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monitoring it. The functional blocks at the GES end of each
P channel are as follows:
a) data scrambler;
b) forward error correction (FEC) encoder;
c) interleaver;
d) timing mark inserter (unique word); and
e) modulator.
The functional blocks at the receive end of each P channel are
complementary to those at the transmit end. The complete
series of functional blocks from transmit end to receive end is
shown in Figure A-1 l of this guidance material.
4.2.2 FUNC~IONAL BLOCKS
4.2.2.1 Data scrambler: A data scrambler is a logical
device which multiplies .the data sequence by a known pseudorandom
sequence. The data is unscrambled at the receiver by
multiplying by the same sequence of random bits. This has no
direct effect on the bit error rate performance on the link.
However, it prevents the possibility of transmitting long
sequences of 1's or 0's over the link. The latter could be
detrimenta1 to the performance of acquisition and tracking
circuiuy.
4.2.2.2 Forward error cormction coding. Satellite communications
are, in general, power-limited due to the limited
resources at the satellite. One can reduce the amount of power
required for communications by introducing forward error
correction coding, which adds redundancy to the transmitted
signal at the expense of requiring more bandwidth. At the
receiver, a decoder uses the redundancy to correct those errors
which normally occur when transmitting at a lower power.
4.2.2.3 Interleaving. Mobile communications are
generally subject to fading due to reflections from nearby
objects. The fading is correlated with time and when it occurs,
can cause a sequence of correlated errors in the data detected
at the receiver. These errors can be corrected by the forward
error correction coding, However, most decoders work best
with uncorrelated errors. The purpose of the (transmit) interleaver
and its inverse, the (receive) de-interleaver, is to
randomize the order of bits presented to the channel compared
to those presented to the decoder, thus reducing any mrrelation
between the errors which may be caused by the channel.
4.2.2.4 fiming mark. A unique word is inserted in each
data stream, at periodic intervals, to provide timing information
to the receiver. This timing information is needed in order
to properly synchronize the de-scrambler, the decoder, and the
de-interleaver with their counterparts on the transmit side. It
also alIows recovery of this synchronization if the signal
happens to be lost momentarily.
4.2.3 P channel frame duration. The frame duration is
either 500 milliseconds (channel rates of 2.4 kbits/s and
higher) or a multiple of 500 milliseconds to provide simple
derivation of a superframe used for R channel slotting and the
reservation TDMA T channel. Scrambling and FEC coding of
rate 112 is used on all P channels. The FEC encoder is not
reset between frames, but starts a new frame in the state
resulting from the previous frame. With the exception of the
0.6 kbitsls P channel, the duration of the interleaver is 500
milliseconds. For the 0.6 kbits/s, the interleaver is 384 bits
corresponding to 2t3 second.
4.2.4 P channel frame f o m t . All P channel frame
formats include a 16-bit field (uncoded) as a format identifier
and for derivation of the superframe that has a duration of
8 seconds. The format identifier is a 4-bit field and is always
set to the value 0001 when the channel is a P channel. The
remaining 12 bits of this field are used as a superframe
counter. To achieve this, the field is subdivided into three 4-bit
fields, of which the first is used to indicate the start of a new
superframe, while the remaining two are used to indicate the
frame number. It is advisable for AES implementations to
make use of all 12 bits to achieve reliable superframe
synchronization in the presence of bit errors.
ALtaehment A to Part I Annex I0 - Aemnarrftrrftcul TelecommunicaCions
4.2.5 For P channels of 4.8 and 10.5 kbitds, a small
number of "dummy" bits are included in the frame just after
the framing bits. This matches the number of bits of the interleaver
to the number of bits required to be transmitted on the
channel.
4.2.6 To facilitate the re-synchronization of an AES
transferring from one P channel to another on the same GES,
all P channels transmitted by the same GES are synchronized.
the appropriate channel rates). These channels are shared with
a number of other AESs on a demand-assigned basis. That is,
these channels are assigned on a conflict-free basis to individual
AESs by the GES for short periods of time. The requests
　
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