term V.24 popularly implies the use of the other RS449 was designed to overcome limitations in RS232C but be- two. Specified for operation up to 20kbit/s via a 25 way D type cause it arrived late and specified pins for ten additional circuits it connector it provides a set of unbalanced (one leg goes to ground has not become widely adopted. V.10 delivers an unbalanced inter- potential), double current (current may flow in either the go or face, with maximum ratings of 100kbit/s and 1000 metres and V.11 return path depending on voltage polarity) interchange circuits. a balanced interface with maximum ratings of 10Mbit/s and 1000 Unbalanced circuits are susceptible to crosstalk and other inter- metres. Maximum distance and speed are mutually exclusive. V.11 ference. The operating distance is capacitance limited to 2500pF electrical interface signals are commonly combined with a 15 way 39/8 Modems D type connector in common use as the method of connection to Unlike other techniques, Lempel-Ziv develops these dictionaries BT's KiloStream service. dynamically as it goes along and stores them in Random Access The goal for the future is to reduce the number and size of these Memory in both modems. Providing up to a fourfold performance interfaces, ideally to an universal DTE/DCE standard, SO a DTE increase, its choice as the CCITT adopted international standard would not care if it were connected to a DCE or another DTE. was due to efficient use of both memory and processing power and Various issues of control signal pass through and timing are the perhaps more importantly its ability to shut the compression algo- principle problems. rithm down when incompressible scrambled, encrypted or random Supremacy in error connection and data compression standards is data is presented to it. currently a straight two horse race between CCITT international Figure 39.8 shows examples of compression ratios possible for standards, and proprietary standards developed by Microcom called various applications. From a commercial viewpoint,