Data Communications Networking Devices - 4th Ed.pdf

1.8 LINE DISCIPLINE ______________________________________________________________ 33MultipointWhen two or more terminal locations share portions of a common line,the line is amultipoint or multidrop line. Although no two devices on such a line can transmitdata at the same time,two or more devices may receive a message at the same time.The number of devices receiving such a message is dependent upon the addressesassigned to the message recipients. In some systems a `broadcast' address permitsall devices connected to the same multidrop line to receive a message at the sametime. When multidrop lines are employed,overall line costs may be reduced sincecommon portions of the line are shared for use by all devices connected to that line.To prevent data transmitted from one device from interfering with datatransmitted from another device,a line discipline or control must be establishedfor such a link. This discipline controls transmission so no two devices transmitdata at the same time. A multidrop line structure is depicted in Figure 1.23b).Although modems are shown on point-to-point and multipoint line structuresillustrated in Figure 1.23,both line structures are also applicable to digital transmissionservices. Dataphone Digital Service facilities support both point-to-pointand multipoint line structures,with the use of modems and acoustic couplersshown in Figure 1.23 replaced by the use of DSUs. T1 and fractional T1 lines,however,are only available as a point-to-point line structure and require the use ofDSUs/CSUs in place of the modems shown at the top of Figure 1.23.Depending upon the type of transmission facilities used,it may be possible tointermix both point-to-point and multipoint lines in developing a network. Forexample,analog facilities and the use of DDS support both line structureswhich enable the development of a mixed network line structure as shown inFigure 1.23c).1.8 LINE DISCIPLINEWhen several devices share the use of a common,multipoint communications line,only one device may transmit at any one time,although one or more devices mayreceive information simultaneously. To prevent two or more devices from transmittingat the same time,a technique known as `poll and select' is utilized as themethod of line discipline for multidrop lines. To utilize poll and select,each deviceon the line must have a unique address of one or more characters as well ascircuitry to recognize a message sent from the computer to that address. When thecomputer polls a line,in effect it asks each device in a prede®ned sequence if it hasdata to transmit. If the device has no data to transmit,it informs the computer ofthis fact and the computer continues its polling sequence until it encounters adevice on the line that has data to send. Then the computer acts on that datatransfer.As the computer polls each device,the other devices in the line must wait untilthey are polled before they can be serviced. Conversely,transmission of data fromthe computer to each device on a multidrop line is accomplished by the computerselecting the device address to which those data are to be transferred,informingthe device that data are to be transferred to it,and then transmitting data to the