Path: utzoo!news-server.csri.toronto.edu!rutgers!usc!elroy.jpl.nasa.gov!swrinde!zaphod.mps.ohio-state.edu!rpi!crdgw1!uunet!motcid!wallach
From: wallach@motcid.UUCP (Cliff H. Wallach)
Newsgroups: comp.dcom.modems
Subject: Re: HST/V.32bis Performance
Message-ID: <4695@cocoa61.UUCP>
Date: 11 Mar 91 19:15:26 GMT
References: <139@ns.UUCP> <3811.27ca371e@hayes.uucp> <141@ns.UUCP>
Organization: Motorola Inc. - Cellular Infrastructure Div., Arlington Heights, IL 60004
Lines: 39

In article <141@ns.UUCP> bob@ns.UUCP (Bob Mathias) writes:

-
-I was using V.42 LAPM during my performance tests for both protocols 
-(V32.bis and HST).  I wasn't aware of the difference in frame size 
-between V.42 LAPM & MNP4 so I decided to do the tests using MNP4 instead.
-This improved my performance for both protocols.  I now get 1732 cps
-for HST and 1723 for V32.bis (less than a 1/2 % difference).   
-

In HST mode an extended form of MNP3 is active.  In addition to
extensions for line reversals are some protocol enhancements that
were intended to reduce the round trip delay of the backchannel.
These enhancements were designed before MNP4 was released.

The standard MNP3 data frame is formatted as:
	1 open flag byte
	7 header bytes
	64 data bytes
	2 crc bytes

HST mode reduced the header to 2 bytes.

Level 4 provided 2 independent modifications to the MNP protocol.  The
header size was reduced to 3 bytes, and the frame size was increased
to 256.  HST mode retained the original streamlined header, but used
the increased frame size.

The different overhead size (5 vs 6) results in a throughput
improvement of about 0.4%.  Another 0.15% improvement results because
every 2 seconds MNP must send an ack frame, while HST mode has a
combined data/ack frame.  Also remember about measurement and roundoff
errors.

When V42bis and HST modulation is negotiated, then the BTLZ data
compression is run with HST mode protocol.


Cliff Wallach				...uunet!motcid!wallach