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used for signalling and control, and for user packet data
transmissions. Standard-length SUs are 96 bits.(12 octets)
used in P, T and C channels. R channel SUs are 152 bits
(19 octets), and the T channel uses a header SU of 48 bits
(6 octets).
Spot beam. Satellite antenna directivity whose main lobe
encompasses significantly less than the earth's surface that
is within line-of-sight view of the satellite. May be
designed so as to improve system resource efficiency with
respect to geographical distribution of user earth stations.
Subsequent signal unit (SSU). In a series of SUs, the signal
unit(s) following the initial signal unit.
Superframe. A recurring, time-structured set of data transmission
frames, which also includes a superframe marker
(see also the definition of "frame").
4.1.1.2 DESCRIPTIOOFN
CHANNEL TYPES
4.1.1.2.1 P channel. Packet mode time division
multiplex (TDM) channel transmitted continuously from the
aeronautical ground earth station (GES) in the to-aircraft
direction to carry signalling and user data. A P channel being
used for sj.stem management functions is designated P,,,,
while a P channe.1 being used for other functions is designated
by Pd. The functional designations P,,, and Pd do not tlesessarily
apply to separate physical chatlncls.
3.1.1.2.2 R channel. Random access (slotted Aloha)
channel, used in the from-aircraft direction to carry signalling
and user data. An R channel being used for system management
functions is designated R,,,, while an R channel being
used for other functions is designated Rd. The functional
designations R,,, and Rd do not necessarily apply to separate
physical channels.
. . -
4.1.1.2.3 T channel. Reservation time division multiple
access (TDMA) channel, used in the from-aircraft direction
only. The receiving GES reserves time slots for transmissions
requested by aircraft earth stations (AESs) according to
message length. The sending AES transmits the message in the
reserved tlme slots according to priority.
4.1.1.2.3 C channel. - Circuit-mode single channel per
camer (SCPC) channel, used in both to-aircraft and fromaircraft
directions. This channel is time division multiplexed to
provide a primary channel, for voice or data traffic and a subband
channel for signalling, supervision and data messages.
The use of the channel is controlled by assignment and release
sigdalling at the start and end of each transaction.
4.1.2 General
4.1.2.1 When aeronautical mobile-satellite service
(AMSS), using near-geostationary orbiting satellites, is
installed and maintained in operation as an aid to air traffic
services. it shall conform with the provisions of 4.1 to 4.10.
4.1.2.2 Requirements for mandatory carriage of AMSS
equipment including the level of system capability shall be
made on the basis of regional air navigation agreements which
specify the airspace of operation and the implementation timescales
for the camage of equipment.
-4.1.2.3 The agreements indicated in 4.1.2.2 shall provide
at least two years' notice of mandatory camage of airborne
systems.
4.1.2.4 Recommendation.- Civil aviation authorities
should co-ordinate, with riatianal authorities and service
providers, those implementation aspects of AMSS which will
permit its world-wide intempembiIi~ and optimiim use, as
appropriate.
Note.- Protiisions on $he allocalion and assignment of 24-
bit aircraft addresses .for use by thc AMSS are contained in
Chapter 9.
4/11/99
No. 74
CHAPTER 10. POINT-TO-MULTIPOINT
COMMUNICATIONS
10.1 SERVICE VIA SATELLITE
FOR THE DISSEMINATION OF
AERONAUTICAL INFORMATION
10.1.1 Point-to-multipoint telecommunication service
via satellite to support the dissemination of aeronautical
information shall be based on full-time, non pre-emptible,
protected services as defined in the relevant CCHT
Recommendations.
10.2 SERVICE VIA SATELLITE
FOR THlE DISSEMINATION
OF WAFS PRODUCTS
1 0.2.1 Recommendation.- System charncteristics
should include the following:
a) frequency - C-bnnd, earth-to-satellite, 6 GHz band,
satellite-to-earth, 4 GHz band;
b) capacz~ with effective signalling rate of not less
than 9 600 bifs/s;
c) bit error rates - better than 1 in 10';
d) forward ermr correctior~; and
e) availability 99.95 per cent.
ANNJ5X 10 - VOLUME 111
7111196
No. 71
CHAPTER 11. HF DATA LINK
11.1 DEFINITIONS AND
SYSTEM CAPABILITIES
Note.- The following Standards and Recommended
Practices are specific to the high frequency data link (HFDL)
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