protected an eight bit CRC contained in the Header Error Control, (HEC), field. The typical header therefore looks like that shown in Figure 2 for the user to network interface as specified by the ATM Forum .
Figure 2: The ATM Cell Header Structure
As ATM will be a broadband service the network will be a high speed one. To lessen the effect of the relatively slow processors within the network only a subset of functions will be carried out in the network. Error detection, correction, and flow control are done at the network edge rather than within the network. At the start of a call to set up a connection in terms of a virtual channel, there is negotiation between the user and the network on the parameters. Once admission is achieved the call is then monitored to ensure that it is compliant with the call setup paprameters. The network may drop low priority cells if congestion is about to or has occured. High priority cells may only be dropped when there are no lower priority cells left to drop. Services not sensitive to cell loss may have some low priority cells and these cells may be dropped. Because of this it is possible to get much higher utilisation than with previous networks.