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All drawings contained in this Recommendation have been done in Autocad.
Recommendation Q.766
PERFORMANCE OBJECTIVES IN THE INTEGRATED
SERVICES DIGITAL NETWORK APPLICATION
1 Introduction
This Recommendation gives the requirements of the Integrated Services
Digital Network (ISDN) application call control service supported by Signalling
System No. 7.
In Recommendation Q.706, the Message Transfer Part performance is
described. The Message Transfer Part supports the ISDN application of Signalling
System No. 7 and provision of a signalling network to support the ISDN
application must take account of the performance of the Message Transfer Part and
the requirements of the ISDN application. For example, taking account of the
message transfer times in Recommendation Q.706 and the requirements for message
transfer times between two ISDN exchanges, a figure may be derived for the total
permissible number of signalling links in tandem for a particular call.
2 Signalling availability
2.1 Signalling route set availability
The availability of a signalling route set is determined by the
availability of the individual components of the signalling network (signalling
links and the signalling points) and by the structure of a signalling network.
The availability of a signalling route set should not be less than
0.99998, corresponding to a downtime of 10 minutes per year for a user signalling
relation.
2.2 Signalling network availability
The availability of the signalling network should be sufficiently high as
to meet the signalling route set downtime objectives stated in S 2.1. The
signalling network architecture selected will strongly influence the
availability. In general, the greater the number of link sets in tandem in a
signalling route set the more redundant signalling paths that will be needed to
meet the availability objective for the signalling route set or user signalling
relation.
3 Signalling dependability
3.1 General
The ISDN application is different from other applications, such as
telephony and data, in that there may be multiple paths involved for any given
ISDN call. There may be several circuits (e.g. telephone conferencing) for either
telephony or data and non-circuit related connections for access to data bases or
for terminal-to-terminal control. This diverse set of uses may require closer
control of the signalling network resources than might be required for other more
simple applications.
3.1.1 Probability of false operation
By means of error detection (see Recommendation Q.703) as well as
transmission fault indication (see Recommendations G.732 [1] and G.733 [2], it is
ensured that, overall, not more than one in 108 of all signal units transmitted
is accepted that, due to errors, will cause false operation.
Fascicle VI.8 - Rec. Q.766 PAGE1
3.2 Probability of signalling malfunction
Unsuccessful calls may be caused by undetected errors, loss of messages,
or messages delivered out of sequence (during emergency situations within the
signalling network) and may result in:
- incomplete call set-up,
- misrouted calls (e.g. connection of wrong numbers),
- calls routed correctly but mishandled (e.g. false clearing),
- inability to access a data base.
Considering the above conditions and the performance for the Message
Transfer Part, no more than 2 in 105 (provisional value) of all ISDN calls should
be unsuccessful due to signalling malfunction.
Note - No more than 1 in 105 of all ISDN circuit connections should be
unsuccessful due to signalling malfunction.
4 Signalling delay
4.1 Functional reference points and transfer time components
Figure ccitt-35600
4.2 Delays
4.2.1 cross-office transfer time, Tcu
Tcu is the period which starts when the last bit of the signal unit leaves
the incoming signalling data link and ends when the last bit of the signal unit
enters the outgoing signalling data link for the first time. It therefore
includes the queuing delay in the absence of disturbances but not the additional
queuing delay caused by retransmission.
4.2.2 user handling time, Thu
Thu is the period which starts when the last bit of the message has
entered the upper layer functions and ends when the last bit of the derived
message has left the upper layer functions.
4.2.3 Objectives for cross-office transfer time, Tcu
The figures in Table 1/Q.766 are the objectives for the cross-office
transfer time Tcu for the ISDN signalling points in the signalling network. These
figures are related to a signalling bit rate of 64 kbit/s.
TABLE 1/Q.766 1)
Cross-office
transfer time Tcu
Message type Exchange call (ms)
attempt loading
Mean 95%
Normal 110 220
Simple (e.g., answer) +15% 165 330
+30% 275 550
Processing intensive Normal 180 360
(e.g., AM) +15% 270 540
+30% 450 900
1) Provisional values.
A processing intensive message is one that arrives at an exchange and
requires detailed examination (and possibly modification) before it is
transmitted to the next exchange.
A simple message is one that requires little or no examination or
modification (typically only label translation) before it is transmitted to the
next exchange.
4.3 Effect of retransmission
PAGE1 Fascicle VI.8 - Rec. Q.766
As a consequence of correction by retransmission, not more than one in 104
signals should be delayed more than 300 ms as a long-term average. This
requirement refers to each signalling link.
This requirement is laid down in order to ensure satisfactory answer
delays.
5 Signalling system limitations
5.1 Labelling potential
5.1.1 Signalling points
The label of the Signalling System No. 7 for the ISDN application provides
the potential to identify 16 384 signalling points.
5.1.2 Number of circuits in a user signalling relation
There may be up to 4096 circuits (4096 channels in each direction) for
each user signalling relation.
5.1.3 Number of SCCP connections in a user signalling relation
There may be up to 224 SCCP connections available at an ISDN signalling
point. All of these may be available for any given user signalling relation, but
must be shared over all signalling relations.
5.2 Number of ISDN call identities at a signalling point
There may be up to 224 (value for further study) simultaneous ISDN calls at
a signalling point with the 224 call identities available. The use of ISDN call
identities is for further study.
References
[1] CCITT Recommendation Characteristics of primary PCM multiplex equipment
operating at 2048 kbit/s, Rec. G.732.
[2] CCITT Recommendation Characteristics of primary PCM multiplex equipment
operating at 1544 kbit/s, Rec. G.733.
Fascicle VI.8 - Rec. Q.766 PAGE1