tances over which they operate. Figure 39.10. V.42bis devices commonly use a method called the The most commonly used medium is still the telephone network. 'trie', which uses both a character and three pointers to identify Links are made by dialling over the PSTN or by using private point information by location. Starting again with our initial character 'S', to point circuits leased from the telephone company. Dial up and the right hand pointer designates the alternative characters at the leased line modems using these voice grade circuits, sometimes second level, 'T', 'A' and 'O'. These are the dependent characters collectively referred to as long haul modems, must compensate for of the parent character 'S'. Similarly 'N' and 'I' are dependent the frequency limitations and the variable quality of the trans- characters of the 'SO' string. mission media. Distance however is unlimited. Since the index value built up by the pointer locations are unique, Baseband modems are not 'true' modems, because they do not the far end modem can use the transmitted, compressed, index value modulate signals in the normally accepted sense of the word. Base- to locate the entry in its own dictionary, with the parent (upward band signals contain frequencies outside the normal 300-3400Hz facing pointer) used to locate the preceding entry. By following the voice grade circuits going right down to DC. Voice grade circuits pointers backwards to the root the whole string is reconstructed in are loaded so that performance for speech is optimised. Baseband its uncompressed form. To maintain synchronisation of information modems therefore cannot use them, because they act like low pass the encoding dictionary updates are delayed by one string to allow filters, eliminating the higher baseband frequencies. Instead they the decoder to store new index values before attempting to decode use unloaded cable where they have the entire bandwidth at their them. In our example, the first time the index value for 'STRI' is disposal, but are subject t