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with other graphic and control characters to provide
additional communication control functions.
8.6.3.3 Three system categories are specified in terms of
their respective circuit characteristics. terminal configurations,
and message transfer procedures as follows:
System category A: two-way alternate, multipoint allowing
either centralized or non-centralized operation and single
ox multiple message-oriented information transfers without
replies (but with delivery verification).
System category B: two-way simultaneous, point-to-point
employing message associated blocking and modulo 8
numbering of blocks and acknowledgements.
System category C: two-way alternate, multipoint allowing
only centralized (computer-to-terminal) operation, single or
multiple message transfers with replies.
8.6.3.3.1 In addition to the characteristics prescribed in
the paragraphs that follow for both system categories A and B,
other parameters that shall be accounted for in order to ensure
viable, operationally reliable communications include:
a) the number of SYN characters required to establish
and maintain synchronization;
Note.- Normally rhe transmitting station sends
three contiguous SYN uhurucrers und the receiving
station detects at leust two before any action is taken.
7/11/96
No. 71
Annex 10 - Aeronaulical ~e'elecommunications Volume XU
b) the values of system time-outs for such functions as
"idle line" and "no response" as well as the number of
automatic retries that are to be attempted before
manual intervention is signalled;
c) he composition of prefixes within a 15 character
maximurn.
Note.- By agreement between the administrations
concerned, it is permissible for supervisory signuls to
contain a stution identification prefix using charucters
selected from colztmns 4 through 7 of lA-5.
8.6.3.3.2 Recommendation.- For multipoint implernenrations
design.ed to permit only centralized (computer-toterminal)
operations, the provisions of 8.6.3.7 should be
employed.
8.6.3.4 BLOCK CHECK CHARACTER
8.6.3.4.1 Both system category A and B shall utilize a
block check character to determine the validity of a transmission.
8.6.3.4.2 The block check character shall be composed
of 7 bits plus a parity bit.
8.6.3.4.3 Each of the first 7 bits of the block check
character shall be the modulo 2 binary sum of every element
in the same bit 1 to bit 7 column of the successive characters
of the transmitted block.
8.6.3.4.4 Thc longitudinal parity of each column of the
block, including the block check character, shall be even.
8.6.3.4.5 The sense of the parity bit of the block check
character shall be the same as for the information characters
(see 8.6.1.2.3 above).
8.6.3.4.6.1 The summation to obtain the block check
character shall be started by the first appearance of either SOH
(start of heading) or STX (start of text).
8.6.3.4.6.2 The starting character shall not be included
in the summation.
8.6.3.4.6.3 If an STX character appears after the summation
has been started by SOH, then the STX character shall
be included in the summation as if it were a text character.
8.6.3.4.6.4 With the exception of SYN (synchronous
idle), all the characters which are transmitted after the start of
the block check summation shall be included in the
summation, including the ETB (end of transmission/block) or
ETX (end of text) control character which signals that the
following character is the block check character.
8.6.3.4.7 NO character, SYN or otherwise, shall be
inserted between the ETB or ETX character and the block
check character.
8.6.3.5 Description of system category A. System
category A is one in which a number of stations are connected
by a multipoint link and one station is permanently designated
as the control station which monitors the link at all times to
ensure orderly operation.
8.6.3.5.1.1 To establish the link for transmission, the
control station shall either:
a) poll one of the tributary stations to assign it master
status; or
b) assume master status and select one or more tributary
(slave) stations to receive a transmission.
8.6.3.5.1.2 Polling shall be accomplished by the control
station sending a polling supervisory sequence consisting of a
prefix identifying a single tributary station and ending in ENQ.
8.6.3.5.1 -3 A tribut'uy station detecting its assigned
polling supervisory sequence shall assume master status and
respond in one of two ways:
a) if the station has a message to send, it shall initiate a
selection supervisory sequence as described in
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