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From: sirbu@GAUSS.ECE.CMU.EDU.UUCP
Newsgroups: comp.dcom.modems
Subject: Re:  Network protocols questions
Message-ID: <8703271618.AA26365@gauss.ECE.CMU.EDU>
Date: Fri, 27-Mar-87 11:18:15 EST
Article-I.D.: gauss.8703271618.AA26365
Posted: Fri Mar 27 11:18:15 1987
Date-Received: Sat, 28-Mar-87 14:11:08 EST
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I'll try to answer a few of your questions.




	Is X.21 really used anywhere?  

X.21 is used all over Europe for connections to X.25 networks.  I think (not
sure) X.21 bis is used in the U.S.



  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?  

The ISDN physical layer interfaces are fairly well defined.  Look at I.430

  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?  

Re:  Forward error correction -- many of the companies offering private
satellite links use it (e.g. American Satellite, ).  It is used in space
probes.  For a good article that I have used in a course see: Edelson, R.E.
et. al.  "Voyager Telecommunications:  The Broadcast from Jupiter"Science
vol 204, No. 1, June 1979 pp. 913-921.


  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, ...  

There was a 1983 Loop Survey which was reported recently in either IEEE TOC
or JSAC.


  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?  

Regarding 802.2.  I believe the IBM token ring network implementation
uses Type 2.