MNP "classes" Microcom Networking Protocol

1 mnp classes

1.1 mnp 1 , 2
1.2 mnp 3
1.3 mnp 4
1.4 mnp 5
1.5 mnp 6
1.6 mnp 7
1.7 mnp 9
1.8 mnp 10

mnp classes

microcom s idea move file-transfer protocol out of host computer , place in modem instead. in doing so, data being transferred error corrected, not file transfers. meant devices no processor, dumb terminals, enjoy error-free link. after connection remote modem, microcom modems played special tone line , listened response; if proper tone received in reply, modems entered error-correcting state.

the original protocol extremely simple , rather inefficient, leading variety of improved protocols referred classes . each class improved performance on earlier versions, retained backward-compatibility reasons.

mnp 1 , 2

the first mnp standard, retroactively known mnp class 1, or mnp 1, simple half-duplex protocol similar xmodem in nature. lacking sliding window support, throughput efficiency low, @ 70%. meant on 2400 bit/s modem, ones microcom sold, throughput limited 1690 bit/s when mnp 1 in use. system created easy possible implement in limited hardware, explains simplicity.

with low-cost processing power improving, microcom introduced mnp 2, full-duplex version of mnp 1 allowed ack messages returned while next outbound packet starting. eliminated pause while modem waited ack returned, adding requirement system needed memory track whether or not ack received within given amount of time. since inter-packet delay reduced, overhead of crc remained, improving throughput 84%.

mnp 3

in normal use, modem can send or receive data @ point in time, mode of operation known asynchronous . modem can determine speed of sender s data listening bits being sent it, , locking clock speed of bits being received. since data can arrive @ time, there no precise timing; clock may have re-adjusted pauses user stops typing (for instance).

unfortunately sort of clock decoding not work unless there @ least transitions between 1 , 0 in data; long stream of 0s or 1s has no transitions in it, making impossible know data particular byte starts. in order avoid problem, additional framing bits added either end of every byte, typically 1 bit on either side known start , stop bits . guarantees @ least 1 1-to-0 transition every byte, more enough keep clocks locked. however, these bits expand every 8 bits of data (one byte) 10 bits, overhead of 20%.

when using file transfer protocol, packets offer own framing. packets send continuous stream of data, clock cannot drift in same way data being sent user typing on keyboard. turning off these framing bits when operating on error-corrected link, 20% overhead can eliminated.

this precisely mnp 3 did. after negotiating , realizing both modems supported mnp 3, framing bits turned off, improving overall efficiency. when using mnp 3, user can expect close ideal 2400 bit/s throughput (versus 1900 bit/s) eliminating overhead.

mnp 4

mnp 4 further improvement on mnp 3, adding variable packet size system referred adaptive packet assembly.

in case of mnp overhead of packet system relatively small, multi-byte crc taking space better used data. using larger packet address this, because crc remains same fixed size , relative overhead reduced compared amount of data. however, when error occur, using larger packets means more data has re-sent. on noisy lines, can slow overall throughput.

with mnp 4 2 modems monitor line dropped packets, , if threshold crossed (selected user), modem drops smaller packet size. means when packet dropped, amount of data has re-sent smaller, leading better throughput. on quality lines, using larger packets means overhead of crc reduced. packets between 64 , 256 bytes, , allowed user force particular size if wished.

mnp 4 introduced data phase optimization, simple change protocol allowed of packet-framing information dropped after link set up, further reducing protocol overhead. combination of these features, along mnp 3 s lack of byte-framing, allowed further increase in throughput efficiency.

mnp 5

an more radical change made mnp 5, introducing on-the-fly data compression in modem. mnp 5, data received computer first compressed simple algorithm, , passed mnp 4 packetizing system transmission. on best-case data system offered 2:1 compression, in general terms 1.6:1 typical, @ least on text. result 2400 bit/s modem appear transfer text @ ~4000 bit/s.

