Data Communications Networking Devices - 4th Ed.pdf

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5.3 GATEWAYS __________________________________________________________________ 585Figure 5.27 illustrates the use of a gateway PC to connect an Ethernet 10BASE-T network to an SNAnetwork. In this example the gateway PC contains a Token-Ring adapter card for connection to a Multistation Access Unit MAU) and anEthernet card for connection to a 10BASE-T wire hub. Although it may appearthat the gateway functions as a bridge, it operates at a much higher level, although itdoes transfer information between the Token-Ring and the Ethernet network. Ifthe Ethernet is operating Novell NetWare, the gateway PC translates LUs into IPXaddresses and vice versa. This address conversion is based upon con®guring thegateway PC software when the gateway is installed. Since LU assignments arede®ned in the NCP in the communications controller, this means that the gatewayinstallation process must occur in close coordination with NCP programmers thatencode the NCP to recognize the gateway as a downstream PU with LUs assignedto that PU. In addition, to enable workstations on the Ethernet to gain full screenaccess to mainframe applications, each workstation desiring such access mustexecute an appropriate terminal emulation program.In the example illustrated in Figure 5.27 one vendor markets a gateway that canbe con®gured to operate as one to eight separate PUs, with each PU capable ofsupporting up to 32 LUs. Since two LUs on one PU are reserved, that vendor'sproduct provides the ability to support up to 254 LUs. This provides the servicingof up to 127 workstations that use separate LUs for screen and printer, or for alesser number of workstations that require the ability to execute multiple SNAsessions in the form of additional LUs.Alternative gateway methodsThe Token-Ring and Ethernet gateways discussed above use the services of a TRAon a control unit, requiring the physical presence of a control unit to interconnect toan SNAnetwork. Recognizing that the additional hardware cost of a control unitcan limit the effectiveness of that gateway method, several vendors, including IBMand third-party vendors such as Eicon Technology of Montreal, Canada, marketalternative solutions which enable different types of LANs to access SNA networksover WAN facilities. Thus, let's look at a few of those alternative solutions.SDLC connectivityAn SDLC gateway consists of a pair of adapter cards installed in a personalcomputer and appropriate software that performs the required conversion from thepacket format used on the local area network to an SNAdata stream. One card is anSDLC adapter which provides the framing for the bit-oriented protocol used bySNA. In actuality, most vendor SDLC gateways include SNA functions in ReadOnly Memory ROM) on the adapter card, which makes the card function as if itwas a series of 3174 or 3274 control units multiple PUs) with each PU associatedwith a group of LUs. This second adapter is typically a Token-Ring or Ethernetadapter, used to connect the gateway to either a Token-Ring or an Ethernet localarea network.
586 ________________________________________________ LAN INTERNETWORKING DEVICESFigure 5.28 Using an SDLC gateway. An SDLC gateway provides access to an SNAnetwork by providing communications between a LAN and a communications controllerFigure 5.28 illustrates the use of an SDLC gateway to obtain access to acommunications controller via a wide area network or an extended distance cable.Concerning the latter, the use of an SDLC gateway may permit the connection of aLAN to a communications controller via a lengthy cable within a building. Thismay be more attractive than the use of the gateway via a control unit TRAwhosecable distance is limited to LAN cabling restrictions.Both RS-232 and V.35 connectors can be obtained with most SDLC adapters.The use of an RS-232 connector limits the SDLC transmission rate to 19.2 kbpswhile the use of a V.35 connector enables digital transmission facilities at 56 kbps tobe used to connect the gateway to the communications controller.Some SDLC gateways are limited to supporting one PU and 32 LUs. OtherSDLC gateways considerably expand upon that basic level of support. For
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Copyright # 1986, 1989, 1992, 1999
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CONTENTSPrefaceAcknowledgementsxxii
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Data Communications Networking Devi
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1.2 LINE CONNECTIONS ______________
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1.3 TYPES OF SERVICES AND TRANSMISS
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1.7 LINE STRUCTURE ________________
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1.9 NETWORK TOPOLOGY ______________
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1.12 ERROR DETECTION AND CORRECTION
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1.13 STANDARDS ORGANIZATIONS, ACTIV
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