Path: utzoo!attcan!uunet!visdc!jiii
From: jiii@visdc.UUCP (John E Van Deusen III)
Newsgroups: comp.dcom.modems
Subject: Re: Microcom's misleading modem ad
Summary: Some reasons for Telebit's uucp superiority
Keywords: telebit trailblazer, V.42, uucp, compress
Message-ID: <534@visdc.UUCP>
Date: 12 May 89 18:33:12 GMT
References: <28111@cci632.UUCP> <1937@edison.GE.COM>
Reply-To: jiii@visdc.UUCP (John E Van Deusen III)
Organization: VI Software Development, Boise, Idaho
Lines: 44

In article <1937@edison.GE.COM> rja@edison.GE.COM (rja) writes:
>
> Most UUCP traffic has already been compressed so Microcom would move
> it at only 9600 bps, while Telebit moves the same data at more or
> less 18K.  The Telebit is still the better choice for UUCP sites.

V.32 modems move data on a good line at 9600 bps in TWO directions at
one time.  Uucp(1) makes no use of that capability.  It is a half-
duplex protocol.  A TB+ moves data on a good line at a theoretical
maximum of 18000 bps in ONE direction.  It uses a significant proportion
of that bandwidth to implement its PEP protocol, but it is still capable
of maintaining a sustained data rate, in one direction, that is greater
than is possible with V.32.

The TB+ is the modem of choice for uucp(1) communication because the
modem implements the g protocol.  When the computer in the master role
sends out a packet, the modem immediately acknowledges that the packet
was received correctly.  In this way, subsequent packets can be
transmitted immediately.  All other modems require that the packet be
verified by the computer running in the slave role.  If is was received
correctly, the acknowledgement must then be transmitted back to the
master before a subsequent packet can be transmitted.  This requires a
lot of time and is required even if the data are transmitted error free
by use of the MNP protocol.

It is not unreasonable to expect uucp transfers in excess of 800 bytes
per second, including all overhead, with a TB+ connected to a 9600 baud
interface.  This is wall time for transmitting multiple files of
precompressed data.  I know of no other modem, regardless of raw
transmission speed that comes close to accomplishing this.

> ... you would need _two_ Microcom modems (one on each end)
> to use their compression ...

Microcom is a very good and innovative company, and I have used their
2400 baud modems day in and day out with zero problems.  That fact not
withstanding, there is a very good possibility that MNP compression is
destined to become obsolete.  For V.42, a form of Lempel-Ziv compression
has been specified.  This is the reason Telebit could almost immediately
claim V.42 compatibility for the T2500; they already support compress(1).
--
John E Van Deusen III, PO Box 9283, Boise, ID  83707, (208) 343-1865

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