Data Communications Networking Devices - 4th Ed.pdf

2.3 LEASED LINE BASED NETWORKS ______________________________________________ 179Although this represents one use of leased lines, in actuality this type of communicationsfacility can be used to construct a number of other types of networksthat can operate over a leased line network. For example, through the use of packetassemblers/disassemblers PADs) instead of the multiplexers shown in Figure 2.4,you could construct a private packet switching network to directly linkorganizational locations instead of relying upon the use of a public packet switchingnetwork. Similarly, the use of Frame Relay Access Devices FRADs) wouldenable your organization to construct a private Frame Relay network. Thus, theuse of leased lines provides a considerable degree of ¯exibility that can range fromdirectly connecting two locations that require a considerable amount ofcommunications to developing a complex network structure for supporting aspeci®c type of private network operating over leased lines.Types of leased linesAs previously noted in Chapter 1, you can consider the use of several types ofanalog and digital leased lines. The primary types of analog leased lines include thevoice grade 3000 Hz bandwidth circuit over which you can transmit data at up to33.6 kbps, and several types of analog wideband circuits that operate at and above40.8 kbps. Due to the near-complete digitization of the backbone communicationscarrier network in the United States, it is now often easier and sometimes lessexpensive to obtain an equivalent digital circuit.Digital leased lines commonly available can be obtained that operate from the2.4 kbps rate of the lowest speed of Dataphone Digital Service through the T3carrier that represents a grouping of 28 T1 lines and provides an operating rate ofapproximately 45 Mbps. In between, you can select from a mixture of fractional T1FT1) and fractional T3 FT3) offerings that enable you to match your organization'sdata transmission requirements to a speci®c leased line operating rate. Forlarge organizations, some communications carriers now support SynchronousOptical Network SONET) connections, permitting operating rates from approximately52 Mbps to 2.4 Gbps.Utilization examplesIn addition to using leased lines to create an infrastructure for running a privatepacket switching or Frame Relay network, leased lines are commonly used on apoint-to-point basis to connect geographically separated locations. Two of themore common uses of leased lines are for use with multiplexers and routers.Through the use of multiplexers a group of terminal devices, to include personalcomputers, can obtain access to a common location, such as a mainframe located ata company headquarters or regional of®ce. Through the use of a router, two ormore geographically separated LANs can be interconnected, providing workstationsconnected to each network with the ability to communicate with one ormore devices connected to the distant network. In this section we will obtain anoverview of the use of time division multiplexers and routers, while detailed