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From: heath@ncrcae.UUCP
Newsgroups: comp.dcom.modems
Subject: Re: Network protocols questions
Message-ID: <2382@ncrcae.Columbia.NCR.COM>
Date: Mon, 30-Mar-87 10:44:39 EST
Article-I.D.: ncrcae.2382
Posted: Mon Mar 30 10:44:39 1987
Date-Received: Wed, 1-Apr-87 01:14:59 EST
References: <12289499503.24.SY.FDC@CU20B.COLUMBIA.EDU>
Reply-To: heath@ncrcae.UUCP (Robert Heath)
Distribution: world
Organization: NCR Corp., Engineering & Manufacturing - Columbia, SC
Lines: 35


1) physical layer -- X.21 is used in the Nordic countries in a network
   call NordicNet.  As for other physical layer interfaces, don't 
   forget RS-422, RS-449, TTY current loop, your LAN interfaces 
   (e.g. IEEE 802.3), optic fiber, etc.

2) forward error correction -- I don't know of any commercial data 
   communications which use forward error correction.

3) bisync -- bisync does have theoretical problems in extremely noisy 
   environments, (these were corrected under its successors HDLC and ADCCP) 
   but it works very well in the workaday world.
   As for stuffing DLE-SYN during bisync transparency, the DLE-SYN's are never
   inserted by the layer above bisync. They are inserted at the data link
   control layer itself, presumably by the same logic which affixes the 
   STX and ETX. The sending bisync layer also parses the outgoing data
   for codes which coincidentally match the DLE character. On finding a DLE,
   it inserts an extra DLE. The receiver disassembles this sequence into 
   only one. This mechanism is widely-used in industry-standard 2780/3780 
   bisync. Nowadays you can buy chips which will do this for you.

4) ADCCP -- as a former contributor to ANSI X3.66, I feel that it is a paper
   model for talking about its subsets HDLC and SDLC. Like many standards,
   it's written too loosely to be implemented as a whole, but does provide a
   mechanism for classifying its subsets.

5) ANSI X3.28 -- properly known as Basic Mode, is implemented in many of 
   NCR's cash registers, teller machines, and terminals.

6) Unbalanced HDLC -- NCR uses a proprietary form of HDLC known as NCR/DLC
   between its POS terminals, teller machines, and terminals.

7) 802.2 Class 2 -- Though it's not popular now, just wait. 
   The full-blown balanced HDLC-like class will become more popular 
   under standards groups working with OSI.