Path: utzoo!mnetor!tmsoft!torsqnt!news-server.csri.toronto.edu!cs.utexas.edu!uunet!motcid!wallach
From: wallach@motcid.UUCP (Cliff H. Wallach)
Newsgroups: comp.dcom.modems
Subject: Re: V32.bis vs HST
Message-ID: <4574@avocado5.UUCP>
Date: 15 Feb 91 21:56:46 GMT
References: <65@ns.UUCP> <3774.27b7e5c6@hayes.uucp>
Organization: Motorola Inc. - Cellular Infrastructure Div., Arlington Heights, IL 60004
Lines: 37

In article <3774.27b7e5c6@hayes.uucp> tnixon@hayes.uucp writes:
-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.
-

-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. 

-	-- Toby
-

Much of the code is common to both modes of operation.  The throughput
differences are a result of the smaller frame size in LAPM mode.

Buffer space is an issue; MNP4 requires 8*256 bytes, LAPM 15*128
bytes.  Pre-V42 HST modems only had 8k of ram, and I had planned for
an easy field upgrade to V42.  This didn't happen after I left US
Robotics.


Cliff Wallach				...uunet!motcid!wallach