 is service dependent: It supports Connections between ATM Plane Management Function and non-ATM interfaces, including adaptation functions such as: Control Plane User Plane continuous bit stream oriented or Protocols & Protocols & CBO, and existing packet mode Functions Functions services (e.g. LAPD) Information is mapped into ATM cells Adaptation Layer Termination modes are flexible and may include: NT, NA, TA and TEs ATM Layer Between these layers is also the Physical Medium Dependent boundary for headers and information Layer fields ATM layer is: Common to all services, and Provides cell transfer capabilities Figure 55.8 B-ISDN protocol model for ATM Conclusions 55/9 and a packet switched signalling mechanism. ATM is a Cell Relay The bus may be looped into a ring format to give a much more technology and offers the major advantage of flexibility over STM. resilient topology than LAN ring approaches. The ring is not closed, For ATM, standards bodies ANSI and CCITT have agreed to use in the way a token ring LAN is. It is configured such that informa- a 53 byte cell, consisting of a 5 byte header and 48 byte information tion terminates at the loop closure point on each side of the 'open- field. The momentum to converge to ATM standards is evident in ing'. If a node or cable segment fails the logical opening is moved the adoption of a 53 byte cell format by the IEEE 802.6 committee to the physical outage, and operation continues without degradation. for Metropolitan Area Networks (see below). DQDB MAN technology is used in the Switched Multimegabit ATM is a wide area networking approach which is why it is more Data Service (SMDS) currently being used in the United States by suited to B-ISDN. The network platforms using ATM will be in the some of the Regional Bell Operating Companies (RBOCs). SMDS form of switching multiplexers or digital cross-connect switches. is designed for access speeds of 1.544Mb/s and 45Mb/s. In Europe The intention is that ATM networks will act as the backbone for ETSI is developing standa