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Transport
protocol
ENTRY PVC TRANSIT RELAY EXIT
CENTRE CENTRE CENTRE CENTRE
ClDlN
Packet
protocol
X.25 Packet
protocol
Datalink
protocol
4
3b
--------------
3a
2
1
-
LEVEL
4
Physical
interface
I I I I 1
-
I
Figure 8-1. CIDIN Protocol Levels
- - A
- F
- \
LEVEL = *
3b
7/11/96
No. 71
-- 4
.- w
LEVEL ' b.
3a
LEVEL ' -
2
LEVEL
1
-
Annex 10 - Aeronautical Telecommunications
1
h- I CIDIN packet (5 256 octets)
1
4
I
I CIDIN TRANSPORT LEVEL r - I
-
I
I
CIDIN PACKET LEVEL II
I
I
I
I = X.25 PACKET LEVEL I
I
I
I
1 I DATA LINK LEVEL I I
1: I I
I
: Communications II
I4 control field +& CIDIN user data field I
I
I
Figure 8-2. CIDIN Terminology
I
I
7/11/96
No. 71
I
,I . I
I
I I
DATA
LINK
CNTL
FIELD
CIDIN
PACKET
HEADER
X.25
PACKET
HEADER
1 I
I
I I
I
I
I
1 I
I 1
I Data link I-frame I I
#
1 1
1 I
I
I I I
:+DLCF ci:
I I
I Link data field
II II
I
I
I I
I I
I
I - L
I i X.25 packet I
I
1
I
I
I
CIDIN
TRANSPORT
HEADER
I I
CIDIN MESSAGE (OR SEGMENT)
DATA
LINK
CNTL
FIELD
Annex 10 - Aeronautical Telecommunications Volume 111
8.6.5.1.2 The data link protocol level
8.6.5.1.2.1 X.25 packets to be transfeaed between two
CIDIN switching centres or a CIDIN switching centre and a
packet switched data nctwork shall be formatted into data link
frames.
8.6.5.1.2.2 Each data link frame shall consist of a data
link control field (DLCF), possibly followed by a link data
field, and shall be terminated by a frame check scquence and
flag (being thc second part of the DLCF). If a link data ficld
is present, the frame shall be denoted as an information frame.
8.6.5.1.2.3 X.25 packets shall be transmitted within the
link dala field of information frames. Only one packet shall be
contained in the link data field.
8.6.5.1.2.4 The data link protocol shall be as described
in 8.6.4 above.
8.6.5.1.3 THE X.25 PACKET PROTOCOL LEVEL (PERMANENT
VIRTUAL CIRCUiT PROCEDURE)
8.6.5.1.3.1 Each CIDIN packet Lo be transferred on
CDTN circuits between CIDIN switching centres shall bc
formatted into one X.25 packet. When a packet switched data
network is used to interconnect two CIDLN switching centres,
it shall be permissible to format the CIDIN packet into more
than one X.25 packet.
8.6.5.1.3.2 The integrity of each CIDIN packet shall be
preserved by the X.25 packet protocol by mapping each
CIDIN packet onto one complete X.25 packet sequence, as
defined in C C I n Recommendation X.25 (1981).
8.6.5.1.3.3 Each X.25 packet shall consist of an X.25
packet header, possibly followed by a user data field (UDE).
8.6.5.1.3.4 The X.25 packet protocol is based on the
application of permanent virtual circuit (PVC) procedures. A
permanent virtual circuit shall be defined as a logical path
between two CIDN switching centres. If a packct switched
data network is used to interconnect two CDIN switching
centres, the procedure shall provide full compatibility with the
procedures to be followed for PVCs according to C C m
Recommendation X.25 (1981). A packet switched data
network providing an X.25 interface to a CIDIN switching
centre may offer a number of options. The following options
shall be selected if available:
a) maximum user data field length of 256 octets; and
b) default window size of seven, or maximum available.
8.6.5.1.3.5 The X.25 packct procedures shall be as
described in 8.6.6.2 below.
8.6.5.1.4 THE CIDIN PACKET PROTOCOL LEVEL
8.6.5.1.4.1 Each transport header and the associated
segment shall be preceded by a CIDIN packet header. No
further segmentation of the CIDIN message shall be used
bctween transport protocol level and CIDW packet protocol
level. Both headers, therelore, shall be used in combination.
Together they shall bc referred to as the Communications
Control Field (CCF). Together with the message segment they
form CIDIN packe~sth at shall be transmitted from entry centre
to exit centre(s), when necessary through one or more relay
centres, as an entity.
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