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term "lowest priority service" denotes the lowest priority used
for safety and regularity of flight, under peak-hour traffic
loading. Performance for priority levels not used for safety or
regularity of flight is not specified.
7.2.2.6 Transit delay and 95 per cent
data transfer delay
7.2.2.6.1 Transit delay is a standard speed of service
performance measure for single DATA packets; transit delay
is defined as an average value. Because the civil aviation
community typically does not rely on average values for the
most critical performance measures, the 95 per cent data
transfer delay is also specified. Under actual operational
conditions, the relationship between the average and 95 per
cent delay values is not fixed, but may depend on the distribution
of traffic. A typical statistical distribution of to-aircraft
delays, under projected peak traffic loading and at the lowest
P channel rate, is illustrated in Figure A-15 of this guidance
material for the highest and lowest priority of data. A typical
statistical distribution of from-aircraft delays under similar
conditions is shown in Figure A-16 of this guidance material.
From-aircraft delays are independent of priority unless two or
more from-aircraft packets contend for resources within a
particular AES. The extent to which such internal contention
may occur within a particular AES will depend on the avionics
architecture of the aircraft in which the AES is installed.
7.2.2.6.2 Transit delay and 95 per cent data transfer
delay are specified on the basis of a standard reference DATA
packet containing 128 octets of subnetwork user data. Actual
delays for shorter packets may be reduced, but not necessarily
in proportion to the packet length. The delay parameters
associated with DATA packets shorter than 128 octets (of
subnetwork user data) should not exceed the corresponding
parameters for 128 octet DATA packets.
7.2.2.7 Throughput
Throughput is the standard speed of service performance parameter
for the transfer of multiple DATA packets. In accordance
with current industry practices, throughput is computed
on the basis of standard reference DATA packets containing
128 octets of subnetwork user data. Thughput is computed
for the transfer of multiple independent packets. Throughput
performance for the transfer of M-bit sequences may be
substantially higher. The subnetwork is expected to support the
minimum throughput values shown in Table A-1 1,
7.2.2.8 Connection estublishment delay,
connection release delay
7.2.2.8.1 Connection establishment delay. The maximum
connection establishment delay is based on a connection
request of the lowest priority, containing a total of 15 octets of
DTE address information and 42 octets of facility fields and
optional hta. The specified value for each channel rate applies
to an equal mix of GES-originated requests and AESoriginated
requests. The maximum connection establishment
delay is the standard speed of service specification for the
connection establishment phase; it is not intended to limit in
any way the future use of optional facilities, user data fields,
or address fields.
7.2.2.8.2 Connection dense delay. The maximum
connection release delay is based on a disconnect request at
the lowest priority. containing no user data, invoking no
optional facilities and carrying no address information. The
specified value for each channel rate applies to an equal mix
of GES-originated requests and AES-originated requests, when
the connection release is not delayed by the presence of
packets in transit on the connection. The maximum connection
relase delay is the standard speed of service specification for
the connection release phase; it is not intended to limit in any
way the future use of optional facilities, user data fields, or
address fields.
7.2.3 F&LlABlUT'l' OF SERVICE
7.2.3.1 Reliability of service is determined by the system
architecture, by the physical layer bit error rate, and by the
average rate of "collisions" on R channels. There is no specification
on the rate of R channel coIIisions. However, the
system performance specifications for both speed of service
and reliability of service a ~beas ed on an average R channel
occupancy factor (i.e. the ratio of occupied R channel slots to
tbe total number of It channel slots during a given interval) of
approximately 0.15. Higher R channel occupancies are
permitted, provided that all of the applicable pdonnance
standards ar~. met.
7.2.3.2 The standard reliability of service parameter for
the transfer of a single DATA packet is residual error probability,
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