Data Communications Networking Devices - 4th Ed.pdf

640 _________________________ WIDE AREA NETWORK DATA CONCENTRATION EQUIPMENTInverse multiplexingInverse multiplexing permits a high-speed data stream to be split into two or moreslower data streams for transmission over lower-cost lines and modems or DSUs.Because of the tariff structure associated with wideband facilities, the utilizationof inverse multiplexers can result in signi®cant savings in certain situations. As anexample, their use could permit 128 kbps transmission over two 56 kbps digitalleased or dial lines at a fraction of the cost which would be incurred when using a T1transmission facility. Figure 6.12 illustrates the use of inverse multiplexers. Thereader should refer to Section 4.3 for additional information on these devices.Figure 6.12 Inverse multiplexing.An inverse multiplexer splits a serial data stream intotwo or more individual data streams for transmission at lower data ratesMultiplexing economicsThe primary motive for the use of multiplexers in a networkis to reduce the cost ofcommunications. In analyzing the potential of multiplexers, you should ®rst surveyusers to determine the projected monthly connect time of each terminal device.Then, the most economical method of data transmission from each individualterminal device to the computer facility can be computed. To do this, direct dialcosts should be compared with the cost of a leased line from each terminal locationto the computer site.Once the most economical method of transmission for each individual terminal tothe computer is determined, this cost should be considered the `cost to reduce'. Thetelephone mileage costs from each terminal city location to each other terminal citylocation should be determined in order to compute and compare the cost of utilizingvarious techniques, such as line dropping and the multiplexing of data by combiningseveral low-to medium-speed data streams into one high-speed line fortransmission to the central site.In evaluating multiplexing costs, the cost of telephone lines from each terminallocation to the `multiplexer center' must be computed and added to the cost of themultiplexer equipment. Then, the cost of the high-speed line from the multiplexercenter to the computer site must be added to produce the total multiplexing cost. Ifthis cost exceeds the cumulative most economical method of transmission for individualterminal devices to the central site, then multiplexing is not cost-justi®ed.This process should be reiterated by considering each city as a possible multiplexercenter to optimize all possible networkcon®gurations. In repeating this process,terminals located in certain cities will not justify any calculations to prove or