thinder@NSWC-WO.ARPA.UUCP (09/24/87)
We have a broadband LAN that has dial-in and dial out modems on it. We would like to install some amount of control over these circuits. I have looked at several vendor offerings but have not found a suitable device. Most of the items I have reviewed are for dial-in use only and involve dial-back as a form of control. The problem occurs on our dial-out circuits, its here that we want to control user access. Currently the most "reasonable" suggestion is to use a data switch, like the Gandalf PACX 2000. Does anyone on the net have any suggestions. I am not on distribution for this group so please contact me directly at: thinder@nswc-wo.arpa or thinder@nswc-oas.arpa Thomas Hinders Naval Surface Warfare Center (301) 394 4225 or 1802 Autovon 290 4225 Thanks, Tom Hinders
edward@ucbarpa.Berkeley.EDU (Edward Wang) (09/09/89)
More specific questions, since no one answered the vague ones last time: What is MNP? I gather it's a packet protocol, implemented on a synchronous line when it's built into a modem. How can I find out the details, like packet format, error detection, handshake method? What kind of protocol does the Telebit use? It seems to do some sort of data compression as well. What algorithm? I've tried looking in the literature (even trade press), but found next to nothing, so any help would be appreciated.
carr@gandalf.UUCP (Dave Carr) (09/19/89)
In article <31194@ucbvax.BERKELEY.EDU>, edward@ucbarpa.Berkeley.EDU (Edward Wang) writes: > > What is MNP? I gather it's a packet protocol, implemented > on a synchronous line when it's built into a modem. How > can I find out the details, like packet format, error detection, > handshake method? > MNP is a synchronous layered protocol which handles parameter negotiation, error correction, and compression. The compression has 2 layers, 5 and 7. MNP level 5 is commonplace. MNP Level 7 is relatively new and expensive. MNP level 5 is character based and achieves 2:1 compression *usually*. For the single user interactive sessions, and maybe for file transfer and mail you get the 2:1. Don't use it for instance on a multiplexer backbone. It won't get even 1.7:1. The details for MNP aren't free, you have to buy them from Microcom. We're under license from them and that's about all the details I can give. Level 5 is something you could write in 2 days if you have the specs. Level 7 is quite a bit more complicated and would takes several weeks to implement (and lots of memory). > What kind of protocol does the Telebit use? It seems to do > some sort of data compression as well. What algorithm? > We're not under contract with these guys, but I don't know the details. I will hazard a guess. What happened about 2 years ago was that most of the modem manufacturers who were producing MNP modems got pissed at Microcom for releasing the MNP specs 6 months after releasing their own modems. This left them 12 months behind in the market. They got off the MNP bandwagon and went to a software house called Adaptive Computer Technologies and bought a package called the ACT CommPressor. It gets about 3:1 in the same situations that MNP Level 5 gets 2:1. It still suffers in some applications. ACT CommPressor software (rights to reproduce) go for about $3,000 US. Now the funny part. CCITT called for papers to standardize modem compression (V.42 bis). Microcom leapt up and said "Pick me. Pick me". ACT and most other modem manufacturers said "Pick us". But alas, CCITT picked a modified Limpel-Ziv (Unix compress,etc) from British Telecom. So, expect the new breed of modems to get about 4:1 compression ! wowzers !. I'll bet the Telebit uses ACT CommPressor software. ---- Dave Carr Gandalf Data Limited Nepean, Ontario, Canada (613) 723-6500
gnu@hoptoad.uucp (John Gilmore) (09/25/89)
carr@gandalf.UUCP (Dave Carr) wrote:
> I'll bet the Telebit uses ACT CommPressor software.
Not true; Telebit uses Unix compress. The modem contains a 68000 and much
of it is written in C; they probably just changed the interfaces a bit
so it would compress blocks on demand, compiled it, and installed it.
Personally I don't see too much point to compression-in-the-modem; hosts
are usually faster than 68000's, and it's more expensive for many hosts
to handle the doubled serial port traffic caused by sending the uncompressed
data over the port to the modem. Better for the host to just compress it
before sending. I suppose it's useful when on a dumb terminal.
John
--
John Gilmore {sun,pacbell,uunet,pyramid}!hoptoad!gnu gnu@toad.com
"Watch me change my world..." -- Liquid TheatreGeoffrey.Welsh@p0.f171.n221.z1.fidonet.org (Geoffrey Welsh) (09/25/89)
> From: carr@gandalf.UUCP (Dave Carr) > Message-ID: <2608@gandalf.UUCP> > ACT CommPressor software (rights to reproduce) go for about $3,000 US. > > Now the funny part. CCITT called for papers to standardize modem > compression (V.42 bis). Microcom leapt up and said "Pick me. Pick me". > ACT and most other modem manufacturers said "Pick us". But alas, CCITT > picked a modified Limpel-Ziv (Unix compress,etc) from British Telecom. Not funny at all. The CCITT V.42bis committee cited problems with MNP5 (may EXPAND the data if it's already been compressed), MNP7, and ACT (they're proprietary; the CCITT won't adopt anything that won't become PD). > So, expect the new breed of modems to get about 4:1 compression! I'm not sure it will be that high. I know that it's possible (the latest version of PKZIP will compress a text file to 15-20% of its original size), but compression "on the fly" is less powerful, especially if you don't want to incurr big delays by keeping enough data in memory to make powerful analyses on them before sending them off. > I'll bet the Telebit uses ACT CommPressor software. Quite possible, but I don't think the TB+ compression algorithm gets significantly better ratios than MNP5. -- Geoffrey Welsh - via FidoNet node 1:221/171 UUCP: {{uunet!}watmath!xenitec!}zswamp!171.0!Geoffrey.Welsh ARPA: Geoffrey.Welsh@p0.f171.n221.z1.fidonet.org
carr@gandalf.UUCP (Dave Carr) (10/05/89)
In article <998.251E4CDB@zswamp.fidonet.org>, Geoffrey.Welsh@p0.f171.n221.z1.fidonet.org (Geoffrey Welsh) writes: > > I'm not sure it will be that high. I know that it's possible (the latest > version of PKZIP will compress a text file to 15-20% of its original size), > but compression "on the fly" is less powerful, especially if you don't want > to incurr big delays by keeping enough data in memory to make powerful > analyses on them before sending them off. > True. But most modem manufactures include the *compression* gained by stripping the stop and start bits (and sometimes parity) to get figures as high as 4:1. Starting at 10-bit characters, you only have to get down to 2.5 bits/character to claim 4:1. This is quite possible. PKZIP is working on 8-bit data. The BT-LZW also learns quicker, and adapts even after the tables fill. It also has a CLEAR TEXT mode, so uncompressable data doesn't expand. I don't know the inner workings of PKZIP, so I can't compare this to V.42 bis. The delay question can be addressed by upping the speed at the terminal side of the modem to 38.4 or higher. The modem will usually buffer data to get higher compression. That is, if the compression is low, the buffers will fill to a higher level, and therefore compression will improve.