Subject: Proposed Implementation Agreement for X.25 Connection Networks

From: "jim (j.) cobban" <[email-masked]>
Date: Tue, 8 Mar 1994 16:15:00 +0000

As a result of a verbal communication from Dennis Wind of IBM I include
conventional analog dial among the networks which can use this addressing
format.  I also noticed an error in the format, the high order bit of byte 1
is always 1, as in Q.931 to indicate that this is the last byte of the prefix
area of the address.

  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 public switched packet data
| networks (PSPDN, X.25), public switched telephone networks (PSTN),
| integrated services data networks (ISDN), frame-relay, and native ATM (as
| opposed to 802.x LAN simulation) subnetworks:
  
  	     +-----------------------+-----------------------+
     byte 0  |              length of address = n            |
  	     +--+--------------------+-----------------------+
|    byte 1  | 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    - 000 - unknown
| 			  001 - international, the address begins with the
  			        country code
| 			  010 - national, the address excludes the country
  			        code
| 			  011 - network specific, for example operator
  			        information number
| 			  100 - subscriber number
| 			  110 - abbreviated number
| 			  111 - 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).  In addition since
| E.164, the addressing scheme of ISDN, is a superset of E.163, the addressing
| scheme of conventional analog dial networks prior to 1995, this proposal also
| includes support of connection networks which use the public switched
| telephone network (PSTN).

---

From: [email-masked] (David Matusow)
Date: Wed, 9 Mar 94 10:43:58 -0500

Well done, Jim!  This looks like a good move forward for generalized 
connectivity and DLC signaling.  The use of the full Q.931 (length 
redundant) provides those of us that already have to support this form a 
simple parsing that is generalized for many types of access.

David