RE: (PM) pm3 disconnects

James Courtier-Dutton (dutton@livingston-ent.co.uk)
Wed, 31 Mar 1999 18:56:52 +0100

Hello
When a TA is in PPP mode is does the following:-
Your Windows 95 PC sends characters to the TA normally over a serial
interface.
These characters are sent as async characters. I.E. one at a time with start
and stop bits round them. When they get to the TA, the TA does something
quite clever.
It looks for PPP frame headers and waits for each PPP frame.
The TA is not doing PPP, it is just looking out for PPP frames from the
Windows 95 PC
When it has a frame, is sends it all as a sync frame over the B-channel of
the ISDN line.
The remote end then sees the PPP frames and decodes them into IP as you
would expect.
If you try to send just text to the TA in PPP mode, the TA never sees the
PPP frame headers, so it ignores the characters. These text characters are
therefore dropped. Some TAs might even send these text characters over the
ISDN-B-Channel, but the remote end will be looking for Frame headers so it
will not catch them.
In this way, each PPP frame is sent as a frame over the ISDN-B-Channel.
If you wish to send text only over ISDN-B-Channel, you will have to use
V.120 so the TA can put each text character is a V.120 frame so it can pass
over the ISDN-B channel.
The ISDN B-Channel will only pass sync frames.
I hope this clarifies things a bit. I can dream can't I :-)
Cheers
James

> -----Original Message-----
> From: owner-portmaster-users@livingston.com
> [mailto:owner-portmaster-users@livingston.com]On Behalf Of Jon Ribbens
> Sent: Wednesday, March 31, 1999 05:56
> To: Richard Morrell
> Cc: Portmaster Mailing List
> Subject: Re: (PM) pm3 disconnects
>
>
> Richard Morrell <portmaster@ednet.co.uk> wrote:
> > > Just to clear up any final confusion: if someone dials in
> using PPP, with
> > > PPP configured on their TA also, they are sending PPP frames
> inside PPP
> > > frames?
> >
> > No. They are always using PPP (unless they are using SLIP, but lets not
> > go there just now). The PPP frames are then either encapsulated inside
> > synchronous ISDN frames or V.120 frames (which go inside the synchronous
> > ISDN frames).
>
> Uhhhhhhhh. My brain hurts.
>
> What does it mean if you set the TA to 'PPP mode' then? Are you saying it
> means 'I am going to use PPP, please do something bizarre'? I had somewhat
> assumed it meant 'Please use PPP to talk to the remote TA'. RFC 1618 talks
> about a load of encoding stuff (NRZ etc) which makes no sense to me unless
> it is the TA which is talking PPP not the computer.
>
> (If the TA is packetizing the data, does it just wait until it
> has finished
> sending data and then take all the data in its input buffer and
> use that as
> the next packet, or what?)
>
> (I tried setting a Courier I-Modem to 'PPP mode' (well, it calls it
> 'Asynchronous to synchronous PPP mode' or somesuch, which makes no sense)
> and although Win95 DUN works over it, I cannot send text over it.
> Which makes no sense either. Unless in PPP mode the TA is expecting PPP
> packets on its serial wire and decoding them.)
>
> Cheers
>
>
> Jon
> --
> \/ Jon Ribbens / jon@oaktree.co.uk
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