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All drawings appearing in this Fascicle have been done in Autocad.
Recommendation Q.702
SIGNALLING DATA LINK
1 General
1.1 A signalling data link is a bidirectional transmission path for
signalling, comprising two data channels operating together in opposite
directions at the same data rate. It constitutes the lowest functional level
(level 1) in the Signalling System No. 7 functional hierarchy.
1.2 Functional configuration of a signalling data link is shown in Figure
1/Q.702.
The terms transmission channel and transmission link are used in Signalling
System No. 7 instead of transfer channel and transfer link used in Signalling
System No. 6.
1.3 A digital signalling data link is made up of digital transmission channels
and digital switches or their terminating equipment providing an interface to
signalling terminals. The digital transmission channels may be derived from a
digital multiplex signal at 1544, 2048 or 8448 kbit/s having a frame structure as
defined in Recommendation G.704 [1], or from digital multiplex streams having a
frame structure specified for data circuits (Recommendations X.50 [4], X.51 [5],
X.50 bis [6], X.51 bis [7]).
1.4 An analogue signalling data link is made up of voice-frequency analogue
transmission channels either 4 kHz or 3 kHz spaced, and modems.
1.5 Signalling System No. 7 is capable of operating over both terrestrial and
satellite transmission links1) .
Figure 1/Q.702 - CCITT 34980
1.6 The operational signalling data link shall be exclusively dedicated to the
use of a Signalling System No. 7 signalling link between two signalling points.
No other information should be carried by the same channel together with the
signalling information.
1.7 Equipment such as echo suppressors, digital pads, or A/m law convertors
attached to the transmission link must be disabled in order to assure full duplex
operation and bit integrity of the transmitted data stream.
1.8 64-kbit/s digital signalling channels entering a digital exchange via a
multiplex structure shall be switchable as semi-permanent channels in the
exchange.
1) The terms transmission channel and transmission link are used in Signalling System No.
7 instead of transfer channel and transfer link used in Signalling System No. 6.
PAGE1 Fascicle VI.7 - Rec. Q.702
2 Signalling bit rate
2.1 General
2.1.1 The standard bit rate on a digital bearer will be 64 kbit/s.
2.1.2 Lower bit rates may be adopted for each application, taking into account
the User Part requirements and the capability of available transmission links.
2.1.3 The minimum signalling bit rate for telephone call control applications
will be 4.8 kbit/s. For other applications such as network management, bit rates
lower than 4.8 kbit/s can also be used.
2.2 Use of bit rates lower than 64 kbit/s
2.2.1 For national telephone call control applications, use of Signalling System
No. 7 at bit rates lower than 64 kbit/s shall take account of the requirement to
minimize the answer signal delay when in-band line signalling systems are
involved (Recommendation Q.27 [8]).
2.2.2 Signalling System No. 7 can be used for direct international application
at bit rates lower than 64 kbit/s between countries which have no in-band line
signalling systems in their national extension networks (see S 2.1.3).
2.2.3 The possible use of Signalling System No. 7 at bit rates lower than 64
kbit/s between countries which have in-band line signalling systems in their
national extension networks is for further study.
3 Error characteristics and availability
Error characteristics and availability requirements will conform to
relevant Recommendations (for example, Recommendation G.821 [9] on digital
circuits). No additional characteristics or requirements will be specified in
this Recommendation.
4 Interface specification points
4.1 Interface requirements may be specified at one of three points, A, B or C
in Figure 2/Q.702. The appropriate point depends on the nature of transmission
links used and the approach toward the implementation of interface equipment
adopted by each Administration.
4.2 For the international application, interface requirements at either Point
B or Point C will apply.
4.3 Interface requirements for an international digital signalling data link
will be specified at Point C in accordance with the specific multiplex structure
used (see S 5.)
4.4 Interface requirements for an international analogue signalling data link
will be specified at Point B on a single channel basis, and thus are independent
of multiplex equipment used. (See S 6.)
