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only SABM, DISC, UA and DM frames. (See 8.6.4.2.7 below
for a description of the commands and responses to which
these framc types refer.)
8.6.4.2.5.2.3 A combined station in ADM shall ~ransmit
a DM when a DISC is received, and shall discard all other
reccived cornmar~d frames except SABM. If a discarded
command framc has the P bit set to "I", the cornbind sta~ion
shall transmit a DM with the F bit set to "I".
2 3 4
N(s)
8.6.4.2.6 Contmlfwldfuncrions urzd parameters. Control
fields contain a command or a response and sequence nurnbcrs
where applicablc. Three types of control fields shall be used
to perform:
5
P
P/F
l M M P / F M M M
0
a) numbered information transfer (]-frames):
6 7 8
S S NR)
b) numbered supervisory functions (S-frames); and
C) unnumbered control functions (U-frames).
The control field formats shall be as shown in Table 8-5. The
functional frame designation associated with each type control
field as well as the control field parameters employed in
performing these functions shall be described in the following
paragraphs.
8.6.4.2.6.1 The I-frame type is used to perform information
transfers. Except for some special cases it is the only
format which shall he permitted to contain an information
field.
8.6.4.2.6.2 The S-frame type is used for supervisory
commands and responses that pcrform link supervisory control
functions such as acknowledge information frames, request
transmission or retransmission of illformation frames, and to
request a Lemporary suspension of transmission of I-frames.
No information Geld shall be contained in the S-frame.
8.6.4.2.6.3 The U-frame type is uscd for unnumbered
commands and responses that provide additional link control
functions. One of the U-frame responses, the frame rcjcct
(FRVR) respotzse, shall contain an information field; all other
frames of the U-frame typc shall not contain an information
field.
8.6.4.2.6.4 The station parameters associated with thc
three control field typcs shall be as follows:
a) Mod~th~Esa.c h I-frame shall be sequentially numbered
with a send sequcnce count, N(S), having value 0
7/11/96
No. 71
Part I Annex 10 - Aeronautical Telecommunications
rhrough modulus minus one (where modulus is the
modulus of the sequence numbers). The modulus shall
be 8. TIIF maximum number of sequentially numbered
I-frames ihat a station shall have outstanding (i.e.
unacknowledged) at any given time shall never exceed
one less zhsn the modulus of the sequence numbers.
This restriction on the number of outstanding frames
is to prevent any ambiguity in the association of transmission
frames with sequence numbers during normal
operation and/or error recovery.
b) The send state variable V(S) shall denote the sequence
number of the next in-sequence I-frame to be trimsmitted.
1) The send statc variable shall take on the value 0
through modulus minus one (modulus is the
modulus of the sequence numbering and the
numbers cycle through the entire range).
2) The value of V(S) shall be incremented by one
with each successive in-sequence I-frame transmission,
but sliall not exceed the value of N(R)
contained in the last received frame by more than
the maximum permissible number of outstanding
I-frames (k). See i) below for the definition of k.
c) Prior to transmission of an in-sequence I-frame, the
value of N(S) shall be updated to equal the value of
VV).
d) The receive state variable V(R) shall denote the
sequence number of the next in-sequence I-frame to be
received.
I) V(R) shall take on the values 0 through modulus
minus one.
2) The value of V(R) shall be incremented by one
after the receipt of an error-free, in-sequence
I-frame whose send sequence number N(S), equals
V(R)-
e) All I-frames and S-frames shall contain N(R), the
expected sequence number of the next received frame,
Prior to transmission of either an I or an S type frame,
the value of N(R) shall be updated to equal the currcnt
value of the receive state variable. N(R) indicates that
the station transmitting the N(R) bas correctly received
all I-frames numbered up to and including N(R) - 1.
f) Each station shall maintain an indcpendent send state
variable, V(S), and receive state variable, V(R), on the
I-frames it sends and receives. That is, each combined
station shall maintain a V(S) count on the I-frames it
transmits and a V(R) count on the I-frames it has
correctly received from the remote combined station.
g) Thc poll (PFj bit sliall be used by a combined station
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