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From: kianusch@ghost.UUCP (Kianusch Sayah-Karadji)
Newsgroups: comp.dcom.modems,comp.sys.mac.comm
Subject: Re: How to increase throughput on 9600 bps modem?
Keywords: n
Message-ID: <17@ghost.UUCP>
Date: 13 Dec 90 19:49:43 GMT
References: <9PF^W}#@ads.com>
Followup-To: comp.dcom.modems
Organization: GHOST (Unigold Remote Site)
Lines: 52

>
>Most binary files transfer from my Mac IIcx through the 9600 V.32
>modem to the remote site (Telebit T2500) at about 300 bps -> 600 bps.
>Text files (like Macintosh HQX files) transfer at around 900 bps.
>ZMODEM claims that 900 bps is 98 percent effeciency.  Ok... let's
>assume ZMODEM is telling the truth when it says that 1000 bps is the
>best I can expect.  Why is it that binary files are doing the snail's
>pace of 300?
>
>There's more to it than that actually.  Very often the performance
>gets as low as 60 bps!  This is apparently because the transmissions
>are very "bursty" in character.  There will be a sudden burst of
>characters to the remote modem and then a LONG 5 second delay of dead
>air and then a little more traffice and then another LONG delay of say
>10 seconds and so on ad nauseum.  Is this because ZMODEM is getting
>NAK packets of some kind back and retransmitting?  Why the LONG
>delays?
>
>Here's another fun bit of info.  Transfers from the remote machine TO
>my Mac are FAST!  Always.  Binary and ascii data crank at 98 percent
>effeciency all the time.  Now why in the world would reception be
>faster than transmission?

... if you modems have compression mode don't use Zmodem
(use Y/X modem instead) ... if you do want to use Zmodem ... turn modem
compression of and use only local error correction... same thing with
compressed files, (zoo, compress, zip, ...) dont use Zmodem or Modem
compression at all.

The reason is that each of those try compress the data... 

	worst scenario... transfering a compressed file, with
	zmodem using an compression modem protocoll...

	zmodem try's to compress ... but actually makes the file bigger ...
	now the modem get's a big-compressed file, which the modem in turn
	tries to compress ... and makes it even more bigger ...  and everything
	slowes down...

happend to me a few days ago... trying to tranfer a compressed (700k) file...
using telebit PEP / COMPRESSION ... it took me 30min for the first 300k ...
I aborted the transfer ... started all over again using PEP only
(no compression) ... and the transfer was compleated in 12 minutes...

Text files are not compressed... so either Zmodem, or Modem-compression will
do a better job...

							Hope This helps
							   Kianusch
-- 
	Kianusch Sayah-Karadji			srhqla!unigold!ghost!kianusch
	 kianusch@ghost.UUCP			pacbell!unicom!ghost!kianusch