4.5 Interface at Point A may or may not appear in particular implementations,
as each Administration may adopt different approaches towards the implementation
of interface equipment. If it does appear in implementations, then the interface
requirements specified in Recommendations V.10 [10], V.11 [11], V.24 [12], V.28
[13], V.35 [14], V.36 [15], X.24 [16] and G.703 [17] (for 64-kbit/s interface)
should be followed as appropriate.
4.6 Implementations which do not follow all the requirements in the relevant
Recommendations cited above should nevertheless take into account those
requirements that are specified for testing and maintenance actions which require
communication between the two ends of a data link. Interface requirements for
testing and maintenance are specified in Recommendation Q.707.
Figure 2/Q.702 - CCITT 34990
5 Digital signalling data link
5.1 Signalling data link derived from the 2048-kbit/s digital path
When a signalling data link is to be derived from a 2048-kbit/s digital
path, the following shall apply:
a) The interface requirements, specified at Point C in Figure 2/Q.702,
should comply with Recommendations G.703 [17] for the electrical
characteristics and G.704 [1] for the functional characteristics, in
particular the frame structure.
b) The signalling bit rate shall be 64 kbit/s.
c) The standard channel time slot for the use of a signalling data link is
time slot 16. When time slot 16 is not available, any channel time slot
available for 64-kbit/s user transmission may be used.
d) No bit inversion is performed.
5.2 Signalling data link derived from the 8448-kbit/s digital path
When a signalling data link is to be derived from a 8448-kbit/s digital
Fascicle VI.7 - Rec. Q.702 PAGE6
link, the following shall apply:
a) The interface requirements, specified at Point C in Figure 2/Q.702,
should comply with Recommendations G.703 [23] for the electrical
characteristics and G.704 [1] for the functional characteristics, in
particular the frame structure.
b) The signalling bit rate shall be 64 kbit/s.
c) The standard channel time slots for the use of a signalling data link
are time slots 67 to 70 in descending order of priority. When they are
not available, any channel time slot available for 64-kbit/s user
transmission may be used.
d) No bit inversion is performed.
5.3 Signalling data link derived from the 1544-kbit/s digital path
(For further study.)
Note - When a signalling bit rate of 64 kbit/s is adopted, the values of
bits should be inverted within the signalling terminal or the interface equipment
in order to meet the minimum mark density requirements of the Recommendation
G.733 [2] based PCM systems.
5.4 Signalling data link established over a digital path made up by digital
sections based on different digital hierarchies
When a signalling data link is to be established between networks based on
different digital hierarchies and speech encoding laws, the following shall
apply:
a) The interface requirements, specified at Point C in Figure 2/Q.702,
should comply with Recommendations G.703 [17] for the electrical
characteristics and G.802 [3] for other aspects, e.g., for interworking
arrangements.
b) The signalling bit rate shall be 64 kbit/s.
c) No bit inversion is performed.
5.5 Signalling data link established over data circuits
When a signalling data link is to be established over data circuits
derived from a 64-kbit/s digital stream having a frame structure as specified in
such Recommendations as X.50 [10], X.51 [11], X.50 bis [12] and X.51 bis [13] the
following shall apply:
a) The interface requirements, specified at Point C in Figure 2/Q.702,
should comply with relevant requirements in one of the above-mentioned
Recommendations, applicable to the environment of the intended use.
b) When 64-kbit/s multiplexed streams are carried on 2048-kbit/s or
1544-kbit/s digital links, Recommendation G.704 [1], should apply.
6 Analogue signalling data link
6.1 Signalling bit rate
6.1.1 Applications of the analogue signalling data link must take account of the
delay requirements described in S 2.2.
6.1.2 For telephone call control applications, the signalling bit rate over an
analogue signalling data link shall be higher or equal to 4.8 kbit/s.
6.2 Interface requirements
In case of 4.8-kbit/s operation, interface requirements specified at the
interface point B in Figure 2/Q.702 should comply with relevant requirements
specified for 4.8-kbit/s modems in Recommendations V.27 [18] and V.27 bis [19].
