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with a "global" beam satellite. This capability provides greater
data communications throughput, faster message delivery and
may decrease operational cost. Level 2 requires one transmit
channel and one receive channel as in Level 1.
1.2.4 The Level 3 capability adds to Level 2 packet data
communications a circuit capability for voice or communications
using a transmit and a receive C channel. These C
channels are capable of operating at channel rates of 10.5 or
21.0 kbitsls to support 9.6 kbitsls vocoders for voice communications.
The C channel can also provide circuit-mode data
communications but this is not considered part of the safety
services. The C channel rate used depends on the use of 112
rate forward error correction (FEC) coding (1/2 rate coding
requires 21 .D kbitsls). Provision has been made to allow use of
lower vocoder rates that could operate at channel rates of 5.25
or 6.0 kbitsts. Level 3 requires one transmit channel (R, T or
C channel) and two receive channels (P channel and C channel).
Simultaneous operation of two-way packet and circuit
communications is not possible, but near-simultaneous operation
is possible by switching between transmit channel types.
1.2.5 Level 4 capability adds to Level 3 capability
additional transmit channel capability to provide simultaneous
operation of two-way packet and circuit communications.
Level 4 capability requires two or more transmit channels
(R or T, and C channel) and two or more receive channels
(P channel and C channel), power control for each channel
carrier and a linear power amplifier. Channel rates are the
same as Level 3. Both R and T channel capabilities are
required on a transmit channel but not simultaneously.
1.2.6 The AMS(R)S is the first aeronautical safety
conimunications service which integrates both voice and data,
as well as non-safety services. This integration of services
must respect message priority. Pre-emption of one data
message by another data message of higher priority is handled
easily within the system without loss of information. Preemption
of a voice call by a higher priority data calI, or vice
versa, will not normally be necessary in an AES which has
two transmitters. However, in a Level 3 AES which has only
one transmitter such a pre-emption may occasionally be
necessary.
1.2.7 Although a Level 4 AES is not required to provide
simultaneous packet data communications with more than one
GES, such a capability is not precluded.
2. BROADBAND RF
CHARACTEIUSTICS
2.1 Use of AMS(R)S bands
2.1.1 Message categories. The transmission sequence at
any aircraft earth station (AES) or ground earth station (GES)
will be ordered in accordance with a given priority scheme. At
the subnetwork interface to the AMSS, the priority scheme for
packet data is as described in Annex 10, Volume 111, Part I,
Chapter 4, Table 4-26. Within the AMSS, this external priority
scheme is augmented with internal priorities assigned to
various signalling and voice-related functions. At the link layer
this augmented priority scheme is referred to as the
Q-precedence number and the resulting internal priority
scheme is given in Table A-3 of this guidance material. This
"Q-precedence" number list conforms to Annex 10 priorities,
which in turn are derived from Article 51 of the ITU Radio
Regulations. The single Q-precedence list includes both voice
and data traffic, and also includes the signalling necessary to
integrate voice and data. The Q-precedence numbers associated
with the signalling were chosen to optimize the over-all system
performance and integrity. '.
2.1.2 Receive frequency band. For historical reasons,
most AESs may be capable of receiving more than the
required band of 1 544 to 1 555 MHz but not the full band
suggested by the recommendations. 'rypically, they cover the
frequency band 1 530 to I 559 MHz, and may not cover
1 525 to 1 530 MHz.
22 Frequency accuracy and
compensation
2.2.1 Frequency accuracy. The Standard contained in
Annex 10, Volume III, Part I, Chapter 4, 4.2.2 reflects a
requirement on the signal received by the GES. There are
several contributors to the frequency error observed at the
GES. These include frequency errors due to the satellite
oscillator, due to relative motion between the aircraft and
spacecraft, due to the local oscillator of the GES (for a closed
loop compensation system) and the local oscillator of the AES.
Efforts are made to reduce the error caused by the first two as
described below. This Standard characterizes that portion of
the frequency emor which is due to the AES and the aircraft
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