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SubjectRe: ISDN, HDLC, LAPD, tutorial/introduction...
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> Meir Dukhan <meir@bis.co.il> writes:
> > on network protocol in general, Layer 2 and 3 of OSI model, and a
> > conceptual/technical description on HDLC, LAPD (is HDLC defined by an
> > RFC
> LAPD is specified in ITU-T recommendation Q.921, which is (at least was)
> on the INFOMAGICS Standards CD (together with lots of other isdn related
> stuff). There are also some ftp servers which have mounted this CD (don't
> know names by heart).

HDLC is defined by ISO 3309-1979/1991 standards, and the control
procedures are described in ISO 4335-1979/1991 standards.
(See RFC 1662) The HDLC itself is derived from IBM's SDLC with
a couple of bit modifications to generalize control, and address
formats "a bit" from its original very restricted lengths.
(I recall 1 control octed + 1 address octet per frame at SDLC.)

CCITT/ITU-T chose those technologies as basis for other
applications, like for a set of Link Access Protocols (LAP*),
of which there exist at least:
LAPB (Balanced: X.25)
LAPM (Modem: V.42)
LAPD (D-channel: Q.921)
LAPF (Frame Relay; Q.9221, I think)

Of these LAPD and LAPF are fair bit alike, although certain
aspects of the control protocols do differ. LAPB and LAPM
are again different animals.

> > Where could I found source code related to these topics ?
> For source code, look at drivers/isdn/hisax/isdn_l2.c and others. For other
> OSI-related protocol sources, look at net/{x25,802,lapb} from a recent
> 2.1.x kernel. The BSD unix sources (netbsd) also contain related stuff.

The OSI protocols are mostly out of vogue these days; Internet
came and concured, but OSI has its places in systems where
other reasons mandate it -- mostly in telecommunication admin
facilities.


If you truly need to dwell into X.25, then you must prepare
to configure about all possible parameters that your X.25
interface allows to configure -- in practice no two X.25
networks from different operators are quite alike...


If you want to dwell into ISDN signaling, you are in for
a huge task. Within Europe we (mostly) use ETSI's editions
of ISDN specifications; often known as "Euro-ISDN", or
perhaps as "NET-5". But as late as 1990 there were no
harmonized ETSI specifications for pan-european use, and
thus Europe has about 30 National ISDN profiles still in
existence.

Naturally USA has their own extension sets, and Japanese ISDN
is -- well, Japanese decided to use practically all possible
extensions bits which CCITT/ITU-T didn't specify, or had left
for "national extensions"...

Due to this kind of profileration of national features the ISDN
switches sometimes interpret some responses/messages from user
equipement in a bit unexpected manners. At one time certain
switches had a bug in their analysis paths which ended in main
signaling processor doing panic() -- the entire switch rebooted,
and as expectable, all connections through it went down :-(
Because the telephone is way more "only well-behaving guys play
here", one should not put just any arbitary code running in
there.

> Henner

/Matti Aarnio <matti.aarnio@sonera.fi>

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