Data Communications Networking Devices - 4th Ed.pdf

6.12 PROTOCOL CONVERTERS ____________________________________________________ 733control unit which enables the 3174 to support asynchronous terminal access. Herethe AEA can be considered to be a built-in control unit protocol converter.Character versus block mode operationSince an IBM 370 networkis based upon the use of intelligent terminals, to be moreeffective protocol converters must support intelligent terminal operations. Tounderstand how this can be accomplished, let us ®rst examine how a synchronousIBM terminal operates when connected to a control unit which a protocol converteris designed to emulate. Then we can use this information to compare and contrasttwo different protocol converter operations to transmission ef®ciency.In an IBM 3270 network, data typed by a terminal operator is transmitted to acontrol unit and echoed backto the terminal. The control unit stores data from eachconnected terminal until a PA, PF, or Enter key is pressed, at which time thecontents of the control unit's buffer are transmitted. This blocking of data until aprede®ned key is pressed that could cause a screen change signi®cantly reducestransmission between control units and a front-end processor, allowing up to 64terminal devices to share access to a front-end processor via a control unit. A secondadvantage obtained from blocking data is the ability of the control unit tocommunicate with the front-end processor using a blockchecking algorithm whichsupports error detection and correction.Figure 6.77 illustrates the typical data ¯ow between a personal computer and aprotocol converter. Some protocol converter manufacturers market softwareemulation programs which blockdata on a personal computer in a manner similarto the personal computer being an intelligent 3270 terminal connected to a controlunit. That is, data is blocked and only transmitted to the protocol converter when aPA, PF, or Enter key is pressed.A second and less sophisticated type of emulation program that works withprotocol converters transmits a character each time a key is pressed. This type ofemulator is a character mode emulator and can signi®cantly increase transmissioncosts and result in the awkward display of characters on the personal computerFigure 6.77 Protocol converter system data ¯ow.Blocked transmission from a personalcomputer to a protocol converter allows error detection and correction between the PC andthe protocol converter.In addition, block mode can signi®cantly reduce the cost ofcommunications and network delays when access occurs via a packet network