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signal units which lnay convey user information and
protocols, signalling, and any necessary preamble.
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:
x ' ~ + x ' '+ x5 + I
Note.- See CCUT (Rcd Book) Recor~ltnerzdationX 25.
Secriorz 2.2.7 for the tnethod of cuicuhtion arzd the bit order:
Direct link service (DLS). A data communications service
which ~ a k e nso attempt to automatically correct errors,
detected or undetected, at the link layer of the satellite
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.
Frame. A structured, repeating time-segment of a cornrnunication
link architecture that provides for time-predictable
communication activities between its beginning and end.
Global beam. Satellite antenna directivity whose main lobe
encompasses the entire earth's surface that is within lineof-
sight view of the satellite.
Initial signal unit (ISU). The first of the series of signal units
followed by SSUs.
Link interface control information (LICI). 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 data unit (LJDU). The total ~nformat~otrna nsterred
in a single interaction across the interface between
the link layer and a link service user. Each LLDU contains
link inlerface control information (LlCl) and may also
contain a single link senrice data unit (LSDU).
Link service data unit (LSDU). A part of the link interface
data unit (LIDU) and is the same as the subnetwork protocol
data unit (SNPDU).
Lone sigizal unit (LSU). A single signal unit comprising the
total message.
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 station (NCS). The entity of the overall
AMS(R)S system that has functional responsibilities to:
co-ordinate cotnmunications traffic and satellite connecrivity
within its satellite region: and provide inter-system
co-ordination with adjacent satellite regions served by
other satellites.
P channel syncltronization. The state of the P channel
demodulator when the P channel unique word is reliably
detected.
ANNEX 10 - VOLUME Ill 5 9/11/95
Annex 10 - Aeronautical Telecommunications Volume IZZ
P channel degrudutionAoss, A declaration that is made when
the P channel bit crror ratc rises above 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 whcn the P chailnel 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).
Reliable link service {RLS). A data communications service
provided by the satellite subnetwork which automatically
provides for error control over its link through error
detection and requested retransmission of signal units
found to be in error.
Satellite region. A geographically defined sub-area within the
view of a satellite within which services can be provided
by that satellite.
Satellite service men. A geographically defined sub-area
within the view of a satellite within which services are
provided by that satellite. Note that a satellite service area
might be sub-divided in tcrms of operational characteristics,
conditions, or limitations for a variety of reasons.
Signal unit (SU). A time-ordered, contiguous set of data
octets used for signalling and control, and for user packet
data transmissions. Standard-length SUs are 96 bits (12
octets) used ,'iFn T and C channels. R channel SUs arc
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
　
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