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not power limited. Alternative applications could include the
use of vocoders that include their own coding, protecting the
important bits more than the others; this could eliminate the
need for coding in the channel modem.
4.5.2 C channel frame. The frame duration at all channel
rates is 500 milliseconds. Carrier activation (burst mode) based
on speech activity is used in the to-aircraft direction. At each
activation, a preamble and unique word are transmitted to
commence the burst, and from then on further unique words
occur every 500 milliseconds. Thus the phasing of the unique
word depends on the instant when activation commences. A
postamble is sent when there is no voice content in the
interleaver block.
4.5.3 Voice activation. In the to-aircraft direction, carrier
activation is controlled by an electrical signal at the C channel
interface. When the C channel is used for voice, this signal is
controlled by the voice encoder. The voice encoder turns the
signal on as soon as it detects voice, but appIies a hangover
time before turning the signal off to avoid excessive tm-off$
between syllables. In addition, the forward carrier is activated
by sub-band channel signalling as required.
4.5.3.1 The channel unit starts a new burst immediately
after the "active" state is signaHed. When the "not active" state
is signalled, the channel unit continues transmission for a
period and then drops the carrier. In the case of channels with
FEC and an interleaver, the channel unit completes transmission
of the current interleaver block, plus another complete
interleaver block, which ends with a postamble to terminate
the C channel carrier bust. In the case of channels with no
FEC or interleaver, the channel unit continues transmission for
20 milliseconds. For both cases, during this period the required
bits are taken from the primary channel interface as usual.
4.5.3.2 The same timing rules apply to the corresponding
control signal from the sub-band signal1ing channel
equipment.
4.5.3.3 In the from-aircraft direction, the carrier is
transmitted continuously during the call regardless of speech
activity. The start of the transmission is the same as for the
forward direction, with a preamble and unique word transmitted
at the beginning, followed by further unique words at
500 millisecond intervals.
4.5.4 Data activation. The capability to use the
C channel for data other than vacoded speech is not required
for safety applications. However, the C channel may be used
for the transmission of data by non-safety services and some
details of its operation are included here. In the to-aircraft
direction the forward carrier is operated in burst mode for
circuit-mode data transfer. Circuit 109 (CF) as defined in
CCIT'T Recommendation V.24 (termed DCD in EL4
RS-232C), from the GES voice band modern controls the
activation interface of the C channel. The interface is initially
activated when the call enters data mode, which is indicated
by the circuit 109 transition to ON state. The interface
normally remains activated until deactivation of data mode by
the AES. If the circuit 109 changes to the OFF state indicating
Ioss of received carrier from the far-end modem, the transmit
circuit-mode interface unit finishes sending the contents of the
plesiochronous buffer and then changes to "not active". The
C channel activation interface is reactivated again when (if)
the circuit 109 changes to the ON state.
4.5.5 Interleaver size. In any data transmission system
where interleaving is required to randomize the errors, the
interleaver size is always a trade-off between the delay
incurred and its effectiveness at randomizing the errors. The
interleaver blocksize for the 21 kbitsls C channel is 384 bits,
which corresponds to 192 information bits. This 192 bits is the
frame s i ,(~20 milliseconds) of most 9.6 kbitsls vocoders. This
direct mapping between vocoder frames and interleaver buffers
minimizes the delay caused by interleaving and its effect on
voice communications. The resulting over-all transmission
delay is around 30 milliseconds (rising to 50 milliseconds at
the lowest channel rate). To match the number of interleaver
bits to the required number of channel bits, a small number of
"dummy" bits are included in the frame just after the framing
bits.
4.5.5.1 The delay at the transmitter arising from the
interleaver depends on the relationship between the voice
frame and the interleaver size. The full de-interleaver delay of
around 20 milliseconds will be experienced at the receiving
　
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