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 MIL-HDBK-232A
5.8.3.3 Handsets. In cases where a hold feature is provided, either through
the use of a multiline telephone set or a separate line selector key strip,
no special handset is necessary. When a hold feature is not available, a
security handset such as the WECO G-10A, G-10B, G-10F, or equivalent, is
essential for audio protection in those situations where the instrument is
left off-hook and unattended for short periods of time while the user obtains
information, files, etc.
5.8.3.4 Signal. Since most standard telephone ringers have microphonic
characteristics, the signaling of incoming calls within sensitive discussion
areas should be accomplished in accordance with one of the following options:
a.
In facilities where the KSU is installed within the secure perimeter,
no special signaling apparatus is required if the KSU includes a local
ring generator and is wired for common audible signaling. Any ringer
or buzzer may be used. This type of installation requires a backup
power source if telephone service is required during commercial power
outages.
b.
In facilities where KSUs are not used or are not installed in the
secure area, the ringer or buzzer should be modified with an approved
isolator kit.
5.8.4 Single-line service. The trend within DoD is to provide single-line
service. With this type of service, each user or subscriber is provided a
dedicated line and instrument. Service may include special functions such as
call forwarding, call transfer, and conferencing. When such systems are
utilized within a CAA, it is recommended that any special function capability
be limited to that area, since extension of service may be detrimental to the
TEMPEST integrity of the facility. All considerations are to be made in
light of operational impact.
5.8.5 Electronic private automatic branch exchange (EPABX). The EPABX is
emerging as the telephone system of the future. An EPABX may be designed to
serve as few as 50 subscribers or may function as a dial central office
serving a specific community of interest with trunking capability to other
exchanges or local offices. The Telephone Security Panel, in a report to the
community, "Computerized Telephone Systems", 30 June 1983 (see appendix D),
issued instructions and standards for implementing computerized branch
exchanges in areas where classified discussions take place. Those standards,
coupled with good RED/BLACK engineering practices, assure a low-risk
installation. It is imperative that systems/devices used in an area
requiring RED/BLACK consideration are provided adequate filtering and
isolation from other equipment or systems.
5.8.6 Key distribution systems. A key distribution system permits a greater
number of users to share limited line capacity. An example of this is an
office with 20 employees and 4 telephone lines. Each employee may have
access to all lines, plus intercom capability. It is recommended that all
instruments coupled to a key distribution system be limited to areas within
the CAA. Installation criteria are as follows:
a.
The KSU must be of U.S. manufacture and installed in accordance with
paragraph 5.7.3.
b.
The KSU should use high-security key telephone unit (KTU) line cards.
c.
All instruments served by the KSU must be within the LEA.
d.
The equipment must be installed and maintained by cleared U.S.
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