lancelot@spock.UUCP (Thor Lancelot Simon) (01/31/91)
In article <1154@vector.Dallas.TX.US> jonb@vector.Dallas.TX.US (Jon Buller) writes: >In article <1991Jan25.214706.185@spock.UUCP> lancelot@spock.UUCP (Thor Lancelot Simon) writes: > >>If you're sending megabytes of data, I would assume you're compressing it. >>But MNP is not really 4800 bps. It is 2400bps with a data compression >>scheme which will get, *at best* 4800bps throughput. If you're sending data >MNP class 4 does no compression, class 5 and class 7 do roughly 2 to 1 >and 2.5-3 to 1 compression respectively. This is all independent of the >transmission speed. Some Telebit modems (T2500?) can do MNP5 at 9600bps >or do v42bis which is a superset of MNP with (your favorite and mine) LZW >compression, very similar to what the 'compress' program does. > >So if you buy a 4800bps MNP4 modem, you will get (almost) 4800bps. A bit >will be lost to packet overhead (about 10 bytes/packet in fact, but I >don't have my docs & code at the moment... If you buy a 4800bps MNP5 >modem you can get between 4800bps and 9600bps depending on your data. I've never seen a real 4800bps modem. I suppose one would use the V32 modulation but at 4800 rather than 9600bps? Is this really much cheaper? In any case, all the "4800bps MNP" modems I've seen always turned out to be 2400bps with MNP 5. You're right, I have no way of knowing that the modem in question isn't 4800bps and MNP4, or that compression is indeed being done at the transmitting end before data is sent. I also was under the impression that because start and stop bits are not needed during a MNP 4 transmission, throughput actually increases because there are more stop bits lost than packet overhead gained. Is that wrong? Anyway, I'm directing this to comp.dcom.modems as it's rather off the subject of mail or UUCP by now. >-- >Jon Buller jonb@vector.dallas.tx.us ..!texsun!vector!jonb >FROM Fortune IMPORT Quote; FROM Lawyers IMPORT Disclaimer; ******************************************************************************* *Thor Simon * Okay, just a little pin-prick...There'll be no more-* *lancelot@spock.UUCP * Aieeeeaaaugh!-but you may feel a little _sick_. * *uunet!hsi!yale!lancelot* ---Pink Floyd * *******************************************************************************