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where:
a) the transmission length is as defined in 6.4.3.1.3.5; and
b) c = the smallest integer greater than or equal to the
value of the fraction.
After extending the data to an even multiple of 1992 bits, the
interleaver shall write the transmission stream into the first 249
octets of each row by taking each consecutive group of eight
bits and storing them from the first column to the 2491h. The
first bit in each group of eight bits shall be stored in the eighth
bit position: the first group of 1992 bits shall be stored in the
first row, the second group of 1992 bits in the second row, etc.
After the FEC is computed on each row, the FEC data (or
erasures) shall be stored in columns 250 through 255. The
interleaver shall then pass the data to the scrambler by reading
ont colnmn by column, skipping any octet which contains
erasures or all fill bits. All of the bits in an octet shall be
transmitted from bit 8 to bit 1.
On reception, the de-interleaver shall calculate the number of
rows and size of the last (potentially partial) row from the
length field in the header. It shall only pass valid data bytes to
the higher layer.
6.4.3.1.6 Bit scrambling. To aid clock recovery and to
stabilize the shape of the transmitted spectrum, bit scrambling
shall be applied. The pseudo noise (PN) sequence shall be a
15-stage generator (see Figure 6-12) with the characteristic
polynomial:
x15 + X + 1
The PN-sequence shall start after the frame synchronization
pattern with the initial value 1101 0010 1011 001 with thc
left-most bit in the first stage of the register as per Figure 6-1.
After processing each bit, the register shall be shifted one bit
to the right. For possible encryption in thc future this initial
value shall be programmed. The sequence shall be added
(modulo 2) to the data at the transmit side (scrambling) and to
the scrambled data at the receive side (descrambling) per
Table 6-4.
Note.- The concept of u PN scrambler is explained in the
International Radio Consultative Committee (CCIR) Report
384-3, Annex III, Section 3, Method I (see Appendix A).
6.4.3.2 CHANNELSE NSING
6.4.3.2.1 Channel busy to idle detection. When a station
receives on-channel, power of at least -87 dBm for at least 5
milliseconds. then:
a) with a likelihood of 0.9, it shall continue to consider
the channel occupied if the signal level is attenuated to
below -92 dBm for less than 1 millisecond; and
2. All figures are located at the end of this chapter.
6/11/97
No. 72
Part I Annex 10 - Aeronautical Telecommunications
b) with a likelihood of 0.9, it shhl consider the channel
unoccupied if the signal level is attenuated to below
-92 dBm for at least 1.5 milliseconds.
Note.- The maximum link throughput available to all
users is highly sensitive to the RF channel sense delay (from
the time when the channel actually changes state until a
station detects and acts on that change) and RF channel
seizure delay Cfrom the time when a station decides to transmit
until the transmitter is suficiently ramped up to lock out other
stations). Accordingly, it is imperative that all efforts are made
to reduce those times as the state-ofithe-art advances.
6.4.3.2.2 Channel idle to busy detection. With a likelihood
of at least 0.9, a station shall consider the channel
occupied within 1 millisecond after on-channel power rises to
at least -90 dBm.
6.5 LINK LAYER PROTOCOLS
AND SERVICES
6.5.1 General information
6.5.1.1 Functionality. The VHF digital link (VDL) link
layer shall provide the following sub-layer functions:
a) media access control (MAC) sub-layer, which requires
the use of the canier sense multiple access (CSMA)
algorithm;
b) a data link service (DLS) sub-layer, providing
connection-oriented point-to-point links using data link
entities (DLE) and connection-less broadcast link over
the MAC sub-layer; and
6.4.3.2.3 Recommendation.- The detection of an c) aan Vd DmLa minataniangse mDeLnEt se nbtiettyw (eVenM Eth)e, wahiriccrha fets taanbdli shthees
occupied channel should occur within 0.5 milliseconds. ground-based systems using link management entities
Note.- A higher probability of false alann is acceptable
(LME).
on the idle to busy detection than the busy to idle detection
because of the effects of the two different errors. 6.5.1.2 SERVICE
6.4.3.3 RECEIVER/TRANSM~~TER
INTERACTION
6.4.3.3.1 Receiver to transmitter turnaround time. A
station shall begin the transmission of the unique word portion
of the training sequence within 1 millisecond after terminating
the receiver function. The total frequency change during the
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