ciated with frame relay and where they fit Synchronous Transport Modules (STMs). There is a hierarchy of in the model and Figure 40.16 shows the utilisation on a frame relay STMs from STM-1 which runs at 155Mbit/s to STM-16 at circuit with different traffic types. Note that this is almost identical 2.488Gbit/s. Again it is the PTTs that are taking the initiative, and to the format of the statistical multiplexer shown in Figure 40.8. services based on this system are on offer in the US as SONET Frame relay. and other fast packet techniques, are covered in more (Synchronous Optical Network). detail in Chapter 55. One of the main benefits of SDH is that it allows plesiochronous networks to operate with relatively inaccurate (1 part in 10-4 and 40.9.2 Synchronous Digital Hierarchy (SDH) therefore inexpensive, clocks. Another is that this service can then be used by other emerging technologies, such as Asynchronous In an attempt to reconcile the three main variants of high order Transfer Mode (ATM) which is the result of the CCITT work on the multiplexers a new set of G. standards have been defined, which can 1.121 standard. transport all types of synchronous traffic. These are G.707, G.708 In ATM data can be transferred without reference to an underlying and G.709 on frame structure; G.781, G.782 and G.783 on SDH clock system. Frame relay is an example of an early form of this but multiplexing equipment; G.784 on SDH management; G.957 on ATM is more generally aligned with the switching and multiplexing SDH optical interfaces: and G.958 on SDH line systems. Other techniques known as 'cell relay'. Unlike frame relay, which uses standards are being proposed for network management and digital variable length frames, cell relay operates with fixed frames (called Channel 1 Channel 1 Virtual Virtual channels channels (DLCI) (DLCI) Direction of data flow Channel 8 Channel 8 Video data LAN data Voice traffic Figure 40.16 Frame relay utilisation when carrying a mix of traffic types 40/18 Multiplexers Virtual Virtual Channels C