y exceed the available composite bandwidth. lishment of X.25. In this situation it is essential that the multiplexer has the facility to Figure 40.8 shows the simplified operation of a statistical multi- flow control the data input, either by use of XON/XOFF or V.24 plexer which operates by scanning the input buffers for data control signals such as DTR or CTS. The decisions by which the presence (indicated by the flag). Data is transferred into a buffer network designer decided how to size a network with the mix of queue waiting to be assembled and transmitted as a frame. input speeds and output speeds were assisted by queuing theory. Input channels Buffer pool Frame assembler Channel 1 Channel 8 Figure 40.8 Simplified operation of a statistical multiplexer 40/8 Multiplexers In summary, statistical time division multiplexers offer asyn- translated to 62kHz in the first group and then to 420kHz in the chronous users the advantages of: second or supergroup. At present the three principal hierarchical systems in existence, Better use of the composite line by only allocating bandwidth are from Europe, US and Japan. when sending data. From Table 40.1 (based on CCITT G.702) it can be seen that each 2. Techniques for encoding additional control and timing infor- system, from a base speed of 64kbit/s, multiplexes in fixed, discrete mation to make the system more flexible. bands, up to the Gigahertz bands, suitable for transmission over 3. Speed translation end to end. microwave and satellite carriers. 4. Intelligent interaction to allow some asynchronous protocols to be emulated locally rather than over the whole link. Table High order systems However some of the disadvantages are: Only asynchronous data was practical. Synchronous proto- Multiplex Europe (khit/s) USA (kbit/s) Japan (kbit/s) level cols could be handled in the same manner as some asyn- chronous protocols in that local emulation of the host end and terminal end would result in only the actual data being trans- mitted over the link and the majority of the c