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to a phase shift of +W0. The phase-encoded A-BPSK data
stream is then filtered with a filter which satisfies the
amplitude and phase versus frequency limits defined by
Tables AZ-1 and A1-2 in Appendix I to Chapter 4.
Aviation-QPSK (A-QPSK). The particular form of offset
quaternary phase shift keyed modulation which is used in
AMSS for channel rates greater than 2 400 bitsls. A-QPSK
is a modulation technique which maps a "0" into a 0
degrees and " 1" into a 180 degrees, or "0" into 90 degrees
and "2" into 270 degrees, alternating between the two
options on successive bits. The encoded A-QPSK data
stream is then filtered such that the modulated spectrum
meets the amplitude mask of Table A1-3 and the phase
mask defined in Table A1-2 in Appendix 1 to Chapter 4.
detected or undetected, at the link layer of the air-ground
communications path. (Error control may be effected by
end-user systems.)
Epoch. A span of time related to the beginning and end, or
lifetime, of an event or a sequence of associated events.
Frmne. A structured, repeating time-segment of a
communication link architecture that provides for timepredictable
communication activities between its beginning
and end.
Global beam. Satellite antenna directivity whose main lobe
encompasses the entire earth's surface that is within line-ofsight
view of the satellite.
Initial signal unit (ISU). The fist of the series of signal units
followed by SSUs.
Link interface control information (WCI). The control
information exchanged between the link layer and any of its
service users as part of the link interface data unit (LIDU).
Link interface &tu unit (1;IDU). The total information
trmsferred in a single interaction across the interface
between the link layer and a link service user. Each LIDU
contains link interface control information (LICI) and may
also contain a single link service data unit (LSDU).
Link service && unit (LSDU). A part of the link interface
data unit (LIDU) and is the same as the subnetwork
protocol data unit (SNPDU).
Burnt. A time-defined, contiguous set of one or more related
units which may convey user infomation and Lone signat unit (LSU). A single signal unit comprising the
protocols, signalling, and any necessary preamble. total message.
Cyclic redundancy check. The last two bytes of each signal
unit form a cyclic redundancy check of the whole signal
unit as follows. The check bits for error detection are
calculated from the first 10 octets of a standard length
signal unit, or from the first 17 octets of an extended length
signal unit or from the first 4 octets of the burst identifier,
using the following generator polynomial:
Note.- See CCLT (Red Book) Recommendation X-25,
Section 2.2.7for the method of calculation and the bit order.
Direct link service (DLS). A data communications service
which makes no attempt to automatically correct errors,
Near-geostationary orbits. Satellites operating in neargeostationary
orbits have an orbit period of 24 hours with
an inclination of up to five degrees from the equatorial
plane.
Network co-ordination s W n (NCS). The entity of the over-all
AMS(R)S system that has functional responsibilities to: coordinate
communications traffic and satellite connectivity
within its satellite region; and provide inter-system c t ~
ordination with adjacent satellite regions served by other
satellites.
P channel synchron&&n. The state of the P channel
demodulator when the P channel unique word is reliably
detected.
ANNEX 10 - VOLUME Ill
Annex 10 - AeronuuticaZ Telecommunications Volume ZZZ
P channel d e g r ~ d ~ o n h sAs .d eclaration that is made when
the P channel bit error rate rises above 10" over an
averaging period of 3 minutes, or more than 10 short-term
interruptions (loss of P channel synchronization for less
than 10 seconds) are experienced in any 3-minute period; or
when the P channel synchronization is lost for more than
10 seconds.
Q number, Q level, Q precedence. A definition of the transmission
precedence of a message or signalling sequence,
using numbers from 0 to 15 (with 15 transmitted first).
used for system management functions is designated P,,,,
while a P channel being used for other functions is designated
by Pd. The functional designations P, and Pd do not
necessarily apply to separate physical channels. -
4.1.1.2.2 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
　
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