Browse Prior Art Database

V.24 Wrap-Testable Block DTR Security Mechanism

IP.com Disclosure Number: IPCOM000107589D
Original Publication Date: 1992-Mar-01
Included in the Prior Art Database: 2005-Mar-22
Document File: 3 page(s) / 114K

Publishing Venue

IBM

Related People

Bergey Jr, AL: AUTHOR [+2]

Abstract

This invention resolves two conflicting requirements encountered when wrap testing (testing by wrapping the outputs back to the inputs) the V.24 (EIA RS-232) interface on computers. It allows DTR-DSR wrap tests and block DTR security mechanisms to coexist in a CCITT V.24 implementation.

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V.24 Wrap-Testable Block DTR Security Mechanism

       This invention resolves two conflicting requirements
encountered when wrap testing (testing by wrapping the outputs back
to the inputs) the V.24 (EIA RS-232) interface on computers.  It
allows DTR-DSR wrap tests and block DTR security mechanisms to
coexist in a CCITT V.24 implementation.

      These two conflicting requirements are part of the prior art:
1.  DTR-DSR Wrap Test Requirement

      The conventional way to test Data Terminal Ready (CCITT V.24
Circuit 108/2, abbreviated DTR) and Data Set Ready (CCITT V.24
Circuit 107, abbreviated DSR) is to wrap the DTR output signal around
to the DSR input.  At one time, this was required by law in Japan.
Although not currently required by law, this wrap is preferred over
other possible ways to wrap-test these signals because of its
customary use and because of the possibility of future laws in a
various countries that may require it.
2.  Block DTR Security Requirement

      The Block DTR mechanism was developed in response to some
serious cases of "hackers" gaining illegal access to computer
systems.  It was found that these illegal accesses could have been
prevented if a mechanism that blocks DTR when DSR falls had been
implemented.

      It is possible for the current, authorized caller to be
disconnected and for another unauthorized caller to re-connect to the
same telephone line.  In some instances, the only indication that
this is occurring is that DSR falls and then rises again.  If DTR is
not blocked, the computer thinks that it is still talking to the
previous authorized user.  The new unauthorized user now has access
to the computer.

      The solution to this problem is to install a circuit that
detects any glitch (fall and rise) of DSR lasting 20 milliseconds or
longer, and block DTR whenever this glitch is found.  DTR must
continue to be blocked until the computer realizes that the current
connection must be terminated, after which the block DTR mechanism is
turned off.  The circuit to do this, essentially a latch that is set
by the fall of DSR and is reset by the computer, is part of the
prior art.
Conflict between These Two Requirements:

      The problem solved by this invention is the behavior of the
block DTR logic when DTR is wrapped to DSR in the prior art.  DSR
being off forces DTR off.  As a result, DTR cannot turn on DSR.
Whenever DTR is wrapped to DSR, the block logic prevents either
circuit from ever turning on.  Thus, in the prior art, DTR wrap to
DSR and the block DTR mechanism ca...