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field indicating "local procedure error" and the
diagnostic field indicating "packet type invalid on
PVCs";
c) if the logical channel is in the flow control ready state,
and the centre is a PVC -transit centre for the PVC
assigned to the logical channel, and the corresponding
logical channel is in the flow control ready state:
execute the procedure for reset on the corresponding
logical channel with the resetting cause field indicating
"remote procedure error" and the diagnostic field
indicating "packet type invalid on PVCs".
8.6.6.2.5 PROCEDUREFO R
INITIAL124TION
8.6.6.2.5.1 To sirnultancously initialize all logical
channels conveyed over a single data link, a centre shall
execute the procedure for restart with the diagnostic code
indicating "no additional information".
c) The window size W is the maximum number of
packets which shall be transferred in a given direction
on a logical channel before waiting for an acknowledgement
from the receiving centre of the data packet
with P(S) equal to the transfer lower window edge.
8.6.6.2.6.2 The data transfer procedure shall operate on
each logical channel in the flow control ready state, independently
from any other logical channel on that data link.
8.6.6.2.6.3 The data transfer procedure shall operate in
each direction of data flow, independently from the opposite
direction of data flow.
8.6.6.2.6.4 When a logical channel enters the flow
control ready state, the receive and transfer lower window
edges shall be set to 0.
8.6.6.2.6.5 W shall normally be 7 but other window
sizes may be negotiated on a bilateral basis.
Note.- Data networks operating in the packet mode and
connected with CIDIN using procedures described in ITU
CCITTRecornrnendation X.25-I981 may require W to be some
other value between 2 and 6, inclusive. Later versions of
Recommendation X.25 will be reviewed as they are released
to uscertain whether or not they should be adopted.
8-6.6.2.6.6 Arithmetic on lower window edges, P(S) and
P(R) shall be performed modulo 8.
8.6.6.2.6.7 Receiving dutu puckets
8.6.6.2.6.7.1 A centre which rcceives a data packet on
a logical channel which is not in the flow control ready state
shall discard the data packet.
8.6.6.2.6.7.2 A PVC transit centre which receives a data
8.6.6.2.6 PROCEDUREF OR packet on a logical channel which is in the flow control ready
DATA TRANSFER state, and the corresponding logical channel is not in the flow
control ready state, shall:
8.6.6.2.6.1 The following descriptions shall apply to the
data transfer procedure: a) discard the data packet;
a) The transfer lower window edge is the oldest P(S) b) execute the procedure for reset on the logical channel
number associated with the transfer (outgoing) direc- from which the data packet was received, with the
tion of packets whose corresponding data packet has resetting cause field indicating "network congestion"
not yet been acknowledged by a P(R) transferred from and the diagnostic code indicating "no additional
the receiving centre. information".
7/11/96
No. 71
Annex 10 - Aeronautical Telecommunications Volume I11
8.6.6.2.6.7.3 A centre shall expect the first data packet
received after a logical channel enters the flow control ready
state to contain P(S) equal to 0.
8.6.6.2.6.7.4 A PVC transit ccntre which receives a data
packet which is less than or equal to the maximum established
length for data packets, and which contains P(S) equal to the
next expected value of P(S), and which contains P(S) less than
the receive lower window edge plus W, shall:
a) increment its next expected value of P(S) and, if this
value differs from the receive lower window edge by
more than one-half W, execute the procedure to update
the receive lower window edge given in 8.6.6.2.6.13;
b) queue the data packet for transfer on the corresponding
logical channel, without waiting for subsequent data
packets;
C) if the P(R) is greater than or equal to the transfer
lower window edge, and less than or equal to the P(S)
to be assigned to the next data packet for transfer:
1) set the transfer lower window edge to equal P(R);
2) if there exist any unacknowledged data packets
which were transferred with P(S) less than the new
value of the transfer lower window edge, execute
the procedure to update the receive lower window
edge on the corresponding logical channel;
b) queue the data packet for processing by the CIDIN
packet level procedures;
c) if the P(R) is greater than or equal to the transfer
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