Data Communications Networking Devices - 4th Ed.pdf

20 _______________________________ FUNDAMENTAL WIDE AREA NETWORKING CONCEPTSnot fractional T1 as they represent special digital services that allow DDS to becarried on a fraction of a T1 circuit at a considerable reduction in cost.Data rates of 64,128,256,384,512 and 768 kbps available under ASDS can beconsidered as true fractional T1 as they represent distinct fractions of a T1 circuit.A 64 kbps data rate represents 1/24th of a T1 circuit since 64 kbps is the data rateof one digitized voice channel on a T1 circuit and that circuit carries 24 digitizedvoice channels.The majority of access to a fractional T1 line requires the use of a T1 local loop.Although the data transmission rate on the local loop is 1.544 Mbps,the fractional T1subscriber in actuality uses one or more 64 kbps channels on the local loop which isrouted to the telephone company central of®ce serving the subscriber. At that locationthe fractions of the T1 local loop used by the subscriber are removed from the T1 lineand input into an interexchange carrier's equipment at the point of presence. Theinterexchange carrier combines the fractions of T1 circuits used by many subscribersinto a full T1 circuit operating at 1.544 Mbps which is then routed through thecarrier's transmission facilities to the point of presence serving the distant location. Atthat point of presence the 64 kbps channels representing the fractional T1 channelused by the subscriber are passed to another telephone company which,more thanlikely,routes the transmission via a T1 line to the subscriber.In addition to T1 lines,AT&T and other communications carriers offer T3digital circuits operating at 44.736 Mbps. A T3 circuit transports a DS3 signalwhich is formed via the multiplexing of 28 DS1 signals. Similar to the recognitionthat many organizations cannot use a full T1,AT&T and other communicationscarriers recognized that only a limited number of organizations can use thecapacity of a full T3 digital circuit. As you might expect,this realization resulted inthe development of fractional T3 FT3) offerings.European offeringsIn Europe,a number of countries have established digital transmission facilities.One example of such offerings is British Telecom's KiloStream service.KiloStream provides synchronous data transmission at 2.4,4.8,9.6,48 and64 kbps and is very similar to AT&T's Dataphone Digital Service. EachKiloStream circuit is terminated by British Telecom with a network terminatingunit NTU),which is the digital equivalent of the modem required on an analogcircuit. In comparison with the T1 circuit used in North America which was basedupon a design for carrying 24 digitized voice conversations,European countriesuse E1 circuits. Such circuits were constructed based upon the placement of 32channels on one circuit and operate at 2.048 Mbps. In the United Kingdom BritishTelecom's E1 service is marketed as MegaStream.DSUsA data service unit DSU) provides a standard interface to a digital transmissionservice and handles such functions as signal translation,regeneration,reformat-