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From: howard@cos.UUCP
Newsgroups: comp.dcom.modems
Subject: Re: Network protocols questions
Message-ID: <188@cos.COM>
Date: Fri, 27-Mar-87 09:07:33 EST
Article-I.D.: cos.188
Posted: Fri Mar 27 09:07:33 1987
Date-Received: Sat, 28-Mar-87 12:10:23 EST
References: <12289499503.24.SY.FDC@CU20B.COLUMBIA.EDU>
Organization: Corporation for Open Systems, McLean, VA
Lines: 67

In article <12289499503.24.SY.FDC@CU20B.COLUMBIA.EDU>, SY.FDC@CU20B.COLUMBIA.EDU (Frank da Cruz) writes:
> In putting together a course on data communication protocols, I've run up
> against some questions that I can't find answers to in any books, journals,
> or magazines (like Data Communications) I've seen.  Can anybody out there help?
> Physical Layer protocols -- every network book I've seen uses CCITT X.21 as its
> only example of a physical layer protocol.  Is X.21 really used anywhere?  I
> suspect not, because it requires special interfaces (as described in X.24),
> which are probably not (yet?) widespread.  
X.21 is fairly common in Europe for providing circuit-switched
data service.  I have used its physical layer as a "simplified
digital interface" in custom building networks.
>How about X.21bis, which works with
> V-series interfaces (e.g. V.24, i.e. RS-232)?  
X.21 bis _is RS-232C, for all practical purposes.
>Are there any other well-defined
> physical layer protocols, or is access to the physical medium (connection
> establishment, maintenance, and release) usually just mushed into the datalink
> layer? 
CCITT V.35 is a common wideband interface; originally, it was intended
for microwave use, but is the common U.S. 56KBPS interface.  Other
standards include RS-449 (a generally dead 37 pin interface making some
comebacks for digitized video conferencing), and the RS-423/422 _electrical
interfaces for longer distances than RS-232.
ISDN physical interfaces also are coming
> I suppose Hayes modem language could be considered a "de-facto"
> standard physical layer protocol...(?)
I would not call Hayes language a physical layer protocol, but a routing
(i.e., OSI network or Layer 3) protocol.  Essentially, it sets up a virtual
call to the modem/network, and goes away.  The physical interface in the
Hayes context is RS-232 (unless there is a directly connected modem)

Physical interfaces, as well as MAC layer interfaces, are defined for
LAN's in the IEEE 802/ISO 8802 series.
> 
> It seems hard to believe that Bisync can actually work under noisy conditions,
> since many of the commands and responses are not error-checked.  Do sites that
> depend on Bisync abound in horror stories?  
YES.
>Also, I don't understand how
> transparent data mode is supposed to work, in light of the fact that SYNs that
> are inserted into the data by the INTERFACE (e.g. idles) are supposed to have
> DLEs stuffed in front of them, presumably by the SOFTWARE that already gave the
> data to the interface to send.  Or are there special Bisync interfaces that
> actually understand all of this...?
Bisync can work, with appropriate software, slowly but adequately until
it dies.  In practice, SYN's are rarely inserted.
> Is ANSI X3.66 (ADCCP) datalink protocol actually used anywhere, or is it really
> just a paper model for talking about its subsets (like HDLC and SDLC)?
In some military nets, yes, but HDLC and SDLC are more common.
ADCCP is the Federal standard (FED-STD-1003).
> Was ANSI X3.28 (sort of an ASCII version of Bisync) ever used anywhere?

Yes, for intelligent async terminals.  Its flow control primitives are
still used.
> For that matter, have there been any recent studies (i.e. later than the 1960s)
> of error patterns on various kinds of communication lines -- the switched phone
> network, leased voice-grade lines, "conditioned" lines, etc.  The distribution
> of errors by burst size (1 bit, 2 bits, etc), the typical bit error rates, ...
See the CCITT G.82x series.  Also, interesting work on error burst analysis,
with some public domain software, is being done by Neal Seitz and associates
at the Institute for Telecommunications Sciences (U.S. Commerce Dept,
Boulder, CO)--(303) 497-3106.

Howard Berkowitz
(703) 883-2812 (voice)

Howard@cos ... via sundc, hadron