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transmission of the unique word shall be less than 10 Hz.
After transmission of the unique word, the phase acceleration
shall be less than 500 Hz per second.
6.4.3.3.2 Transmitter to receiver turnaround time. The
transmitted power level shall decay at least by 20 dB within
0.3 milliseconds after completing a transmission. A station
shall be capable of receiving and demodulating with nominal
performance, an incoming signal within 1.5 milliseconds after
transmission of the final information symbol.
6.4.4 Physical layer system
parameters
6.4.4.1 The physical layer shall implement the system
parameters as defined in Table 6-5.
6.4.4.1.1 Parameter P1 (minimum transmission length).
Parameter PI defines the minimum transmission length that a
receiver shall be capable of demodulating without degradation
of BER.
6.5.1.2.1 Connection-oriented. The VDL link layer shall
provide a reliable point-to-point service using a connectionoriented
DLS sub-layer.
6.5.1.2.2 Connection-less. The VDL link layer shall
provide an unacknowledged broadcast service using a
connection-less DLS sub-layer.
6.5.2 MAC sub-layer
6.5.2.1 The MAC sub-layer shall provide for the transparent
acquisition of the shared communications path. It makes
invisible to the DLS sub-layer the way in which supporting
communications resources are utilized to achieve this.
Note.- The service specification for the MAC sub-layer is
modeled on the MAC Service Dejinition (IS0 DP 10039).
6.5.2.2 MAC SERVICES
6.5.2.2.1 Multiple access. The MAC sub-layer shall
implement a non-adaptive p-persistent CSMA algorithm to
equitably allow all stations the opportunity to transmit while
maximizing system throughput, minimizing transit delays, and
minimizing collisions.
6.5.2.2.2 Channel congestion. The MAC sub-layer shall
notify the VME sub-layer whenever channel congestion is
detected (see 6.5.2.3.2).
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No. 72
Annex 10 - Aeronautical Xelecommurtications Volume ZZZ
6.5.2.3 MAC service system parameters. The MAC service
shall implement the system parameters defined in
Table 6-6.
6.5.2.3.1 Timer TMI (inter-access delay timer). Timer
TM1 shall be set to the time (TM1)th at a MAC sub-layer will
wait between attempts to access the channel. This timer shall
be started if it is not already running and the channel is idle
after an unsuccessful access attempt. The timer shall be
cancelled if the channel becomes busy. When the timer expires
another access attempt shall be made.
6.5.2.3.2 Timer TM2 (channel busy timer). Timer TM2
shall be set to the maximum time (TM2) that a MAC sub-layer
will wait after receiving a request to transmit. This timer shall
be started if it is not already running, when the MAC sublayer
receives a request for transmission. The timer shall be
cmcelled upon a successful access attempt. When the timer
expires, the VME shall be informed that the channel is
congested.
6.5.2.3.3 Parameterp (persistence). Parameter p (0 < p
< 1) shall be the probability that the MAC sub-layer will
transmit on any access attempt. An access attempt shall be a
Bernoulli process with a probability of success equal to p.
6.5.2.3.4 Counter MI (maxivzurn number of access
attempts). Counter M1 shall be set to the maximum number of
attempts (MI) that a MAC sub-layer will make for any
transmission request. This counter shall be cleared upon:
system initialization, Timer TM2 expiring, or a successful
access attempt. The counter shall be incremented after every
unsuccessful access attempt. When the counter reaches the
maximum number of attempts (MI), authorization to transmit
shalI be granted as soon as the channel is idle.
6.5.2.4 DESCRIFTIOONF PROCEDURES
6.5.2.4.1 Channel sensing. Before attempting to access
the channel, the MAC sub-layer shall verify that the channel
is idle.
6.5.2.4.2 Access attempt. An access attcmpt shall be
made when Timer TM1 expires, the channel transitions from
busy to idle or a transmission request arrives from the DLS
while the channel is idle.
6.5.3 Data link senice sub-layer
6.5.3.1 The DLS shall support bit-oriented simplex
air-ground communications using the aviation VHF link
conlrol (AVLC) protocol specified in this section.
Note.- The DLS is derived from HDLC, as specified by
IS0 3309, IS0 4335,lSO 7809, and IS0 8885. Any definitions
r;rS service are derived from the OSI Data Litlk Service Definition
IS0 8886.3. AVLC is a variant of HDLC and derived
from, but is not fully spec$ed by, options 1, 3.2, 4, 7, and 12
of JSO 7809. Explicit rejkmnces to these documents are made
　
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