> -----Original Message-----
> From: jon@chalk.oaktree.net.uk [mailto:jon@chalk.oaktree.net.uk]On
> Behalf Of Jon Ribbens
> Sent: Friday, March 26, 1999 01:36
> To: James Courtier-Dutton
> Cc: portmaster-users@livingston.com
> Subject: Re: (PM) pm3 disconnects
>
>
> James Courtier-Dutton <dutton@livingston-ent.co.uk> wrote:
> > Groups of characters are enclosed in frame marker bytes making
> up a frame.
> > This saves on bits.
> > Async sends 8 bit Bytes with at least 1 start and stop bit
> making a total of
> > 10 bits per character.
> > Sync sends groups of 8 bit bytes (say 256 bytes) enclosed in
> frame markers,
> > (normally about 3-5 bytes)
>
> This confuses me greatly. I thought ISDN inherently provides a method of
> sending bits. Why do you need start or stop bits, which are usually sent
> to synchronise clocks? The ISDN system must already have such
> synchronisation
I was just explaining what Async and sync were generally.
ISDN uses Sync frames. Each frame contains control bits and 1 byte for each
B channel and a few bits for the D channel.
This then gives you 3 clear channels at the higher layer so no more
syncronisation is necessary.
This is fine as far as it goes, but for data to pass over the D and B
channels it should be put in frames. D channel used LAPD, B channel uses PPP
frames. (Each PPP packet has a frame header)
The B channel continuously sends padding characters when no data is needing
to go down the channel, so you nead at least a packet header to tell you
when some data has arrived.
> at a lower layer, otherwise it could not talk on the D-channel to
> the switch.
> Why do you need *more* synchronisation within the B channel itself?
>
> Or is it that you *don't* need the extra synchronisation, and
> this is why you
> should use PPP over ISDN and not V.120, in that using an
> asynchronous protocol
> over ISDN is just pointless?
Yes, it uses the PPP header as the frame header in the B channel, therefore
not waisting any bytes.
Whereas V.120 encapsilates each character in an LAPD frame.
>
> Presumably PPP is the best protocol to tell our customers to use
> to connect
> to our PM3? If their TA does not support PPP, what is the next
> best protocol
> to use?
If there TA does not support PPP, I would throw it away, because they would
have to then use V.120
I don't think the PMs support V.110 and X.75 yet. (not sure here)
>
> Cheers
>
>
> Jon
> --
> \/ Jon Ribbens / jon@oaktree.co.uk
>
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