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Network Working Group                           T. N. Pyke, Jr.Request for Comments  230                       NBSNIC 7647                                        24 September 1971Category: C5Reference #203                       TOWARD RELIABLE OPERATION                OF MINICOMPUTER-BASED TERMINALS ON A TIP       The present protocol for communication between a TIP andattached terminals requires character-oriented transmission andprovides for no error control. In the design of this protocol, it wasapparently assumed that the majority of terminals attached to a TIPwould be interactive, be normally used in a character-by-charactermode both for transmission to and from the terminal, and normallysupport a human user who would in effect be in the communication loop.The human user would thus be in a position to detect any significanttelecommunication-induced errors both by direct observation of thecharacter stream and, more importantly, by examining the computeroutput in the context of his ongoing interaction.       The effectiveness of this means for error detection andinitiation of corrective measures when necessary is not adequate inthe following cases:     a. For terminal-TIP communication at a medium or     higher data rate (say 1200 bps or higher) it is quite possible     that the human will skim computer output and not be an     effective character-by-character error detector. In     particular, when both user input and computer output     contain numerical data it is possible that significant     undetected errors could occur.     b. For terminals located at a distance from the TIP     and connected either by a private line or the switched     network more errors may be introduced than with a     terminal local to the TIP (see Note 1). When a large     number of user terminals are connected to TIP's through     telecommunications facilities, whether within a single     organization or, even more likely, when users and user     groups not needing the full TIP capability are connected     to a remote TIP, this problem may arise.                                                                [Page 1]

     c. For terminals containing a substantial amount of logic,     including possibly a minicomputer, a human user is very     likely not in the direct terminal-TIP communications loop.     This case is important, since both alphanumeric and full     graphics terminals containing minis are now becoming     popular.     d. An interesting potential application of the network is to     provide support for minicomputers used for process     control and other laboratory measurement functions. In     providing software support for such minis as well as     acquiring data from them usually there is no human user     in the communication loop.     e. A number of sites already offer a remote job entry     service. Although the present sites assume that the unit     record devices such as card readers and line printers are     files within a multiprogrammed system at another site, it     appears natural that remote batch terminals be attached     to the network through TIP's. Here again, there would be     no human in the loop between the terminal and the TIP.       In addition to some degree of error control on these types ofterminal loops, it may be desirable to provide for block-oriented datatransmission, at least for terminals of types (d) and (e) and possibly(c) above. It is possible that error control utilizing blocktransmission can be superimposed on the present TIP-terminalcommunication protocol. Data blocks, including error control and blockdelimiting information, can be multiples of a single character inlength. The communication channel would still not be as fully utilizedas for conventional synchronous block communication, since start andstop bits for each character would need to be transmitted. This lossis not substantial and does occur now for 2000 bps TIP-terminalcommunication.         There are at least two ways to implement such a protocol ontop of the existing TIP-terminal communication protocol. In bothcases, the remote terminal would have to handle both originate andreceive error and block control procedures:     a. Through an addition to TIP software, the controlled     communication loop could terminate in the TIP, thus     providing error control only where it is most needed,     between the TIP and the terminal. This, however, would     involve additional TIP software and a block buffering     capability which may put an excessive load on the TIP.                                                                [Page 2]

     b. The other end of the block transmission error control     loop could be in the serving host system, either in an     applications program or in system support software.       If the remote end of the block transmission error control loopis in the serving host, then this software could possibly be used forhost-to-host, end-to-end error control in addition tohost-host-terminal end-to-end error control. For host-to-hostcommunication, however, there would be a slight loss in efficiency dueto the imbedded character-oriented format, unless an option wereprovided in which start/stop bits were not required.-------------------------------Note 1: The most recent published data concerning data transmissionerror performance of the switched telecommunications network isprovided in the 1969-70 Connection Survey conducted by BellLaboratories. The results are published in The Bell System TechnicalJournal, Vol. 4, No. 50, April 1971.  In this survey, 12 receiving and92 transmitting sites in the U.S. and Canada were used with standardBell System Dataphone datasets used at both ends.  At both 1200 and2000 bps, approximately 82% of the calls had error rates of 1 error in10^5 bits or better, assuming an equal number of short, medium, andlong hauls.       The results of this survey for low-speed, start/stop datatransmission at rates up to 300 bps indicate a character error rate of1 error in 10^4 characters or better on 77.6% of all calls made withinthe survey. It is interesting to note that only 48.3% of the low-speeddata tests completed were error-free. These tests were nominally 40minutes in length.       For voice grade private line data channels, the Bell Systemtechnical reference, "Transmission Specifications for Voice GradePrivate Line Data Channels," dated March 1969 reports "When a BellSystem dataset is combined with the recommended channel, the expectedlong term average error rate of the system is 1 error in 10^5 bits orbetter during normal transmission conditions. "       [ This RFC was put into machine readable form for entry ]       [ into the online RFC archives by BBN Corp. under the   ]       [ direction of Alex McKenzie.                   12/96   ]                                                                [Page 3]