a. ASSEMBLY LANGUAGE IMAGES PRECOMPRESSED FILES RANDOM DATA SOURCE DATABASE/ SPREADSHEET TEXT FILES AVERAGE COMPRESSION RATIO Figure 39.8 Examples of compression on files using V.42 bis Types of modem 39/9 V.42bis dictionaries do not search from start to finish but use a 'tree' type data structure somewhat similar to a telephone directory. In that way the same information will be identically stored when- ever it is placed in either the encoding or decoding dictionaries. V.42bis uses the concepts of nodes to store information. The initial character is stored as a 'root' node which has 'leaf' nodes associated with it on paths radiating out from the root. Suppose the modem tries to find the code word 'STRING'. The initial character S is read and the entry located. T, the next character is read and a search started amongst the immediate offshoots or 'leaves' of the root node (see Figure 39.9). If found, the location is recorded and the next charac- ter read. This is repeated until the 'I' character when this particular root node has been exhausted. At this time the compressed index value of the location for 'STR' is transmitted, the algorithm adds an 'I' leaf node to the string 'STR' and starts afresh to look for a new root node associated with 'I' which may be elsewhere in the diction- ROOT NODE Character Pointers LEAF NODE Figure 39.10 V.42 bis data structure signals and use them to modulate an analogue signal capable of LEAF NODE being transmitted down a telephone line, where another modem could demodulate the analogue signal and reproduce the computer's original digital signal faithfully. The modem took its name from this MODulating and DEModulating process. Today there are a number of devices called modems that do not Figure 39.9 V.42 bis dictionary tree modulate or demodulate a carrier signal in the same way as the original dial up modem. In order to classify devices, we have to consider the type of line they use, the mode of transmission and the Implementing this concept into a memory structure is shown in dis