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4.6.5 T channel reservation protocol
Note.- In 4.6.5, the wse of the terms "AES" and "GES"
refer to the link layer functions for the T channel reservation
protocol within the AES and the GES.
The link layer functions for the T channel reservation protocol
shall be as defined in 4.6.5.2 and 4.6.5.3.
4.6.5.2 AIRCRAFEA~R TH STATION (AES)
4.6.5.2.1 Upon receipt of an LIDU containing an LSDU
exceeding 33 octets, the AES shall route the LDU to the T
channel protocol.
Note.- An LIDU containing an LSDU of 33 octets or less
may be routed to R channel protocol or T channel protocol.
4.6.5.2.2 Before passing the LDU to the T channel transmission
protocol, the AES shall assign an available (not
currently assigned) reference number (Appendix 3 to Chapter 4,
Item 46) to it. If a reference number is not assigned to an LIDU.
the LIDU sha1.l be stored until one is assigned to it. Each
assignment of a reference number shall be to the oldest received
LlDU without a reference number. The reference numbers for
assignment to T channel LDUs shall be defined in pairs as
(0.1); (2,3); (4,s); (67); (8.9); (10,ll); (12,13); (14,15). Once
released, a reference number from a pair shall not be reassigned
until the other number from the pair has been assigned and
released. The AES shall not assign a reference number from a
pair whose other number has just been released until a reference
number from each pair that was available for assignment before
the release has been assigned. The assigned reference number
shall be used by both the T channel transmission protocol and
the T channel reservation protocol.
4.6.5.2.3 To request a reservation for the transmission of
an SU set, the AES shall send a reservation request (REQ) SU
to the GES indicating the reference number, Q number and the
length of the SU set to be transmitted.
4.6.5.2.4 The AES shall transmit a REQ SU on the R
channel (Appendix 2 to Chapter 4: Figure A2-47) or on the T
channel (Appendix 2 to Chapter 4, Figure A2-7) to the GES
in accordance with the following criterion:
The T channel shall be used if the AES is currently transmitting
a T channel burst and there is enough room to
accommodate the REQ SU or if there is a T channel burst
due to start in the next 8-second time window; otherwise,
the R channel shall be used.
4.6.5.2.5 When the T channel is used for transmitting a
REQ SU to the GES, the requested length shall be increased
by one SU.
4.6.5.2.6 After sending a REQ SU to the GES, the AES
shall do the following:
a) If the T channel was used for a REQ SU transmission,
and neither a corresponding reservation ( E S ) SU
(Appendix 2 to Chapter 4, Figure A2-23) nor a corresponding
reservation forthcoming (RFC) SU (Appendix
2 to Chapter 4, Figure A2-24) is received from the
GES within tA6 seconds (Appendix 4 to Chapter 4)
from the time the REQ SU was sent on the T channel,
the AES shall send the REQ SU again on the T or R
channel, in accordance with 4.6.5.2.4, with the
requested transmission length equal to the length last
requested or incremented by one if T channel is used.
The AES shall then proceed as in 4.6.5.2.6.
b) If the R channel was used for the 'REQ SU
transmission, and neither a corresponding RES SU nor
a corresponding, RFC SU is received from the GES
within tA7 seconds (Appendix 4 to Chapter 4) from
the time the REQ SU was sent on the R channel, the
AES shall command the selection of an R ,channel
frequency and send the REQ SU again on the T or R
channel, in accordance with 4.6.5.2.4, with the
requested transmission length equal to the length last
requested or incremented by one if the T channel is
used, The AES shall then proceed as in 4.6.5.2.6.
However, if tA7 seconds elapse after the fifth
consecutive transmission of a REQ SU on the R channel
without receiving any corresponding RES SU or
.RFC SU from the GES, the AES shall abort the
reservation procedure for the corresponding LlDU and
shall discard the number of SUs specified in the last
transmitted KEQ SU. If an ISU is amongst the SUs
discarded, the AES shall then discard the complete SU
set headed by the discarded ISU even if this shall
result in discarding more SUs than the number of SUs
specified in the REQ SU. The AES shall then send a
transmission status indication LIDU (Table 4-25)
indicating failure to the appropriate link service user in
the AES corresponding to that completely discarded
SU set. If an ISU is amongst the SUs discarded, the
AES shall then discard the complete SU set headed by
the discarded ISU even if this shall result in discarding
　
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