Subject: Proposed Implementation Agreement for X.25 Connection Networks

From: "jim (j.) cobban" <[email-masked]>
Date: Tue, 8 Mar 1994 11:47:00 +0000

In order to implement a connection network there must be an agreement on the
format in which DLC addresses appear in the TG Descriptor control vector
(X'46').  Although there are unresolved concerns with regard to the
implementation of connection networks over transports which do not have a
consistent set of metrics, I would like to start the ball rolling by
proposing the following format for DLC address on X.25, ISDN, frame-relay,
and native ATM (as opposed to 802.x LAN simulation) subnetworks:

	   +-----------------------+-----------------------+
   byte 0  |              length of address = n            |
	   +-----------------------+-----------------------+
   byte 1  |     type of address   |   numbering plan      |
	   +-----------------------+-----------------------+
   byte 2  |                   address[0]                  |
	   +-----------------------+-----------------------+
	...
	   +-----------------------+-----------------------+
   byte n  |                  address[n-2]                 |
	   +-----------------------+-----------------------+

This is based upon the address format in the Q.931 (ISDN signalling)
specification:

   length of address  - the number of bytes (octets in ITU-TS speech) of
			address information which follows.  This may seem
			redundant, since the address is contained inside a
			DLC Signalling subfield, which has a length.  However
			it may prove necessary to include additional routing
			information in the DLC signalling subfield after the
			address itself.  Both X.25 and Q.931 include the
			length field as part of the address.

   type of address    - 0000 - unknown
			0001 - international, the address begins with the
			       country code
			0010 - national, the address excludes the country
			       code
			0011 - network specific, for example operator
			       information number
			0100 - subscriber number
			0110 - abbreviated number
			x111 - extension

   numbering plan     - 0000 - unknown
			0001 - ISDN E.164 (E.163)
			0011 - PSPDN X.121
			0100 - Telex F.69
			1000 - national standard
			1001 - private numbering plan
			1111 - extension

   address[i]	      - IA5 (7 bit ASCII) character with 0 parity.

Q.931 encoding, rather than the X.25 encoding which is identical except that
each address "digit" is only 4 bits long, is chosen because it permits
representing any valid X.25 (actually X.121) address, but is also the basis
of the representation of addresses in frame-relay (technically the frame
mode bearer service of ISDN where classical ISDN describes the circuit
and packet mode bearer services), and ATM (which was called broadband ISDN
until the promoters realized that was the kiss of death).