Path: utzoo!mnetor!uunet!ncc!alberta!ubc-cs!van-bc!sl
From: sl@van-bc.UUCP (pri=-10 Stuart Lynne)
Newsgroups: comp.dcom.modems
Subject: Re: hayes 9600 vs. trailblazer
Message-ID: <1720@van-bc.UUCP>
Date: 17 Apr 88 20:56:32 GMT
References: <8804110136.AA16978@ucbvax.Berkeley.EDU> <15612@onfcanim.UUCP> <280@telebit.UUCP> <4435@hoptoad.uucp>
Reply-To: sl@van-bc.UUCP (pri=-10 Stuart Lynne)
Organization: Public Access Network, Vancouver, BC.
Lines: 43

In article <4435@hoptoad.uucp> gnu@hoptoad.uucp (John Gilmore) writes:
>rls@telebit.UUCP (Sr. Systems Engineer) wrote:
>> 1.  The Telebit TrailBlazer uses Lempel-Ziv data compression, when the S110
>>     register is set appropriately.


>>     Now, if we compress in the computer, we can send the COMPRESSED output out
>>     at 19.2K...			   But if we did compression in the
>>     modem, we'd always be limited by the link speed to 19.2K. OK?
>
>It's sad but true that the bottleneck in sending ASCII data between
>systems through a Telebit modem is getting to be the 19200 max speed on
>the serial cable.  Telebit really should support 38,400 baud.
>
>Jerry Aguirre mentioned that compressing data on the host burns less
>CPU cycles too; this would be because running "compress" over say 500K
>of data is cheaper than squirting 188K (the data saved) out the serial
>port.  The inner loop of compress must be a *lot* smaller than the
>inner loop of the serial drivers on his system, though this might not
>be true on all systems.

A back of the envelope calculation shows this to be true for low end cpu's such
as the 68010 I'm running on (10Mhz).

For transmitting a file at 9600bps, my machine can sustain the character
flow at full speed, consuming something on the order of 50% of the cpu cycles
available. This indicates that my effective output bandwidth is on the order
of 2kb per second. Compress will quite happily consume on the order of 12kb of
data per second when compressing.

If we average a 30% reduction overall in a file by compression we have a net
cost of:

	cpu cycles = cost of transmission + cost of compression

				= COT*.7 + COT / 6
				= COT*.7 + COT*.166
				= COT*.866

Giving us a net savings of about 13% (pun intended).

-- 
{ihnp4!alberta!ubc-vision,uunet}!van-bc!Stuart.Lynne Vancouver,BC,604-937-7532