echniques make use of variable-sized packets and this leads to significant variations in the arrival times of the packets of a particular data stream. Because each physical connection may carry traffic from many individual data streams, it occurs every now and then that a specific packet is queued behind a number of large packets from other data streams that are waiting to be sent out on the physical connection. A further consequence is that switching is carried out by software that will eventually constrain the speed and performance of the network. We saw in the 1980s that sooner or later all services would be inte- grated into a common network, which was called broadband ISDN (B-ISDN). However, the two main network technologies, packet-switched for data and circuit-switched for voice services, could not support the other main service type and the ITU decided to develop ATM. Data Communications ATM is a cell-relay technology, which uses small fixed-size frames called cells. Cell relay transmits frames with constant length, 53 octets, and provides both variable-bit-rate (VBR) service that is optimum for data trans- mission and constant-bit-rate (CBR) service for voice and video applications. CBR is not available in frame-relay technology. ATM defines the structure of cells, continuous transfer of cells, and cell switching. Isochronous service is available by reserving certain fixed capacity of ATM cells from the network. ATM cells are packed into an SDH frame, STM-1, or into a SONET frame and then the physical data rate may reach 155 Mbps or higher. Significant advantages of cell-relay technology follow from the use of fixed-size small packets or cells instead of packets with vari- able lengths. The consequences of this principle are as follows: Delays in the network are much lower and more predictable. By ensuring that the cells from a specific data stream occur at regular intervals in the cell stream, it is possible to provide guaranteed band- width with low delay and jitter just as in circuit-switched networks