Data Communications Networking Devices - 4th Ed.pdf

516 _________________________________ WIDE AREA NETWORK TRANSMISSION EQUIPMENTTable 4.39Split stream unit modes of operationDDS rate Mode SSU Channel Data Rate1 1 2 3 49600 1 96002 2400 72003 7200 24004 4800 48005 2400 4800 24006 4800 2400 24007 2400 2400 2400 24004800 1 48002 1200 36003 3600 12004 2400 24005 1200 2400 12006 2400 1200 12007 1200 1200 1200 12002400 1 24002 600 18003 1800 6004 1200 12005 600 1200 6006 1200 600 6007 600 600 600 600This example shows the use of a device marketed by AT&T which is called a splitstream unit SSU). The SSU is similar to the multiplexer incorporated into nowobsolete Bell System 209 data sets, in that the user can select various combinationsof data transfer rates on up to four individual channels, up to the maximumcapability of the DDS line. The SSU plugs directly into the DSU, with operationalsettings at one-half or one-quarter the speci®ed DDS data rate. The unit provideslocal loop and remote loop testing of each individual channel and very effectivelylifts the four speed restrictions of DDS service. Table 4.39 lists the operationalmodes of SSUs designed for use on 2400, 4800, and 9600 bps DDS circuits.Of course, you can also obtain a variety of multiplexers manufactured byindependent communications vendors that can be used in place of SSUs. SinceDDS is a synchronous network that operates at a series of ®xed data rates, you canalso employ multiplexers as a means to concentrate data for communications over aDDS line. You can also use multiplexers to convert asynchronous data intosynchronous data stream that is compatible with the data format for which DSUsare designed.KiloStream serviceAlthough British Telecom's KiloStream service is similar to DDS, there areseveral signi®cant differences that warrant discussion.