dick@softart.UUCP (Richard B. McMurray) (01/26/88)
> I've seen ads claiming that with the various MNP levels that the throughput > on a 2400 baud modem is that of 9600 baud. > > Is it hype or is it real? > > Tony Holden > ihnp4!killer!tony MNP Classes 3, 4 and 5 do improve your modem's throughput beyond what a normal modem can do, but you won't see 9600 bps throughput on a 2400 bps modem in normal usage. - Class 3 transmits data in SDLC format without start and stop bits so it takes 8 bits to transmit a byte rather than the 10 bits required for async data. This would improve your throughput to 10/8 or 125% of what async data could be transmitted, ignoring protocol overheads. - Class 4 reduces the protocol overheads incurred by Classes 1-3. - Class 5 does data compression so the average character will need less than eight bits. Class 5 includes run length encoding, so if great numbers of the same character are transmitted your modem will only have to transmit a tiny number of bits. So, if you take two 2400 bps MNP Class 5 modems and transmit a constant stream of spaces to one modem at 9600 baud, the other modem will issue a (virtually) constant stream of spaces out at 9600 baud. If you transmit a stream of "the quick brown fox jumps over the lazy dog", you will get a different throughput. I have recently been testing MNP modems in Class 4 (no data compression) and Class 5 (data compression). Here are the results of tests transmitting a stream of data into one 2400 bps modem at 9600 bps and measuring the time from the receipt of the first to the last character on the receiving modem. Class 4 (quick brown fox) 122% of async speed Class 5 (quick brown fox) 162% Class 5 (stream of ' ') 395% Depending upon your input data a 2400 bps modem will behave as if it it was anywhere between a 4000 bps and a 9600 bps modem. The limitation on the high end is due to the speed that I sent data to the modem, ie: if I had sent spaces at 19.2 Kbps it would have looked like a 19.2 Kbps modem. In normal use with a 2400 bps modem, you can expect up to the equivalent of 2900 bps with MNP Class 4 and 4000-5000 bps with MNP Class 5. ------------ Dick McMurray watmath!softart!dick Softart Microsystems Inc.