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C) positive acknowledgement loss: the RQA SU refers to
an SU set which has been processed just before.
When the initial SU set has been lost, the GES determines the
initial SU set length from the message length parameter in the
RQA SU in order to reserve and send the correct T channel
capacity along with the request for complete renmsmission.
The T channel reservation protocol is used to request
reservations fw the transmission of data on the T channel.
6.4.2.1 The T c b lre servation protocol in the AES
assigns reference numbess to the D U s received from the
subnetwork layer for transmission on the T channel. The
reference numbers ace assigned in such a way that no two
consecutive LJDUs with the same Q number transmitted from
an AES to a GES have the same reference number. The
reference number assigned to an LSDU is used by the
T channel. reservation protocol in the request Eor reservation
(REQ) SU and by all the associated signalling and by the
Tchannet transmission protocol in the &ta SUs and by
the associated acknowledgements and request for acknowledgement
SUs.
6.4.2.2 The REQ SU is transmitted to the GES on the T
channel or on the R chame1. Every time the REQ SU is
transmitted on the T channel, the requested length in the REQ
SU is incremented by one in order to account for the T channel
capacity used to transmit the REQ SU.
6.4.2.3 The timer tA8 in the AES is wed to supervise the
receipt of expected T cbnel reservation (RES) SUs
associated with either previously received reservation
foahcorning (RFC) SUs or received TACK SUs, Due to the
priority scheme for the transmission of SU sets, resulting in
higher priority SU sets potentially occupying reservation
assignments for lower priority SO sets, there will be instances
of time whete multiple tAS timen associated with the same
SU set might have been initiated at the AES, Upon receipt of
an awaited RES SU for that SU set, the AES must stop the
corresponding tA8 timer (Annex 10, blurne IU, Part I,
Chapter 4, 4.6.5.2.6 and 4.6.5.2.7). However, since a one-toone
correspondence between a tA8 timer and a received RES
SU is not guaranteed, a selection of a most suitable tA8 timer
must be made. Such selection is implementation dependent.
For instance, the selection could be made dependent on the
time-out vatues of all outstanding tA8 timers, such that the tA8
timer with the shortest remaining time to time-out is selected
regardless of the size of reservation in the received RE3 SU.
In this case, care must be taken in the implementation to
request reservations for any resulting deficit in the total size of
outstanding reservations associated with an SU set at the AES
when dl tA8 timers have ken stopped. Other criteria for the
selection may be used; however, the constraint that at least the
total size of all outstanding reservations associated with any
given SU set must be, received prior to releasing the reference
number assigned to that SU set must always be observed.
6.4.2.4 Reservations m y rsd be received in chtoaabgical
order. The minimum inter-reservation gap h e e n two
reservations on two dBerent T channels is made adequate to
allow for frsquency (T channel) switching at the AES. A
reservation (RES)S U d e 6 1o~ne or more burst allocrrtions on
one T channel. The starting frame number of the initial burst
is encoded within the RE3 SU and is known as belongifig to
one of the sixteen frames -following the reception of the RES
SU (the time widow). Thereafter, h r h r reservations (if any)
occur every 2BI frames mtil the m b e r of reserved bursts is
reached. BI is the ending of the burst interval information
element within the RES SU. The AES computes the starting
fathe for each reserved burst using both the starting f m e
number of tbe initial btltst and the tmrst interval information.
Once a rese~edbu rst has elapsed, the AES deletes it from its
time plan.
6.4.3.1 The GES assigns mmvatiom in response to
requests for remations fmm its loggedsn AESs. The GES
also assigns reservations for retrmmissions of missing SUs of
an SU set transmitted by the AES. So, whenever the T channel
transmission .protocol at the GES transmits a TACK SU set
comprised of one or more TACK SUs indicating ermrs, the
T charmel reservation protocol at the GES makes reservations
for the retransmissions.
6.4.3.2 The GES assips reservations to an AES for my
one of the four T channels that may have been assigned to the
ABS at the time of log-oa The GBS indicates the T channel
on which the reservation has been made to the AES in the RES
SU. A procedure to select a T charmel, out of the possible four
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