On Sat, 05 Aug 2000 14:17:23 +0000 Bastiaan Edelman wrote:
> in practise the speed of a normal telephone connection wil not exeed
> 19k2 ... often lower than that.
> Your computer is blowing the bits into the modem faster than it can
> handle (115k vs. 56k) and the modem is spitting it in the telephone line
> at a spead much higher than the line can handle.
> In the end the computer is at least 8 times faster than the line...
> with stalling and other trouble as a result.
> What is the point in setting the computer to speeds that modem and line
> cann't handle anyway...
>
With respect, I'm NOT sure that this is absolutely correct. The modem
is 'intelligent' and and knows how fast the data that it has is going
down the telephone line. It won't therefore request data from the PC
faster than it can send it down the line, even though the actual link
between the modem and the PC can handle data at very much higher speeds
than the telephone line speed. When data is to be transferred between PC
and modem, the two boxes carry out a sort of dialogue that goes:
(Modem:) Send me some data!
(PC:) Here it is....
(Modem:) Stop! Too much....
(Modem:) Right - carry on....
(PC:) Here it is, then....
(etc.)
The original query that started this subject had to do with the link
speed from Modem to PC being TOO FAST for the PC to collect the data
from the modem. Although the 16550 UART has a buffer, it seems that
either Arachne doesn't use it or that Arachne is still too slow to empty
the buffer before it overflows. This results in missed characters in the
communication and so the communication protocol gets completely
confused. Sometimes you see a dead giveaway that this is happening if
you watch the terminal screen during dial-up with the serial port speed
set to 115200. The messages that appear, sent from the modem to the PC
and reproduced verbatim, have characters missing - a good indication
that all that the modem is sending isn't being seen by Arachne. I've
found that setting the serial speed to 57600 on a 486DX4-100 cures the
problem, using a 16550-equipped serial port.
Since data CAN come down the line in quite fast bursts sometimes,
it's a good idea to have the port speed going as fast as possible. Bear
in mind that the port speed isn't equal to the line speed for at least
two reasons: firstly, the data coming down the line may be compressed.
The modem de-compresses it so that there is more data to send to the PC
than came down the line. To get that expanded data into the PC at the
same rate that the compressed version came down the line requires that
the data is transferred to the PC at a faster rate, obviously. Secondly,
the PC and modem exchange control messages along the serial link as
well. All told, there's more data going between the modem and PC than
there is going along the line.
Hope this helps!
Ron.
-- Arachne V1.66, NON-COMMERCIAL copy, http://arachne.cz/
