Data Communications Networking Devices - 4th Ed.pdf

7.2 DATA COMPRESSION PERFORMING DEVICES ____________________________________ 757router. Although only one spare modem or DSU is shown here, a second and itsassociated line facilities could be added, since the output side of the 4 by 4 switchcan interface one additional device.As shown by the circles, router 1 normally transmits data through modem 2 orDSU 2, and router 2 via modem 1 or DSU 1. Either router can be connected to thespare modem or DSU and associated line, should its primary modem, DSU or linefail. If a router fails, the other one can be connected to the failing device's primarymodem or DSU and line, thus ensuring the continuation of full throughput. If eachrouter communicates through more than one link, the use of an 8 by 8 or a 16 by 16switch or the chaining of more than one matrix should be explored.From the preceding examples, the utilization of switches provides the systemdesigner with a low-cost option when designating a teleprocessing network thatrequires a level of redundancy which can range from a single redundant componentrequirement through multilayers of redundancy.7.2 DATA COMPRESSION PERFORMING DEVICESAlthough data compression is correctly considered as a technique used to reducethe duration of a transmission session, other bene®ts may be derived from the use ofdata compression performing devices. Since compression results in the transmissionof a lessor number of characters in comparison to sending uncompressed data,the probability of a transmission error occurring is reduced. Similarly, a reductionin the number of characters can reduce the cost of transmission. Examples of costreduction can range from the ability to connect additional terminals to a multidropline to the use of a packet network where one of the cost elements associated with theuse of the network is the number of characters transmitted and received by aterminal device.From the preceding, it is obvious that the use of compression performing devicesshould be considered whenever possible since their use can boost network ef®ciencyas well as potentially reduce the cost of operating a network. In this section, we will®rst review a few of the methods that can be employed to compress data. This willbe followed by an examination of the bene®ts associated with the utilization of datacompression performing devices. Using the previous information, we will thenexamine several compression performing data communication devices to investigatehow they can be effectively used, focusing our effort upon the bene®ts that canbe derived by their use.For a detailed description of over 20 methods of data compression readers arereferred to the author's book Data and Image Compression, 4th edition, alsopublished by John Wiley & Sons Ltd.Compression techniquesWhile the number of algorithms that have been developed over the years tocompress data may exceed 100, in essence compression techniques can be subdividedinto two classesÐcharacter and statistical encoding.