Data Communications Networking Devices - 4th Ed.pdf

2.3 LEASED LINE BASED NETWORKS ______________________________________________ 181Figure 2.6 Switching at a remote location M modem, T/PC terminal or personalcomputer, TDM time division multiplexer, DSU data service unit). Through the use of a dataPBX or post selector a common set of communications facilities can be shared. In thisexample the rotary and dial-in lines can provide common access to a minicomputer or to aTDMthe data PBX, which is also known as a port selector and whose operatingcharacteristics are described in Chapter 6, allows travelling sales personnel to dialone common rotary number. Upon access to the data PBX they obtain the abilityto route their connection directly into the minicomputer at location 3 or into a porton a TDM which will multiplex data traf®c from that location to the mainframecomputer. To provide continuous access to the mainframe for data entry clerks,the network designer determined that a control unit whose operation is describedin Chapter 6 should be installed at location 3. Through the use of a control unit atthat location terminals or personal computers can be directly cabled to that devicewhich allows a clustered group of terminal devices to share access to the mainframeon a poll and select basis. Since both the control unit and a portion of the linesfrom the data PBX must be routed to the mainframe, a TDM was installed toshare the use of a high-speed transmission facility from the new location to thelocation where the mainframe is installed.In the network con®guration illustrated in Figure 2.6, it is assumed that becauseof the transmission requirements of the control unit as well as the maximumnumber of interactive dial-in users that could be routed to the TDM, the TDMwould require an operating rate beyond that obtainable with an analog transmissionfacility. Thus, a DSU is illustrated in this example which enables transmissionat 56 kbps over a DDS circuit.Although the two prior examples represent a small selection of the many ways inwhich leased lines interconnecting different types of transmission devices can beused to construct wide area networks, they illustrate the basic concept ofconnecting devices and transmission facilities to develop networks, which is therationale for this book. Of course, the apparent simplicity of linking equipmentand facilities in actuality is a time consuming and tedious task which requires adetailed understanding of the operational characteristics of different types ofcommunications equipment and their networking constraints and limitations.Thus, subsequent chapters in this book are devoted to providing a detailedunderstanding of how different types of communications products operate, whythey can be used to satisfy the communications requirements of organizations, andwhen they should be considered.