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20-1
INTRODUCTION
The Project 25 features extend the standard
functions of the R2670 to provide an all-in-one
servicing instrument for Project 25 digital radio
systems.
20-2
BASIC OPERATION
The R2670 can emulate the same digital
environment used by Project 25 radios in clear
and secure mode. This capability allows the
analyzer to interoperate with conventional Project
25 radios. Standard test equipment is unable to
decrypt secure audio transmissions from Project
25 radios because the audio signals are converted
from their native analog format to a digital,
ciphered representation prior to transmission.
A Project 25 digital radio provides for both clear
and secure voice and data. When operated in
secure mode, the radio encrypts communications
prior to their transmission. Only intended
subscribers, with compatible hardware plus
matching security keys, can decrypt the message.
In this way, users of a system can be partitioned
into groups, each with their own security key.
The R2670 will accept an external operating key
from any compatible keyloader.
In its Project 25 mode, the R2670 provides
display screens for monitoring bit error rate
characteristics of a radio under test and also
generates bit error rate test patterns to support
this mode of operation.
The R2670 provides a clear scope display of
audio signals, either the input to the vocoder
(generate mode) or the output from the vocoder
(monitor mode).
20-3
TEST CONSIDERATIONS
Project 25 digital radio systems encrypted
communications have a need to maintain even
higher standards of RF signal quality than normal
FM radio systems. Encryption subjects the
Project 25 signals to additional processing. In
Section 20
DESCRIPTION
order to work reliably, the radios must be tuned
to preserve critical factors such as operating
frequency response and transmitter deviation.
Operating in secure or standard mode, the R2670
bundles a wide assortment of sophisticated test
features in a single instrument.
20-4
SPECIAL TERMS
The following list contains brief definitions of
special terms that are used in association with
Project 25 radios and test equipment:
algorithm - In secure systems, an algorithm is a
distinct method for translating clear information
(input) to a secure version of the information
(output). The same algorithm is used to interpret
the message. Project 25 systems use the
following algorithm:
DES-OFB
DES-XL
DVP-XL
DVI-XL
BER test - In generate mode, the analyzer
modulates the RF carrier with Bit Error Rate test
pattern to test the Unit Under Test receiver. The
test pattern received by the Unit Under Test will
be compared in the unit under test with a stored
version of the test pattern to compute a BER.
This will only be possible if the Unit Under Test
has this test mode capability.
In monitor mode, the analyzer monitors the
received 1011 Hz tone test pattern, compares it
with a stored version, and provides a metering
display of bits received, number of bits expected,
and bit error rate percentage.
cipher - to convert information to a seemingly
random pattern for transmission or wireline
delivery in order to protect sensitive information.
Ciphered voice and data can only be understood
by means of a special key.
255
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