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USR X2 Modems . Do they Work??? Or just hype.

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Andrew Gordon

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May 22, 1997, 3:00:00 AM5/22/97
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ButtUgly wrote:
> Let's look at a typical example. For a 33.6kb connection operating at
> N-8-1 (no parity, eight data bits and one stop bit - the "typical"
> modem connection):
>
> 8 data bits
> +1 start bit
> +1 stop bit
> ---------
> 10 bits per frame

This is _not_ correct for a typical modem connection using V.42,
where the data is packetized and transmitted with HDLC framing
- no stop/start bits. There is a small overhead for the framing,
but much less than 2 bits/byte.

Your logic is applicable the PC serial port however.

John Navas

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May 24, 1997, 3:00:00 AM5/24/97
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[POSTED TO comp.dcom.modems]
4ms2u$m...@michaels.demon.co.uk (Michael Salem) wrote:

>In a previous message there is a calculation of the bytes/second (BPS) as a
>function of the bps (bits/second rate of a modem). The BPS is given as
>exactly one-tenth of the bps. As the message says, there is also
> "Noise on the modem connection, ISP delays and lost packets
>which
> "all contribute to a much lower realistic data transfer rate.
>
>Additionally, good modem protocols (e.g., MNP) improve matters by not
>requiring all the start and stop bits (before compression is taken into
>account).
>
>I found that on a good 28800 bps ZMODEM link data flowed at a fairly
>reliable 160KB/minute. Downloads from Web sites go at the same speed
>in the best of cases, down to much much much lower.


On a good link you should be able to do much better than that,
over 190 KB/min with compressed data.


>Generalising, the actual download speed in KB/minute is about 5.5 times
>the connection speed in kbps.


More like about 6.75.


>This is useful, rather than theoretical,
>knowledge, as you can estimate the best-case time to transfer a file.
>
>E.g., 1MB at 31.6kbps will take at least 1000/(31.6 * 5.5) = 6 minutes.


You should be able to do it in less than 5 minutes.


p.s. Please stop the inappropriate cross-posting of this thread!

--
Best regards,
John mailto:JNa...@NavasGrp.Dublin.CA.US http://www.aimnet.com/~jnavas/
28800 Modem FAQ: http://www.aimnet.com/~jnavas/modem/faq.html

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