Section 2 - Commodore Computers

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112 COMPUTEt COMPUTE! May, May. 1982, Issue 2.t1 76 Telecommunications: Sending Sending Programs Over The Phone Michael E. Day Michael E. Day Chief Engineer Edge Technology Edge Technology One use for r a modem is to transfer data between yo ur own com pUler and another one. This mun your own computer and another one. This mun dane aspect of telecommunications cann be one of the more interesting and rewarding uses of both the more inte resting and rewardin , uses of both the modem and your computer. The usual method of getting programs into your computer is to either enter them by hand through your keyboard, or to obtain a disk or cassette with the desired program on it. While this is finee if the program desired is readily y available, it can make things a bit difficult if it is not. If, for instance, a fri end of you rs has a BAS IC program instance, a friend of yours has a BASIC program you u would like to have, the usual procedure is for him or her to make a copy of the program on him or he r to ma ke a copy of th e program on cassette or disk and give it to you. Ifit is not too large, youu might get a printed copy. If you have a cassette and your r friend has a disk, the usual response is to not bother. Ifboth of you had a way to transfer the e program over the phone, though, you could easilyy get the program. Another interesting aspect of this is that the program can be transferred to you instantly. With the modem all it takes is a phone call. In I n order to make these calls, there must be some agreement as to how you will transfer this information. The actual mechanics can be quite complex. An agreemell t about how to ma ke the complex. An agreement about how to make the actual al transfer is called a communicationformat com.munical.ionfonllat or protocol. /)roto(o/. Although there area certain basic requirements needed to make the transfer, r, there is no standard format for r the actual details of the transfer. Making A Link The general structural requirements of data transfer are standardized to some extent. This is largely due to the basic requirements of an actual transfer. TheT first thing that is done is to establish the communications link. ThisT is doned when you call your friend, make the arrangement nt to do the trans fer, r, and turn on the modems to begin the transfer. Next, the computers rs must synchronize themselves, and then, finally, make the actual transfer. r. T he transfer itself is broken into small pieces The transfer itself is broken into small pieces called records. The records generallyy consist of 128, 256, 2">6, or 1024 characters (bytes). The actual record size is normallyy chosen to fit the particular system that the transfer program is runningg on and, in fact, is one of the reasons for the lack of standardization of these transfer programs. One popular operating system that is used on some computers is CP/M". CP/M*. Because CP/M P/M'was originally y based on the I IBM standard eight inch noppy floppy disk which uses a storage method of 128 bytes per record. So, naturally eenough, the record length that is usually chosen for the data transfer through the modem is 128 bytes too. This doesn't mean that the transfer has to take place in 128 byte records. But a decision had to be made as to what the record length would be aand, since the system stored data in 128 byte groups, this was chosen. It could just as easilyy be 256 or 1·024 1024 bytes. Actually a 128 byte record is a fairlyy reasonable size since, at 300 baud, (the numberr of characters sent per r second) takes a little over four seconds to transmit a record. 256 bytes would take over eight seconds, and 1024 bytes would take over 30 seconds. TThe idea here is to keep the transferr size down so thal, that, if an error does occur, nOltoo not much tin1e time is wasted retransmitting it. On the otherr hand, youu don't want it broken down into such small pieces that the overhead involved in handling the records significantly retards the transfer time. Overhead time can be considered the time it takes to acknowledge the receipt off the record. In a simple transfer program this would be a single character. Another part of the overhead that must cha racte r. Another part of the overhead that must be considered is the turnaround und lime time of both the computer systems and the phone line. On a local computer systems and the phone line. On a local call this generallyy averages out to about three or four character times (assuming 300 baud). On a long distance call, , this can stretch out to eight to twelve character times. (If f the call is via satellite, it will be around 40 character times.) will be around 40 character times.) Assuming that the call is local, this means that Assuming that the call is local, this means that the overhead would be about five characters. For 128 byte records this would be about 4% overhead. For 256 byte records it wo uld be 2%, and fo r 1024 For 256 byte records it would be 2%, and for 1024 byte records it would be 1/2%. This has to be balanced against the expected error rate. The phone line e has an average errore rate of about one error in every 10,000 bytes of data that is transferred. If every 10.000 bytes of data that is transferred. If the phone line is weak or 0 1' noisy, it can get much worse. With 1024 byte records, this means that about one out of every ten records will be bad, (a 10% error rate) so the 1/2% transfer rate is lost in the 10% error rate. With 256 byte records, the error rate is down to 2.5%, and with 128 byte records it is 1.25%. Assuming a 128 byte record format, and ac- 10% error rate) so the 1/2% transfer rate is lost in the 10% error rate. With 256 byte records, the error rate is down to 2.5%, and with 128 byte records it is 1.25%. Assuming a 128 byte record format, and ac-
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