In addition, the following shall apply:
a) Application of either Recommendations V.27 [18] or V.27 bis [19]
depends on the quality of the analogue transmission channels used.
Recommendation V.27 [18] shall apply only to transmission channels
conforming to Recommendation M.1020 [20], while Recommendation V.27 bis
[19] to transmission channels conforming to Recommendation M.1020 [20]
or of lower quality.
b) Full duplex operation over a 4-wire transmission link should be
adopted.
c) If a separate modem is to be used, the interface requirements specified
in Recommendations V.10 [10], V.11 [11], V.24 [12] and V.28 [13],
applicable at Point A in Figure 2/Q.702, should be followed as much as
possible.
PAGE1 Fascicle VI.7 - Rec. Q.702
References
[1] CCITT Recommendation Functional characteristics of interfaces associated
with network nodes, Vol. III, Rec. G.704.
[2] CCITT Recommendation Characteristics of primary PCM multiplex equipment
operating at 1544 kbit/s, Vol. III, Rec. G.733.
[3] CCITT Recommendation Interconnection of digital paths using different
techniques, Vol. III, Rec. G.802.
[4] CCITT Recommendation Fundamental parameters of a multiplexing scheme for
the international interface between synchronous data networks, Vol. VIII,
Rec. X.50.
[5] CCITT Recommendation Fundamental parameters of a multiplexing scheme for
the international interface between synchronous data networks, Vol. VIII,
Rec. X.51.
[6] CCITT Recommendation Fundamental parameters of a 48-kbit/s user data
signalling rate transmission scheme for the international interface
between synchronous data networks, Vol. VIII, Rec. X.50 bis.
[7] CCITT Recommendation Fundamental parameters of a 48-kbit/s user data
signalling rate transmission scheme for the international interface
between synchronous data networks using 10-bit envelope structure, Vol.
VIII, Rec. X.51 bis.
[8] CCITT Recommendation Transmission of the answer signal, Vol. VI, Rec.
Q.27.
[9] CCITT Recommendation Error performance on an international digital
connection forming part of an integrated services digital network, Vol.
III, Rec. G.821.
[10] CCITT Recommendation Electrical characteristics for unbalanced
double-current interchange circuits for general use with integrated
circuit equipment in the field of data communications, Vol. VIII, Rec.
V.10.
[11] CCITT Recommendation Electrical characteristics for balanced
double-current interchange circuits for general use with integrated
circuit equipment in the field of data communications, Vol. VIII, Rec.
V.11.
[12] CCITT Recommendation List of definitions for interchange circuits between
data-terminal equipment and data circuit-terminating equipment, Vol. VIII,
Rec. V.24.
[13] CCITT Recommendation Electrical characteristics for unbalanced
double-current interchange circuits, Vol. VIII, Rec. V.28.
[14] CCITT Recommendation Data transmission at 48 kbit/s per second using
60-108 kHz group band circuits, Vol. VIII, Rec. V.35.
[15] CCITT Recommendation Modems for synchronous data transmission using 60-108
kHz group band circuits, Vol. VIII, Rec. V.36.
[16] CCITT Recommendation List of definitions for interchange circuits between
data terminal equipment (DTE) and data circuit-terminating equipment (DCE)
on public data networks, Vol. VIII, Rec. X.24.
[17] CCITT Recommendation Physical/electrical characteristics of hierarchical
digital interfaces, Vol. III, Rec. G.703.
[18] CCITT Recommendation 4800 bit/s per second modems with manual equalizer
standardized for use on leased telephone-type circuits, Vol. VIII, Rec.
V.27.
[19] CCITT Recommendation 4800/2400 bit/s per second modem with automatic
equalizer standardized for use on leased telephone-type circuits, Vol.
VIII, Rec. V.27 bis.
[20] CCITT Recommendation Characteristics of special quality international
leased circuits with special bandwidth conditioning, Vol. IV, Rec. M.1020.
Fascicle VI.7 - Rec. Q.702 PAGE6