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From: SY.FDC@CU20B.COLUMBIA.EDU.UUCP
Newsgroups: comp.dcom.modems
Subject: Network protocols questions
Message-ID: <12289499503.24.SY.FDC@CU20B.COLUMBIA.EDU>
Date: Thu, 26-Mar-87 12:17:01 EST
Article-I.D.: CU20B.12289499503.24.SY.FDC
Posted: Thu Mar 26 12:17:01 1987
Date-Received: Sat, 28-Mar-87 05:13:50 EST
Sender: daemon@ucbvax.BERKELEY.EDU
Distribution: world
Organization: The ARPA Internet
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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.  How about X.21bis, which works with
V-series interfaces (e.g. V.24, i.e. RS-232)?  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?  I suppose Hayes modem language could be considered a "de-facto"
standard physical layer protocol...(?)

Forward Error Correction.  This is talked about a lot ("when you have long
delays, then you can use FEC..."), but what networks or datalink protocols
actually use it?  What form of it do they use -- Hamming code, or what?  How
many bit errors per character can they detect and correct?  What's the
per-character overhead?  Since these codes can typically correct only one bit
error per byte, there must also be allowance for retransmission.  Do they also
use a block check in addition to per-character FEC?  etc etc.  Where can I read
about this?

Does anybody know what kind of protocols and error control techniques are used
to send back data from space (like from Uranus and Neptune), where corruption
is very likely, and retransmission is very expensive?

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?  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...?

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)?

Was ANSI X3.28 (sort of an ASCII version of Bisync) ever used anywhere?

Outside of IBM land, and in peer (host-to-host) networks, does anyone use the
unbalanced versions of the HDLC-like datalink protocols?

Has anyone really analyzed checksums to the same degree as CRCs have been
anaylized?  The certainty with which they can catch different kinds of errors,
the probability that they will let various kinds of errors go undetected, etc.,
and their effectiveness compared with CRCs under real conditions.

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, ...

IEEE 802.2, Logical Link Control for LANs, describes two datalink options:
Type 1 (connectionless, no acknowledgement or error control), and Type 2
(a nearly full-blown balanced HDLC-like affair).  Does anyone use Type 2 in
real life?

Enough for now... (as you can see, I haven't got past the lower two layers
yet).  Thanks in advance to anyone who's willing to provide pointers!  - Frank
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