Data Communications Networking Devices - 4th Ed.pdf

702 _________________________ WIDE AREA NETWORK DATA CONCENTRATION EQUIPMENTline connection, the scanner will insert or delete control characters, perform errordetection and correction, and perform such functions as bisynchronous codetranslation.The line interface coupler provides the physical connection between the IBMcommunication controller and a directly connected terminal or communicationsfacility terminated by a modem or digital service unit. Several types of LICs areavailable for selection, with their actual use based upon such factors as the linespeed, type of line interface RS-232/ITU V.24 or ITU V.35), and line protocol.Some LICs can be installed in either LABA or LABB bases, while other LICs canonly be installed in type B line attachment bases.The third major subsystem of the 3725 is the maintenance and operatorsubsystem MOSS). This subsystem has its own microcode and is used to detectand isolate failures within the controller. Since the microcode in MOSS operatesindependently of the rest of the controller, it can function even when the controlleris inoperative. Up to two external operator consoles can be connected to the 3725maintenance and operator subsystem.Due to the wide acceptance of local area networks within organizations, IBMdeveloped a Token-Ring interface coupler TIC) which allows 3725 and morerecently developed communications controllers to be connected to Token-Ringnetworks. The TIC on a 3725 can be considered as a gateway device and representsone of four methods by which Token-Ring networks can be connected to IBMcommunications controllers. Readers are referred to Section 4.3 for additionalinformation concerning Token-Ring gateways.IBM 3745The 3745 communications controller was introduced by IBM during the late 1980s.A series of 3745 models are marketed by IBM which extended the useful life of thiscommunications controller as well as added a networking capability which wassigni®cantly more versatile than its 3725 predecessor.The 3745 has the same functional structure as the 3725, consisting of a maintenanceand operator subsystem, control subsystem, and communications subsystem.The key differences between the 3745 and 3725 are in the number and types ofcommunications links supported by the newer front-end processor, which is alsoreferred to by IBM as a communications controller.Although the exact con®guration of a 3745 determines the actual number of low,medium, and high speed lines and LAN connections it can support, its capabilityfar surpasses the 3725. For example, a 3745 can support a maximum of 896 low andmedium speed transmission lines at data rates up to 256 kbps per line, as well as upto 16 high speed T1 or E1 lines operating at 1.544 Mbps and 2.048 Mbps,respectively. In addition, a 3745 can be con®gured to support a maximum of eightToken Ring and four Ethernet LAN connections through the installation ofappropriate LAN adapters in its communications subsystem.Similar to a 3725, a 3745 can be channel-attached to host computers and linkattachedto other communications controllers and control units. Unlike the 3725that was restricted to supporting Token-Ring LANs, the 3745 can also support