Data Communications Networking Devices - 4th Ed.pdf

718 _________________________ WIDE AREA NETWORK DATA CONCENTRATION EQUIPMENTconnected, and the method of attachment channel- or link-attached), and thenumber of devices the control unit is to support.Two of the most popular IBM control units throughout the mid-1980s were the3274 and 3276 devices. The 3274 enables up to 32 devices to be connected while the3276 is a table-top control unit with an integrated display station that permits up toseven additional devices to be connected to it. In 1987 IBM introduced its 3174control unit which can be obtained in several different con®gurations and can becon®gured to support 64 devices eliminating VN like the 3274 which supports amaximum of 32 devices. Other major differences between the 3174 and 3274 are theavailability of a Token-Ring option and a protocol conversion option for the newer3174 control unit. The token-ring option permits the control unit to be connected toIBM's token-ring network, while the protocol conversion option permitsasynchronous ASCII terminals to be connected to the control unit in place ofnormally more expensive EBCDIC devices.Since the attachment of terminal devices to an IBM control unit requiresconstant polling from the control unit to the terminal, either dedicated or leasedlines are normally used to connect the two devices together. Therefore the 3270Information Display System is considered to be a `closed system'. In addition,many IBM control units require terminals to be connected by the use of coaxialcable. This normally precludes the use of asynchronous terminals using the PSTNto access a 3270 Information Display System.Devices marketed by other computer vendors vary considerably in comparison tothe method of attachment employed by IBM. Most non-IBM control units permitterminal devices to be connected via an RS-232/ITU V.24 interface while thiscapability is only available on certain 3174 and 3274 models. Thus, terminal devicesremotely located from the main cluster of terminals can be connected to suchcontrol units via tail circuits as illustrated in Figure 6.65. Another differencebetween IBM and other computer vendor control units concerns the method ofattachment of the control unit to the host computer system. IBM control units canbe channel-attached or link-attached, whereas most non-IBM control units arelink-attached to the vendor's front-end processor, even when the control unit islocated in close proximity to the computer room.Unit operationOne of the major differences between a control unit and modem- and line-sharingunits is the fact that the former is not a passive device. The 3270 data stream that¯ows between the control unit and the host computer system contains orders andcontrol information in addition to the actual data. Both buffer control and printerformat orders are sent to the control unit, where they are interpreted and actedupon. Buffer control orders are used by the control unit to perform such functionsas positioning, de®ning, modifying, and assigning attributes and formatting datathat is then written into a display character buffer that controls the display ofinformation on the attached terminal. Thus, the terminal display information ¯owsfrom the host computer to the control unit in an encoded, compressed format,where it is interpreted by the control unit and acted upon. Similarly, printer format