> -----Original Message-----
> From: Richard Morrell [mailto:portmaster@ednet.co.uk]
> Sent: Thursday, March 25, 1999 06:14
> To: Jon Ribbens
> Cc: James Courtier-Dutton; Patrick Kormann;
> portmaster-users@livingston.com
> Subject: Re: (PM) pm3 disconnects
>
>
> On Thu, 25 Mar 1999, Jon Ribbens wrote:
>
> > James Courtier-Dutton <dutton@livingston-ent.co.uk> wrote:
> > > ISDN PPP gives better performance than V.120, this is because
> V.120 adds a
> > > lot of overhead because V.120 is Async, and ISDN is sync.
> >
> > Does anyone have a good URL or ISBN for a reference to learn about what
> > all this actually means?
>
> There are two general comms books I'd recommend:
>
> 1) "Computer Networks" by Andrew S Tanenbaum, ISBN 0-13-394248-1
> 2) "Computers in Communications" by Gorden Brebner, ISBN 0-07-709198-1
>
> > What is the difference between "sync" and "async" in this context?
>
> When sending data between two entities you need to be able to maintain the
> structure of the data (e.g. bytes, packets, frames, etc). There are two
> ways of doing this:
>
> 1) Asynchronous transmissions are sent without precise clocking. Such
> signals generally have different frequencies and phase relationships.
> Asynchronous transmissions usually encapsulate individual characters in
> control bits (called start and stop bits) that designate the beginning and
> end of each character.
>
> 2) Synchronous transmissions are sent with precise clocking. Such signals
> have the same frequency, with individual characters encapsulated in
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)
So you see there are fewer waisted bits per byte of actual data.
> control bits (called start bits and stop bits) that designate the
> beginning and end of each character.
>
> In an ISDN context, asynchronous means that the timing signals are sent in
> the B-channel along with the rest of the data and synchronous means that
> the timing signals are sent in the D-channel (which means you get your
> full 64kbps for data).
>
> > Why would you use PPP over ISDN and how does it work?
>
> Because this usually means "PPP over ISDN, synchronous" as opposed to
> V.120 which is asynchronous.
>
> > What's V110,
>
> V.110 is a bit rate adaptation protocol defined by International
> Telecommunication Union (ITU). V.110 provides a standard method of
> encapsulating data over GSM and ISDN networks.
>
> > V120
>
> V.120 is an asynchronous protocol for encapsulating data over ISDN.
>
> > and X75?
>
> Not too sure. AFAIK, it's an "inter-networking" protocol that's use to
> join X.25 networks together. I'm a bit fuzzy on what it has to do with
> ISDN.
>
> > What do the different values for 'Line Speed' mean in the PM3? (e.g.
> > '64000', '64000/async')
>
> 64000 is synchronous 64k and 64000/async is asynchronous 64k (i.e. V.120).
>
> --
> Richard Morrell
> richard@ednet.co.uk
> System Administrator - edNET: Internet Access for Edinburgh -
> www.ednet.co.uk
>
>
>
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