Re: SocketModem query (3)

From: Grant Edwards (grante_at_visi.com)
Date: 08/18/04 

Date: 18 Aug 2004 14:53:23 GMT

On 2004-08-18, Unbeliever <alfkatz@remove.the.bleedin.obvious.ieee.org> wrote:

>> 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?
>
> 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).

That's why he asked if there was some other way the two modems
do flow control.

I'm pretty user the v.NN protocols do provide modem to modem
flow control. A few Usenet postings Google found confirm my
recollections:

Quoting from

http://groups.google.com/groups?hl=en&lr=&ie=UTF-8&th=103ae0f4d8f24127&rnum=5

    If the modems have negotiated an error correction protocol
    (such as MNP or V.42) then the sending modem will indeed
    receive a flow control signal when the receiving computer
    tells the receiving modem to halt (whether via hardware
    flow control or software flow control). Modem error
    correction protocols include a mechanism for flow control
    messages between the modems, and if the receiving modem's
    buffers fill up it will signal to the sending modem that
    trasmissions should stop.

http://groups.google.com/groups?hl=en&lr=&ie=UTF-8&selm=telecom17.86.10%40massis.lcs.mit.edu
    
    Pretty close. All high-speed (>=9600bps) modems use
    synchronous links between the modems. On top of this, they
    use LAP-M (which is an HDLC-based protocol) and V.42 or
    V.14 to adapt asynchronous traffic to the modem's
    synchronous interface.

    The LAP-M protocol is basically a reliable transport
    protocol which uses retransmission timers and sequence
    numbers to achieve reliability. In this protocol, flow
    control is done by having the receiver indicate that it's
    willing to receive more data, and not necessarily by a
    "flow off" message as with XON/XOFF. (Though, confusingly,
    V.42 does provide a way to carry RS-232 signals end-to-end.
    Sigh!)

http://groups.google.com/groups?hl=en&lr=&ie=UTF-8&selm=01058004.qnkf9z%40oberon.notable.com
    
    Yes, the modem will communicate the "halt of data" to your
    modem. How it is done (in-band or out-of-band) is part of
    the modem specs that we hear about all the time. Usually
    you don't need to worry about the specifics of how the
    modems communicate this info back and forth.

I don't have a copy of the CCITT specs, so I can't confirm the
details. I do remember testing end-to-end flow control once
with v.92 modems, and it seemed to work fine for me.
    
> 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

Avoid Xon/Xoff at all costs (even if you have to learn to
solder so you can build a cable). Xon/Xoff is implimented very
inconsistently from one OS to the next, and your chances of
getting end-to-end Xon/Xoff to work correctly through a pair of
modems is sufficiently close to zero to be ignored.

Just use RTS/CTS hardware handshaking on both ends between the
modems and the computers, and then let the modems do their own
flow control. 

-- 
Grant Edwards                   grante             Yow!  Did we bring enough
                                  at               BEEF JERKY?
                               visi.com

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