🚀 go-pugleaf

In article <12656004461.12.BILLW@mathom.cisco.com> BILLW@MATHOM.CISCO.COM (William "Chops" Westfield) writes:
>Aren't X.21 and X.25 vastly different technologies?  circuit switched
>vs packet switched?



It's really a worthwhile point, which I think is practical rather
than pedantic, to remember that X.21 and X.25, respectively,
are interfaces to networks rather than network specifications.

X.21 specifies a circuit-switching interface to a network, which
really could be packet-switching internally.  X.25 specifies a
packet-switching interface to a network which could be an X.25-like
virtual call connection-oriented network, or DECnet, or SNA, or
a LAN...

To confuse things further, X.25 cites X.21 as is physical interface
specification.  Really, what is meant by this is the Level 1 (physical)
part of X.21, which specifies a 15-pin connector and pin assignments.

At the next level of confusion, very few people, certainly in North
America, use the X.21 connector specification (the CONNECTOR, but
not the logical pin definitions, IS used for T1 and other applications).
Instead, people use X.21 _bis_, which is more or less RS-232C.
[Actually, X.21 bis points to the physical connector specification in
V.24].  RS-232C, V.24, and X.21 bis will interoperate in practice if
one is careful, and sensitive to issues such as the screw thread
on the locking connector (as opposed to the compatible pins).  The
EIA standard specifies an English 4-40 screw thread, while the 
international specs specify a metric screw thread.  At COS, we
finessed this in our testing and stack specifications by referring
to X.21 bis, but asking the implementor to state whether he was
using English or metric threads.
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