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end.
4.5.6 Voice channel delay. To maintain voice circuit
quality, the CCITT recommends that the end-to-end delay
should be limited to 440 milliseconds or less. Given that the
nominal satellite delay is 270 milliseconds, the voice
Attachment A to Part I Annex 10 - Aeronaulical Telecommunications
processing system delay should not exceed 65 milliseconds.
This assumes that the terrestrial network is allotted 40 milliseconds
of delay, and the modem and RF sub-systems are
allotted 65 milliseconds of delay. It is to be noted that with
current voice processing algorithms, the processing delay tends
to increase as the vocoder rate decreases. Consequently, this
end-to-end delay objective may be difficult to meet with
low-rate vocoders, e.g. 2.4 and 4.8 kbits/s.
4.5.7 Sub-band C channel. The data transferred by the
sub-band C channel includes control and signalling for the call
set-up of the C channel, continuity checks, and power control.
In each frame, the first 24 half-octets of the sub-band
C channel are combined to fom a standard length 96-bit
signal unit. Successive groups of 24 half-octets are combined
in the same way, if available. The 25th data field of the last
voicetdata ' block in the frame is discarded. When the
C channel is activated, and there are no other signal units to
be sent, fill-in signal units are sent.
5. LINK LAYER P CHANNEL AND
R CHANNEL PROTOCOLS
5.1 General
The P and R channels are used for signalling and data
communications in the to-aircraft and from-aircraft directions,
respectively. Data transmission on the R channel is restricted
to short messages (less than 33 octet link service data units
(LSDUs)). This section of the guidance material describes
issues related to the R and the P channel link layer protocols.
5.2 SU set generation
5.2.1 In both the aircraft earth station (AES) and the
ground earth station (GES) link layers. an SU set cannot be
generated from a received LIDU as described in Annex 10,
Volume 111, Part I, Chapter 4, 4.5.3.2.3 until a reference
number is assigned to it, except for system table broadcast
LIDUs and LIDUs received from the circuit-mode service entities
to be transmitted, either on the R channel at the AES, or
the P channel at the GES. System table broadcast LIDUs do
not require reference numbers. Circuit-mode services LlDUs
are assigned application reference numbers by the circuit-mode
service entities at the AES and the GES, thus an SU set is
generated upon receipt of the LIDU. For all LIDUs exchanged
between the link layer, at the AES or the GES, and any of its
service users, no formats have been specified in the SARPs.
Such formats are implementation dependent.
5.2.2 When an LIDU received from the subnetwork
layer is assigned a reference number, the LSDU of the LIDU
is encoded in the data field of the ISU/SSU(s) of a P channel
SU set or in the data field of the SUs of an R channel SU set.
The number of SUs needed in the SU set depends on the
LSDU length. Each SU, except the last one, will contain the
maximum permitted number of user data octets. The number
of user data octets in the last SU which belong to the set may
have less than the maximum length. In a P channel SU set, the
number of octets in the final SU (ISU or SSU) is encoded in
the "No. of octets in the final SSU" field of the ISU. On the
R channel, the number of octets in the last SU is encoded in
the SU's "SU type" field (Annex 10, Volume 111, Part I,
Appendix 3 to Chapter 4, Item 67).
53 SUs transmission according
to precedence
At the AES and the GES, in order to meet the SUs transmission
requirements of Annex 10, Volume 111, Part I, Chapter 4,
4.5.8.2.3, the R and P channel protocols may employ several
first-in-first-out (FIFO) queues, one queue per valid Q number,
to store the SUs prior to transmission. For the R channel, the
SUs of an SU set would be queued according to the ascending
order of their sequence indicators (Annex 10, Volume IU, Part
I, Appendix 3 to Chapter 4, Item 58). For the P channel, the
SUs of an SU set would be queued with the ISU, or RTX SU
for a retmsmission set, first followed by the SSUs according
to the descending order of the SSUs sequence numbers. At the
instance of transmission, an SU would be selected from the
highest priority queue with one or more SUs in it.
5.4 P channel protocol
5.4.1 The P channel link layer protocol is used in the toaircraft
direction for processing the transmission of: 1) the
LIDUs received from the GES management, 2) the LIDUs
received from the circuit-mode services with routing parameters
　
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