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transmission tl~at is independent of any
encoding.
b) A data link is the logical association of two interconnected
stations, including the communication control
capability of the interconnected stations.
c) A station is a configuration of logical elements, from
or to which messages are transmitted on a data link,
including those elements which control the messagc
flow on the link via communication control
procedures.
d) A combined station sends and receives both commands
and responses and is responsible for control of the data
link.
e) Data communication control procedures are the means
used to control and protect the orderly interchange of
information betwecn stations on a data link.
f) A component is defined as a numbcr of bits in a prescribed
order within a sequence for the control and
supervision of the data link.
g) An octet is a group of 8 consecutive bits.
h) A sequence is one or more components in prescribed
order comprising an integral number of octets.
i) A field is a series of a specified number of bits or
specified maximum number of bits which perfoms the
functions of data link or communications control or
constitutes data to be transferred.
j) A frame is a unit of data to be transferred ovcr the
data link, comprising one or more fields in a
prescribed order.
k) A common ICAO data interchange network (CDIN)
switching centre is that part of an automatic AFTN
switching centre which provides for the entry, relay,
and cxit centre functions using the bit-oriented link
and CIDIN network procedures specified in this
section and includes the appropriate interface(s) with
other parts of the AFTN and with other networks.
8.6.4.2 BIT-ORIENTEDDA TA LINK CONTROL PROCEDURES
FOR POINT-TO-POINT, GROUND-GROUND DATA
INTERCHANGE APPLICATIONS EMPLOYING
SYNCHRONOUS TRANSMISSION FACILITIES
Note.- The followirlg link level procedures are the same
as the LAPB link level procedures described in 1TU CCZn
Recomm,endarion X.25, Section 2, Yellow Book (1981 version).
Later versions oj' Recommendation X.25 will be reviewed as
they ure released to ascertain whether or not they should be
adopted.
8.6.4.2.1 Frame formut. Frames shall contain not less
than 32 bits, excluding the opening and closing flags, and shall
conform to the following format:
8.6.4.2.1.1 A frme shall consist of an opening flag (F),
an address Geld (A), a control field (C), an optional information
field (I), a frame check sequence (FCS), and a closing
flag sequence (F), and shall be transmitted in that order.
FLAG
F
Note.-- In relation to CZOZN, the opening jag, the Felds
A and C, the FCS and the closing Jag form together the Data
Link Control Field (DLCF). The field I is denoted as the Link
Data Field (LDF).
8.6.4.2.1.1.1 The flag (E) shall be the %bit sequence
01 11 11 10 which delimits the beginning and ending of each
frame. It shall be permissible for the closing flag of a frame to
also serve as the opening flag of the next frame.
ADDRESS
A
8.6.4.2.1.1.2 The address (A) field shall consist of one
octet, excluding 0 bits added to achieve transparent transmission,
which shall contain the link address of the combined
station.
8-6-4.2.1.1.3 The control (C) field shall consist of one
octet, excluding 0 bits added to achieve transparent bansmission,
and shall contain the commands, responses, and frame
sequence number components for the control of the data
link.
CONTROL
C
8.6.4.2.1.1.4 The information (I) field shall contain
digital data which may be presented in any code or sequence
but shall not exceed a maximum of 259 octets, excluding 0
bits addcd to achieve transparent transmission. The I field shall
always be a multiple of 8 bits in length.
8.6.4.2.1.1.5 The frame check sequence (FCS) shall
consist,of two octets, excluding 0 bits added to achieve transparent
transrrrission, and shall contain the error detecting bits.
INFORMATION
I
8.6.4.2.2 A frame check sequence (FCS) shall be
included in each frame for the purpose of error checking.
7/11/96
No. 71
FCS FLAG
F
Part I Annex 10 - Aeronautical Telecommunications
8.6.4.2.2.1 The error checking algorithm shall bc a 8.6.4.2.3 Achievement of transparency. The frame
cyclic redulldancy check (CRC). format contents (A. C, link data field, and FCS) shall be
capable of containing any bit configuration.
8.6.4.2.2.2 The CRC polynominal (P(x)) shall be
8.6.4.2.3.1 The following rules shall apply to all frame
contents, except flag sequences:
8.6.4.2.2.3 The FCS shall be a 16-bit sequence. This a) the transmitting station shall examine the frame con-
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