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From: tnixon@hsfmsh.UUCP (Toby Nixon)
Newsgroups: comp.dcom.modems
Subject: PEP vs MNP vs v.32 (& v.42bis)
Message-ID: <3843@hsfmsh.UUCP>
Date: 17 Aug 90 22:26:46 GMT
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Organization: Hayes Microcomputer Products, Inc.  Norcross, Georgia
Lines: 88

In article <32867@cup.portal.com>, Carra_its-me_Bussa@cup.portal.com 
writes:

- I'm planning on getting a high-speed modem.  There SEEM to be 3 types:
- 
- USR HST modems using MNP 5 (giving 19.2)
- v.32 modems using v.42bis (Hayes V-series & others)
- Telebit PEP modems

Actually, there are several other types of 9600bps modems.  There 
are only two international standards: V.29, which is half-duplex on 
dial-up lines and full-duplex on four-wire leased lines, and used 
for Group 3 fax transmission; and V.32, which is asynchronous or 
synchronous full-duplex on two-wire dialup or leased lines.  There 
are a plethora of proprietary modulation schemes, including Hayes 
V-series fast-turnaround ping-pong, Telebit DAMQAM multicarrier, USR
HST assymetrical, Microcom MNP6 (which is fast-turnaround V.29),
Racal-Vadic 9600VP (which is 2400bps duplex with switching into V.29
slow-turnaround half-duplex as required), EVI Fastcomm (a different
asymmetrical scheme), etc. 

- HST (5+) uses compression to increase their speed over the single
- carrier. 

The HST uses an asymmetrical scheme with real 14,400bps modulation 
in the forward direction and 450bps in the reverse direction.  It's 
OK for file transfers, but not that great if you're changing the 
direction of the line very often (especially echoing).  They also 
support compression on top of this.  The USR Dual Standard also has 
V.32 modulation.

- V.32 runs at 9600 FDX, w/v.42bis to double it's speed over the
- single carrier. 

V.32 is 4800 or 9600 duplex.  V.42bis data compression can quadruple 
throughput, depending on the data you're sending.

- PEP uses multiple carriers (unlike Hayes/USR HST) and can slowly(?) 
- lower the baud and/or change working carriers as the line
- changes. 

PEP is the error control protocol used in Telebit modems;  DAMQAM is 
the modulation scheme.  The point is not that it "slowly lower[s] 
the [bps] rate", but that it measures the quality of the circuit and 
assigns power and bits to each carrier depending on the measured 
capacity of that portion of the channel.  The increments of 
degradation are smaller, but not necessarily the speed at which 
changing line conditions are recognized (line conditions changing 
very rarely during the course of a connection).

- USR has the dual v32/HST modem; Hayes has the Ultra modem.  Both of
- these do MNP5 and v.32, and can talk to each other (right?) 

Right.  The currently-shipping HST Dual Standard also does V.42bis, 
so you can get 38,400bps throughput between it and an Ultra 96.

- The telebit modems use PEP, but the 1000 is PEP 9600, the + is
- 9600/v.32, and the 2500 is PEP/v.32/HST (?)  (MNP 5??) 

I think it's the 1500 that is V.32-only.  The 2500 DOES NOT have HST 
asymmetrical modulation, just DAMQAM, V.32, and the lower speeds.

- So I can use the (external?) T2500 and talk to any modem known to
- man? 

No.  It won't talk in high-speed mode with modems from other 
manufacturers that have only their proprietary modulation scheme.  
You'll only be able to talk to modems that also have V.32.  
Fortunately, that number is growing; virtually all manufacturers 
I've mentioned above now have modems that support both their 
proprietary schemes AND real CCITT V.32 modulation.

- Just trying to get my facts straight before I plunk down a megagram
- of pennies 

Somehow, I think you'll be due change back from 2,200 pounds of 
pennies, regardless of which modem you buy!  :-)

	-- Toby

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Toby Nixon, Principal Engineer     Fax:    +1-404-441-1213  Telex: 6502670805
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