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delay is around 30 milliseconds (rising to 50 milliseconds at
the lowest channel rate). To match the number of interleaver
bits to the required number of channel bits, a small number of
"dummy" bits are included in the frame just after the framing
bits.
4.5.5.1 The &lay at the transmitter arising from the
interleaver depends on the relationship between the voice
frame and the interleaver size. The full de-interleaver delay of
around 20 milliseconds will be experienced at the receiving
end.
4.5.6 Voice cham1 delay. To maintain voice circuit
quality, the CCTlT recommends that the end-to-end delay
should be limited to 440 milliseconds or less. Given that the
nominal satelte delay is 270 milliseconds, the voice
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 woders, e.g. 2.4 and 4.8 kbits/s.
4.5.7 Sub-band C channel, me data tmnsferred by the
sub-band C channel includes control and signalling for the call
set-up of the C channel, continuity checks, and power control.
En each f m e , the first 24 half-octets of the sub-band C
channel are combined to form 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 voicd
data 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,
AMachmenl A to Part I Annex 10 - Aeronautical Telecommunic&ns
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 (AS) and the
ground earth station (GES) link layers, an SU set cannot be
generated from a received LIDU as described in Annex 10,
blume JII, 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 LDUs do
not require reference numbers. Circuit-mode services LIDUs
are assigned application reference numbers by the circuitmode
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 whlch belong to the set may
have less than the maximum length. In a P channel SU set, the
number d 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" fietd (Annex 10, Volume III, Part I, Appendix
3 to Chapter 4, Item 67).
Sd SUs transmission accordlag
to pmedence
At the AES and the GES, in order to meet the SUs
transmission requirements of Annex 10, Volume III, 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 IlT, Pact I, Appendix 3 to Chapter 4, Item 58). For the
P channel, the SUs of an SU set would be queued with the
　
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