ues, and higher reliability, due length are fixed. This fixed boundary is inefficient because voice to the self-routing nature of individually addressed packets. The and data slots cannot be interchanged to accommodate statistical main benefit of TDMs is that dedicating bandwidth provides the fluctuations in voice and data traffic. For more efficiency, the high throughput and low delays needed for voice, video and high voice/data boundary can be designed to be movable, and the frame speed data communication. Cell Relay switching delivers the best length dynamic, but not without complicated network analysis and of both worlds by bringing the economy and reliability of packet increased complexity of switching architecture. switching to traditional TDM applications, such as voice, and the high throughput and low delays of TDMs to packet applications. Hybrid switching is not really an integrated solution because not only are two different switches built, but a bandwidth management Some manufacturers have introduced hybrid TDM and packet system is required that can deal with two separate types of data. In switched platforms as an attempt to capture the benefits of both effect, there are two separate networks piggybacked onto one trunk. technologies in one product, as in Figure 55.7. The product can cost twice as much as a truly integrated Cell Relay 55/8 Fast Packet Switching PACKET SWITCH FRAME BANDWIDTH MANAGER FIXED DYNAMIC Figure 55.7 Hybrid switch approach, with less of the simplicity and bandwidth economies to 55.3.1 Asynchronous Transfer Mode show for it. There are two recognised branches of Cell Relay technology, CCITT recommendation I.121 Asynchronous Transfer Mode Asynchronous Transfer Mode (ATM) and Distributed Queue Dual (ATM), is one of two approaches defined in Broadband ISDN. The Bus (or Queued Packet Synchronous Exchange). Let us deal with other is Synchronous Transfer Mode (STM), which is an extension these each in turn. of traditional TDM principles providing fixed bandwidth channels Adaptation layer