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254 Chapter 8
C:\Spk\Ref\RefGuideRevE\Enc-dec.fm
Signaling Encoder and Decoder - Screens and Control Fields
AMPS-TACS, NAMPS-NTACS Encoder and Decoder
AMPS-TACS, NAMPS-NTACS Decoder
The AMPS-TACS, NAMPS-NTACS decoder acts like a base station
receiver by analyzing Reverse Control Channel (RECC) and Reverse
Voice Channel (RVC) message streams for various cellular telephone
formats.
The decoder can also be used to analyze Forward Control Channel
(FOCC) and Forward Voice Channel (FVC) data from the base station.
Decoder Mode Differences
The AMPS-TACS and NAMPS-NTACS decoder modes are essentially
the same for analyzing reverse control channel (RECC) information.
However, the voice channel (RVC) information for NAMPS-NTACS is
displayed differently than AMPS-TACS information. Fields and decoder
measurements that are only used for either mode are noted in their
descriptions.
Interpreting Decoded Data
RECC and AMPS-TACS RVC Measurements
See “NAMPS-NTACS Reverse Voice Channel Measurements” on page
256 for information about NAMPS-NTACS RVC measurements.
After being armed, the measurement begins when the last bit of Word
Sync has been received. The Gate Time field specifies the measurement
period for all RECC measurements and the AMPS-TACS RVC
measurement.
The received bits are displayed as hexadecimal (hex) characters. If the
last bits received do not complete the last hex character, the received
bits are used as the most significant bits in the character, and the
remaining bit positions are filled with zeros.
For example; if the last bits received are “01”, two zeros are added to
the right to produce the binary number 0100. The hexadecimal
equivalent, 4, is displayed.
The first two hex characters of the RECC data displayed contain the
7-bit Digital Color Code of the Seizure Precursor. The characters are
right-justified so the farthest bit to the left for the first hex character is
always 0. The first word of the RECC message begins in the third hex
character of the displayed data.
All bits of the RECC and RVC data streams received after the initial
Word Sync are displayed, including Parity and additional Dotting and
Word Sync sequences.
The decoder does not check for any errors in the received data stream.