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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 infonnafion (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 (LIDU). The total information transferred
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 data irnil (LSDU). A part of the link interface
data unit (LIDU) and is the same as the subnetwork protocol
data unit (SNPDU).
Lone signal unit (LSLI). 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 communications traffic and satellite connectivity
within its satellite region; and provide inter-system
co-ordination with adjacent satellite re&ions served by
other satellites.
P channel synchronization. The state of the P channel
demodulator when the P channel unique word is reliably
detected.
ANNEX 10 - VOLUME 111 5 911 1/95
Annex 10 - Aeronautical Telecommunications Volume 111
P channel degradationhss. A declaration that is made when
the P channel bit error rate rises above lo4 over an
averaging period of 3 minutes, or more than 10 shon-tern
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).
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 area. 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 terms of operational characteristics,
conditions, or limitations for a variety of reasons.
Signal unit (SU). A time-ordered, contiguous set of data
octets u s d 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 (1 9 octets), and the T channel uses a header SU
of 48 bits (6 octets).
Spot beam. Satellite antenna directivity whose main lobc
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.
~u~erfnzmeA. recumng, time-structured set of data transrnissim
frames, which also includes a superframe marker
(see also the definition of "frame").
4.1.1.2 DESCRIPT~OOF N
CHANNEL TYPES
4.1 . I .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 system management functions is designated P,,,,
while a P channel being used for other functions is designated
by P,. The functional designations P,,, and P, 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
functions is designated %,,, while an R channel being
used for other functions is designated R,. The functional
designations Rs,, and R, do not necessarily apply to separate
physical channels.
4.1.1.2.3 T channel. Reservation time division multiple
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