On Sunday 08 July 2001 14:45, Tony Alfrey wrote:
> Both distros now set at 38K. I'll try 57K, but they already act
> differently.
You MUST set your "line speed" to "one more" than your "connection speed".
Period. How much 'more' is immaterial. If you fail to do so, you will never,
and can *never* transmit or recieve at the 'connection speed', only something
slower.
The reason for this is blindingly obvious, after the event. Worth some
paragraphs to explain it.
All modems negotiate transparently, between themselves, for compression and
transmission methods. The earliest compression methods: MNP levels 2 to 7,
have long since been incorporated into whatever CCITT Vxx 'standard' is
relevant this week.
One of these MNP levels dictates that the data sent on the 'connection' is
stripped of stop and start bits (which are pointless between modems) and only
genuine 'data bits' are sent. (A slight lie since they are sent as quadrature
phased packets, not bits)
You and your modem are NOT directly connected together. Instead, a UART is
employed between you and the modem. This is true even for 'internal modems'.
Reasons are good. Uarts, specifically the Natsemi INS1650 and follow ons, is
a set in concrete protocol, with set in concrete registers and signals and
handshakes. Modem protocols change on each new chip release.
So, when you set your 'line speed' it is the speed between you and your uart.
UART:= universal, ASYNCHRONOUS, reciever, transmitter. Asynchronous means
stop and start bits are sent for each byte of data. As a bare minimum, your
line speed is ONLY 8 bits / 10 bits = 80% of the actual transfer capable.
Your modem is spending 20% of its time, doing nothing.
When you also factor in additional compression methods, simplest of all would
be RLE, your modem is underworked unless you absolutely flood it with data.
The actual value use must be AT LEAST one more than connection speed. You can
safely set it at 115k (or more if you have it) because the uart will tell the
application when it's full / busy.
In a nutshell. Both line speed and connection speed are rated in bits, kbps
(forget about baud). You are behind the eight ball to begin with because you
have 2 bits too many for modem transmission.
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