Re: SocketModem query (3)

From: Unbeliever (alfkatz_at_remove.the.bleedin.obvious.ieee.org)
Date: 08/18/04 

Date: Wed, 18 Aug 2004 21:25:57 +1000

"Roger" <enquiries@rwconcepts.co.uk> wrote in message
news:VlHUc.105$cb5.82@newsfe5-gui.ntli.net...
> I'm messing around with a SocketModem at the moment, trying to develop
some
> basic driver software for an AVR. Between the modem and uC is a simple RTS
> flow control signal. My question is this: Assume the modem is receiving
data
> down the phone line and passing it to the uC (i.e. nRTS is driven low by
the
> uC). The receive buffer in the uC becomes too full and the nRTS signal is
> set high to temporarily stop more data being sent. What happens to the
flow
> of data down the phone line? Will my receiving modem somehow signal up the
> line to the sending modem to stop sending data? If not how is a fully
> flexible link capable of handing downloads and bottlenecks at the
receiving
> processor implemented?
>
> TIA.
>
> Roger.
>

CTS/RTS handshaking isn't used across the PSTN boundary. It is used between
the modem and the DTE (usually by the modem to indicate that its buffer is
temporarily full). To tell the upstream device to stop sending in the
unlikely event that you've found a modern microcontroller that can't keep up
with a modem or are doing a shipload of processing or slow I/O on the data
received consider using XON/XOFF handshaking
(http://searchnetworking.techtarget.com/sDefinition/0,,sid7_gci213406,00.htm
l and apparently pronounced EKS-AWN EKS-AWF - possibly in some of the
hillier souther regions of the USA) or something similar.

Cheers,
-
Alf
alfkatz@remove.the.obvious.ieee.org
(Good, Fast, Cheap - Pick any two).

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