| Document: FSC-0075
  | Version:  001
  | Date:     23rd October 1993
  | Author:   Jan Ceuleers

				   ISDN capability flags in the Nodelist
								 A proposal
						 by Jan Ceuleers, 2:292/857
					   version 0.4, October 3rd 1993


	 1 Introduction

	 The  Integrated Services Digital  Network is a  worldwide overlay
	 network, offering the same services  as the PSTN (Public Switched
	 Telephone Network) and  more. Its	basic bearer  capability is  a
	 digital  bit stream  of 64000	bits/s,  as opposed  to the  audio
	 channel with a 3.1 kHz bandwidth provided by the PSTN.

	 Transferring data across the ISDN can be done in one of two ways:

	   -  by using	the telephony services the ISDN  provides. In this
		  mode,  a standard  modem	can  continue  to  be  used.  Some
		  modulation schemes currently in use are V.32bis, PEP, ZyXEL,
		  HST,	V.32  etc. We  already have  nodelist flags  for these
		  cases.

	   -  by using	ISDN's 'native'  mode. In this  case, a  number of
		  protocols  (either or not  ISDN-specific) are used,  such as
		  V.110, V.120, X.75 etc.

	 This  document  aims to  define  the way  in  which nodes	are to
	 advertise their ISDN  capabilities in the nodelist,  irrespective
	 of whether or	not ISDN-only nodes should	be in the  nodelist in
	 the  first place.	This latter  problem is  to be	solved by  the
	 politicians.

	 Descriptions of ISDN equipment and compatibility with POTS (Plain
	 Old  Telephone Service) equipment	are beyond	the scope  of this
	 document. More detailed information on ISDN is also not provided;
	 readers  are referred to the literature (e.g. 'Computer Networks'
	 by Andrew Tanenbaum).



	 2 Different flavors of the same protocol

	 Some ISDN protocols have different flavors, some  of which differ
	 only marginally, but others are quite distinct.

		  For  the techies among the readership: examples of both
		  cases can be	found in  the 1988	definition of  V.110.
		  There  are two variants of the  frame structure used in
		  the  56kbps  synchronous	mode  (marginal  difference),
		  while there  is quite  a major  difference between  the
		  synchronous and asynchronous versions of V.110.


				   ISDN capability flags in the nodelist


	 Since FidoNet	applications are essentially  character-based, the
	 asynchronous versions	of protocols  will be  preferred over  the
	 synchronous-ones(1).  This  applies  to V.110	and  V.120	and to
	 any other such protocol.

	 If there is an option, 8 data bits, no parity and 1 stop bit will
	 be used in preference over all other possible combinations. (This
	 is in line with the FOSSIL spec).


	 3 Speeds

	 Some protocols (such  as V.110) can be used  at different speeds.
	 Certain implementations of  these protocols may not  support some
	 of these speeds.

	 The  baud	rate field	in	the nodelist  should not  be  used for
	 indicating the  maximum speed an  ISDN node is capable  of, since
	 ISDN capability  flags could  (technically) co-exist with	normal
	 modem capability flags(2).


	 4 Nodelist flags

	 ISDN-related  nodelist  flags	consist of	a  prefix,	a protocol
	 indicator and an optional (set of) suffixes.

	 The prefix is the capital letter I (for ISDN).

	 The protocol indicator is one of the strings defined in paragraph
	 5 below.

	 The suffix indicates the way in which the implementation deviates
	 from the preferred  implementation, as indicated in  paragraphs 2
	 and 3. The possible suffixes are:

		  Onnn		The  only bit  rate supported  =  nnn *  100 (e.g.
					IV110O384 means that this node only supports V.110
					asynchronous at 38400 bps and at no other speeds.




		  1.  As  a  matter  of  fact, there  are  no  provisions  for
	 advertising the synchronous versions of such protocols.

		  2. ISDN technology indeed allows for the possibility of both
	 modem	 and  ISDN-specific  datacom   connectivity  on  the  same
	 'telephone' number.


				   ISDN capability flags in the nodelist


		  Pnnnnn	Maximum  packet size supported in bytes. This is a
					layer 2  packet protocol  parameter. Communication
					between two nodes is only possible if either end's
					maximum  packet  size  is  not	exceeded.  Leading
					zeroes are to be omitted.

		  Rnnn		Highest  bit  rate	supported =  nnn  *  100 (e.g.
					IV110R192 means  that this node  does not  support
					V.110 asynchronous at 38400  bps, but does support
					all other standardised	rates up to  and including
					19200 bps)

		  Wn		Window size. The window size must be less than the
					modulo value (i.e. in modulo 8, the maximum window
					size is 7).

	 If more than one  suffix is used, the suffixes will  be sorted in
	 ascending order.


	 5 Protocols

	 This section defines the meaning of the base protocol indicators.
	 The  aim  is to  have	this  base	protocol indicator	cover  the
	 majority of cases, so that suffixes will only rarely be required.


	 5.1 V.110

		  The  protocol  indicator	is V110.  When	specified  without
		  suffixes,   the	IV110	nodelist  flag	 indicates	 V.110
		  asynchronous capability  at bit  rates up  to and  including
		  38400 bps.


	 5.2 V.120

		  The protocol	indicator  is  V120.  When	specified  without
		  suffixes,  the   IV120   nodelist   flag	 indicates	 V.120
		  asynchronous	capability. Due to the nature of the protocol,
		  the O and R suffixes are irrelevant.

		  There  is no	explicit mention  of frame	size in  the V.120
		  specifications. However, since Q.921 is the layer-2 protocol
		  of V.120, one might assume the frame size of Q.921, which is
		  260 bytes. Frame sizes larger than that should be negotiated
		  between sysops.


				   ISDN capability flags in the nodelist


	 5.3 T.90 (X.75)

		  The protocol indicator is T90. Base protocol parameters are:

			   modulo: 8
			   window size: 2
			   packet size: 2048 bytes

		  Currently, there	is no standardized method  for negotiation
		  of the modulo mode (Recommendation ITU-TS T.90 reserves this
		  subject for further  study), all	T.90-capable nodes	should
		  answer   in  modulo-8  mode.	It	is	therefore  useless	to
		  advertise  modulo-128  capability.   This  also  limits  the
		  maximum window size to 7.

		  Some	implementations have  a maximum  frame	length of  130
		  bytes  and  a  maximum  window  size of  1.  This  would	be
		  documented  as  IT90P130W1.  The 1992  version  of  the T.90
		  standard specifies a method for in-band negotiation of frame
		  length and window size.


	 5.4 Other protocols

		  Additional protocols can be added to this document (and thus
		  assigned	 a	 nodelist	flag)	if	sufficient	 technical
		  information is made available.

		  Neither X.25 on B nor on D have been added, because there is
		  no room in the nodelist for the X.25 address.


				   ISDN capability flags in the nodelist


	 6 Conversion from old to new

	 The ISDNA, ISDNB and  ISDNC nodelist flags are already  in use in
	 zone 2.  The table below  shows the relationship between  old and
	 new.


						  old				   new
						 ISDNA				IV110O192

						 ISDNB				IV110O384

						 ISDNC				   IT90
					  ISDNA,ISDNB			  IV110

					  ISDNB,ISDNC		  IV110O384,IT90



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