Path: utzoo!mnetor!tmsoft!torsqnt!news-server.csri.toronto.edu!bonnie.concordia.ca!uunet!hayes!tnixon
From: tnixon@hayes.uucp
Newsgroups: comp.dcom.modems
Subject: Re: V32.bis vs HST
Message-ID: <3774.27b7e5c6@hayes.uucp>
Date: 12 Feb 91 12:55:33 GMT
References: <65@ns.UUCP>
Organization: Hayes Microcomputer Products, Norcross, GA
Lines: 43

In article <65@ns.UUCP>, bob@ns.UUCP (Bob Mathias) writes:

> One of the bbs's that I use just upgraded to the new USR Dual Standard modem
> (V32.bis capable).  Finally I would get a chance to see how much faster
> V32.bis is to HST.  I downloaded a number of files using Ymodem-G with 
> both V32.bis and HST.  And lo and behold, HST proved to be faster.  On
> a 300k zipped files I got 1727cps for HST and only 1682cps for V32.bis.

Did you repeat this multiple times, and are these the average of 
several attempts?  Or did you just try one time for each modulation?
The reason I ask is that these figures are in fact the same within
+/- 22 cps; this is only +/- 1.3%.  The CPS calculations of most
comm programs can easily vary by this much, since most are based on
full-second increments (which, on a 300k file transmitted at
1704cps, could easily put you off by +/- 1% by itself). 

Another, actually more likely, explanation is that USR is using 
256-byte frames in HST mode (the default for their modified MNP4), 
and 128-byte frames in V.32bis mode (the default for V.42 LAPM).  
With 256-byte frames, the maximum throughput of a 14,400bps modem is 
about 1722cps (including 7 bytes of overhead per frame, and the 
typical 1 bit inserted every 64 for transarency purposes); with
128-byte frames the maximum throughput is about 1678cps (excluding 
data compression in both cases).  This aligns closely with the
figures you see.  The benefit of using the smaller frame size is
that you recover faster when there is line noise, but pay a small
throughput penalty otherwise.  If you configured the modem to
negotiate to a 256-byte frame size in LAPM, the throughputs would be
identical. 

Another factor could be some anomaly in USR's inplementation that 
causes them to have slightly less throughput in V.32bis than in HST. 
There two modulations provide a 14,400bps pipe, so there's no reason 
for them to have different throughput, all other conditions being 
equal.

	-- Toby

-- 
Toby Nixon, Principal Engineer    | Voice   +1-404-449-8791  Telex 151243420
Hayes Microcomputer Products Inc. | Fax     +1-404-447-0178  CIS   70271,404
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