Path: utzoo!utgpu!water!watmath!clyde!cbosgd!mandrill!hal!ane From: ane@hal.UUCP (Aydin "Bif" Edguer) Newsgroups: comp.dcom.modems Subject: Re: MNP make for a faster modem? Summary: YES, MNP MAKES FOR A FASTER MODEM Keywords: MNP NSA TNT Message-ID: <209@hal.UUCP> Date: 26 Jan 88 15:55:43 GMT References: <3027@killer.UUCP> <6678@agate.BERKELEY.EDU> <6701@agate.BERKELEY.EDU> Reply-To: ane@hal.UUCP (Aydin "Bif" Edguer) Organization: Biometry Computing Facility Lines: 88 In article <6701@agate.BERKELEY.EDU> chapman@eris.UUCP (Brent Chapman) writes: >In article <6678@agate.BERKELEY.EDU>, ??? writes: >#In article <3027@killer.UUCP> tony@killer.UUCP (Tony Holden) writes: > >#>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? > >#It's hype. MNP is an error detection and correction protocol. A modem >#running >#at 2400 baud with MNP will probably actually have slightly (_very_ slightly) >#lower throughput than without MNP, because of the extra bits needed for the >#protocol. I happen to think it's worth it, though, because of the error >#correction. > >I've been informed that higher levels of MNP include compression as well as >error detection and correction. Even so, I find a 4 to 1 compression ratio >hard to believe under any "real" conditions, especially in interactive use. >The "compress" program, which is pretty good according to the people I know >who know about such things (I don't), rarely manages a 4:1 ratio, and then >only on large, fairly repetitious files; I doubt the ability of any modem >to do much better, because it can't use _too_ large a "block size" for >compression because of response time considerations. [I'd be happy to >be shown to be wrong, however...] I have yet to see an ad for a modem that transmits information at 9600 baud over a 2400 baud link and I would be inclined to disbelieve it... HOWEVER... MNP DOES NOT SLOW DOWN _CORRECT_ DATA TRANSMISSION! MNP SPEEDS UP _CORRECT_ DATA TRANSMISSION! Please note I said correct data transmission. MNP is first and foremost an error correction protocol and thus over a noisy line, it will send a packet till it is correct and thus could slow down the total time of transmission. (but it will be right) How does it speed up transmission? Good question. I would say RTFM but there isn't one available. So instead I will include an excerpt from another document. It only covers up to Class 4 MNP. Microcom has licensed people up to Class 6 MNP (and I saw a listing for a Class 8 MNP modem - I think someone made a typo but I never bother to call....) Aydin Edguer !cbosgd \ !decvax -+ !mandrill.CWRU.edu!hal!ane !sun / ================================ From the Bytecom MNP Introductory document.... MNP is an acronym for Microcom Networking Protocol, which was devised to provide a method of error detection and a means to correct erroneous data received. Data are grouped together and sent in packets called Link Transfer (data) frames and are acknowledged by Link Acknowledgement frames. Each data frame contains a 16 bit Cyclic Redundancy Check (CRC) character. Any received data frame that contains a CRC that does not match the locally computed CRC is said to be "broken". Detection of a broken frame causes the modem to send a negative acknowledgement to the remote system, which then retransmits the frame. The likelihood of receiving an undetected error is extremely small when this method is employed. There are several levels of operation available, which are referred to as Service Classes. The service class refers to framing techniques that the modem uses to transfer data. Class 1 uses standard asynchronous framing techniques for data transfer during half duplex operation, which is not used by full duplex modems. Class 2 uses standard asynchronous framing techniques for data transfer during full duplex operation. Class 3 uses synchronous framing techniques for data transfer during full duplex operation. Asynchronous data framing start and stop bits are "stripped" from each character prior to transmission, resulting in a 20% reduction in the number of data bits to be transmitted when using ten bits per character. This produces a data transfer rate effectively greater than the modem's data transmission rate (bps), which is 2600 bps compared to 2400 bps, respectively. Class 4 optimizes performance by using more efficient frame headers and allowing for larger data frame capability. Data frame size is adjusted in size as the modem detects higher data error rates. Data frames are reduced in size when the modem detects higher rates of data frame retransmissions.... When the modem detects less data frame retransmissions, the data frame size is increased. Effective data transfer rates up to 2900 bps compared to 2400 bps are possible when this technique is utilized.