this dramatic increase in throughput allowed microcom modems remain competitive models other companies otherwise nominally faster. instance, microcom produced 1200 , 2400 bit/s modems using commodity parts, while companies usrobotics , telebit offered models speeds 19200 bit/s.

however, improvement in performance available if modems on both ends supported mnp. made system attractive sites installing modems @ both ends of links; dial-up services bulletin board systems (bbs) there no compelling reason use microcom device when end-user unlikely have one. in cases user in control of both ends of link, microcom s proprietary modems less interesting models other companies offered higher real world throughputs.

in order create market microcom modems, starting mnp 5 took radical step of licensing entire mnp suite free. idea dramatically increase number of modems mnp installed, making real microcom modems more attractive. moreover, newer standards improved performance offer better performance when there microcom modem @ both ends of link.

unfortunately plan backfired. introduction of improved lapm compression system in v.42bis standard outpaced microcom s own advancements, diluting value of real microcom model zero. using v.42bis , commodity parts, huge number of low-cost modems better performance microcom s available. although microcom continued introduce newer standards, largely ignored , microcom stopped being force in market.

mnp 6

the introduction of v.32 led number of standard 9600 bit/s modems, of offered mnp 5. further differentiate becoming commodity market (although not until introduction of v.32bis suprafaxmodem 14400 in 1991), microcom created mnp 6.

mnp 6 s main feature statistical duplexing, dedicate more or less of bandwidth 1 side or other of modem link. instance, if 1 machine sending large file, other end send small amount of information, ack , nak messages. in case modems give of channel possible sender, offering one-way bandwidth 19,200 bit/s. did not require changes modulation system: 9600 bit/s modem had full 9600 bit/s channel in both directions, total of 19200 bit/s; mnp 6 allowed more or less of bandwidth given 1 side or other, instead of leaving fixed @ 9600 both ways.

this basic concept used in industry, having formed basis hayes s express 96 protocol, usrobotics hst telebit s pep, , (briefly) compucom speedmodem. of these standards found difficult survive in v.32bis dominated market, and, them, mnp 6 largely ignored.

a less notable addition mnp 6 universal link negotiation. introduction of additional modulation modes, notably v.32 , later additions, modems on either end of link had spend increasing amount of time negotiating common standard. instance, v.32bis modem first send tones line try 14.4 link; if failed after time, try 9600, 2400 , 1200 bit/s. since each of these standards defined minimum period of time try link, delay grew on 10 seconds.

uln avoided delay negotiating link @ 2400 bit/s no error-correction turned on. although eliminated compatibility older 1200 bit/s modems, point in time extremely rare. once connection made, occurred quickly, both modems sent small identification string remote modem. both modems examined string , selected fastest common mode. caller re-negotiated once @ higher speed.

mnp 7

mnp 7 introduced new compression algorithms claimed improvement 3:1 compression on text files. however, time mnp 7 introduced, v.42bis standard offering 4:1 compression.

mnp 9

mnp 9 (there apparently no 8 released) improved universal link detection add newer high-speed modes, otherwise identical mnp 7.

mnp 10

mnp 10 introduced new error-correction protocol designed work on noisy phone lines used in eastern europe. unlike earlier versions mnp 4, mnp 10 monitored line quality , adjusted packet size if conditions improved.

in 1991 microcom licensed mnp 10 rockwell international use in extremely popular modem chip sets. since modems exception of usr s models used rockwell chipset 1995, mnp 10 became deployed (if not used). usr added mnp 10 v.everything series modems, making universal.

mnp 10 later expanded mnp 10ec, ec standing extended cellular . series of modifications allowed mnp 10 deal transmission pauses when cell phone moves 1 cell another, interpreted errors in line. using mnp 10ec, these pauses correctly identified not errors , , link speed remains higher. success led at&t paradyne-created competitor, etc.

mnp 10ec particularly attractive in cellular role due inclusion of uln link-negotiation method introduced in mnp 6 (and improved in mnp 9). on cellular network air-time billed, faster setup saved money. mnp 10ec had limited life span, cell networks turned variety of all-digital systems no longer required modem connect computer.