X.25, X.31 vs Q.921/Q.921 and Networking Style in General
mart...@bacchus.uucp.uucp
>Do the obvious thing: chuck it all and use the DoD Internet Protocol
>suite for this application. It is certainly clear that you need some
>sort of transport that makes no assumption about the reliability of
>the data link layers, and the X.* stuff doesn't seem to qualify.
While I have enjoy Padlipsky's writing, you have missed the point (or
I have accidentally mislead you), the X.25, X.31, LAPD, Q.921/Q.931
suite of protocols is not comparable to TCP/IP but is rather
comparable to 1822. The ISDN switch is basically comparable to an IMP
(or PSN nowadays I guess). One might argue about the virtual circuit
orientation of the X.25 but in fact an ISDN switch is much more
complicated than an IMP and you really need something more complex
than 1822. As for the circuit orientation, from personal experience
in dealing with the FCC, I believe tariffing a reliable virtual
circuit access protocol in which the remote end point is specified is
much easier than tariffing an access protocol like 1822. Of course,
the existence of a level 3 reset is relatively stupid in a virtual
circuit oriented access protocol, but the level 3 reset was *demanded*
by the PDNs because of the limitations of their switches.
Running TCP/IP on top of X.25 is just as reasonable as running TCP/IP
on top of 1822 but as far as the PTTs and the private long distance
carriers are concerned, they have no interest in an internetting layer
or a transport layer built on top of it. A true internetting layer
makes tail-end-hop-off (popping in and out between public and private
networks where the two end point are accessing the catenet via public
phone lines in order to decrease phone bills) and such a specification
will never come out of CCITT.
Now the proprietary network on which I am working was developed before
the modern TCP/IP based Arpanet came into existence. As I understand
it the pre-TCP/IP Arpanet basically used a predecessor of 1822 as an
end to end protocol, so that my contractee's decision to go with X.25
as the network communications protocol was not unreasonable especially
given that the major portion of their business is overseas where until
recently no one did TCP/IP. Of course not learning anything from the
Arpanet experience is pretty close to criminal but the company is
being punished in its sales, so justice is being served.
Granted, there is no real reason to run X.25 on an unrestricted
digital 64kbps CSC bearer channel except that the CSC may equally
probably be going to a PSDN as to one of the proprietary network hosts
where one of the network hosts is acting as the DCE and pretends to be
a PSDN. This is reasonable because if some other manufacturer
supports some service via a PSDN, my contractee can directly offer
that service to the other manufacturer's equipment because my
contractee's equipment can pretend to be a PSDN.
But my contractee is also trying to sell real data communications via
a ISDN-PBX-LAN and my feeling is that while the network hosts can
pretend to be PSDNs, among friends (non-alien hosts), the host that
pretends to be a DCE does not have to imitate the non-obligatory
procedures which PSDNs use like VC reset, some of the network-level
restarts, the various worthless level 3 DCE timers.
If they do complete imitation of the PSDN non-obligatory procedures, a
CSC based ISDN-PBX-LAN running the proprietary data communications
network should quite quickly become fragmented into topologically
disconnected islands of network connectivity with evil consequences
like dead-lock, loss of network services and inability to use to use
some of the niftier features of Q.921/Q.931 [of course maybe they are
trying to prevent the ultimate convergence of phone and computer
hacking :)].
I would really like some genuine X.25/PSDN expert to tell me my
suggestion is perfectly reasonable and is in fact the right way to do
data communications in the environment of a CSC based ISDN-PBX-LAN
using the X.25 suite of protocols. Of course, if I am wrong I would
like to be told and be told why. The network-export at my
contractee's has yet to show any understanding of data communications
at a level more sophisticated than that of a PAD.
Yaqim Martillo