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 MIL-HDBK-232A
all possibility that on-hook audio will be transmitted out of the PCZ on the
telephone wires. Properly designed and constructed station equipment can be
type-accepted by the community as intrinsically fulfilling the
isolation/disconnect requirement without additional support.
CBX configurations incorporating the accepted isolation/disconnect measures
need not be subject to any installation or operational restrictions unless
consideration is being given to concerns other than on-hook audio.
Isolation of the station equipment from all uncontrolled lines forms the
basis for the community's program to prevent an on-hook audio compromise of
national security information. The use of specialized isolation/disconnect
devices or station equipment to achieve this isolation, however, for large
CBX installations often involves unmanageable economic, logistic, and/or
operational burdens and becomes impractical.
The Telephone Security Panel has developed a set of installation standards,
conformance to which will permit a CBX system to be used without the special
isolation/disconnect or station equipment normally required, These standards,
which are listed below, are predicated upon having located the CBX within the
PCZ so that the CBX may be used as the means of isolation. The entire local
telephone system is strictly organized to assure that no means is provided
for on-hook audio to be present on any external lines. This approach also
provides the opportunity to deny external access both to internal calls and
to call detail information for the individual subscriber stations: benefits
not available with CENTREX or uncontrolled CBX systems using conventional
isolation methods.
TSP GUIDELINES FOR COMPUTERIZED TELEPHONE SYSTEMS
I.
Minimum standards for CBX on-hook audio protection.
1.
Physical security measures: A physical control zone (PCZ) with
appropriate physical security is required over the entire area of
concern.
1.1 The CBX is located in the PCZ. If the CBX supports station equipment
located in a nonconterminous PCZ, then those stations are not
protected by the isolation provided by the CBX installation and they
must be provided with special isolation/disconnect devices or station
equipment.
1.2
If the PBX supports any nonapproved or unprotected station equipment
within the area of concern, all lines, intermediate wiring frames,
and distributed CBX equipment modules (to include voice and data
links) associated with these stations are contained within the PCZ.
1.3
All program media (tapes, disks, etc.) are provided positive
physical protection against unauthorized alteration. A certifiably
correct master program is always maintained under secure conditions
to be available as a check of the operating program and a means of
removing possible or identified software security deficiencies.
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