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All drawings appearing in this Fascicle have been done in Autocad.
Recommendation Q.709
HYPOTHETICAL SIGNALLING REFERENCE CONNECTION
1 Introduction
This Recommendation specifies how the elements of a signalling connection
are combined to meet the signalling requirements of the networks that it
supports. Included are parameters for signalling transfer delay in both national
and international networks, and overall signalling delay that such combinations
will produce, together with the availability required, to enable the performance
of the network served by the signalling network to be maintained.
A probabilistic approach is been taken, i.e., limits are specified for
mean and 95% of connections. These figures will apply to the normal operation of
a signalling network. No consideration is given to the "unusually long"
signalling paths that are found in some signalling networks. Any unusual routing
caused by some network structures and/or reconfigurations due to network failure
are considered to be covered in the remaining 5% of connections.
The hypothetical signalling reference connection (HSRC) for international
working is specified in this Recommendation by defining the constituent parts of:
i) the international section,
ii) the national section.
In any combination of those sections to produce an overall hypothetical
signalling reference connection, it is necessary to consider what impact each of
the component parts (international and two national sections) have on each other
and the full hypothetical signalling reference connection. This means that
certain national or international limits such as the maximum number of signalling
transfer points allowed in a signalling relation (see Recommendation Q.705, S
5.2) require modification and account of this has been taken in this
Recommendation.
2 Requirements of networks served by the signalling connection
To meet the requirements of services carried on the network served by the
signalling network, the signalling connection performance should be closely
aligned with those requirements. Since these services are ultimately to be
carried on an ISDN, the hypothetical signalling reference connection is based
upon the hypothetical reference connection produced for that network
(Recommendation G.801).
However, for a considerable time the majority of services in the network
served by the signalling network will be telephony-based and account must
therefore be taken of the reference connection for conventional telephony
application (Recommendation G.101).
3 Hypothetical signalling reference connection components for
link-by-link signalling
3.1 General
The components of an hypothetical signalling reference connection are
signalling points and STPs which are connected in series by signalling data links
to produce a signalling connection (Note 1). The number of signalling points and
STPs depend on the size of the network. Two limits are prescribed to cover mean
or 95% cases. Separate cases are allowed for large countries and average sized
countries (Note 2). This section outlines the considerations involved in
formulating a hypothetical signalling reference connection for link-by-link
signalling and details the number of hypothetical signalling reference connection
components and the delays they produce.
Note 1 - The term signalling point is used to designate use of the user
function in a signalling point: whether or not STP function is presented
irrelevant in this context. The term STP is used to designate use of the STP
function in a signalling point: whether or not user function is present is
irrelevant in this context.
Fascicle VI.7 - Rec. Q.709 PAGE1
Note 2 - When the maximum distance between an international switching
centre and a subscriber who can be reached from it does not exceed 1000 km or,
exceptionally, 1500 km, and when the country has less than n x 107 subscribers,
the country is considered as of average size. A country with a larger distance
between an international switching centre and a subscriber, or with more than n x
107 subscribers, is considered as of large size. (The value of n is for further
study.)
3.1.1 Number of signalling points in the hypothetical signalling reference
connection
The number of signalling points in the hypothetical signalling reference
connection has been determined by considering the maximum number of links allowed
by the Telephone Routing Plan (Q.13/E.171). These Recommendations define "last
choice" backbone routes and only a small proportion of traffic take these routes.
Traffic generated in metropolitan areas, generally the largest source of traffic,
usually takes far fewer links to an international switching centre. Even for
rural areas a connection to the international switching centre will not generally
be required to follow the backbone route.
Limitation of the number of signalling points required will reduce the
signalling delay, considering that signalling point delay, forms the largest
component of signalling delay.
3.1.2 Number of STPs in an hypothetical signalling reference connection
The number of STPs in the hypothetical signalling reference connection is
a function of the number of signalling points, and the signalling network
topology used to connect these signalling points. The number of STPs should be
kept to a minimum in order to limit the signalling delay. In some signalling
relationship, associated signalling may be used for which no STPs are required.
In others, one or more STPs may be used. For international signalling
relationship, it is recommended that no more than 2 STPs be used in a signalling
relation. (See Recommendation Q.705, S 5.2.)
3.1.3 Signalling network availability
The availability of a signalling connection is an important network
parameter. It is necessary for the availability to be significantly better than
the availability of the component being controlled (e.g. a circuit). A figure of
10 minutes down time per year maximum unavailability is recommended for any
particular signalling route set (Recommendation Q.706, S 1.1).
This corresponds to an availability of 0.99998, which can be achieved by
the use of suitable network redundancies.
3.1.4 Signalling message transfer delay
Signalling message transfer delay is another important network parameter.
It affects call set up delay and also affects network response time to service
requests made during a call. In this Recommendation, the transmission propagation
delays are not included (see S 7.2).
3.2 International component of hypothetical signalling reference connection
The international component of the hypothetical signalling reference
connection includes all international signalling points in the connection and the
STPs carrying signalling messages between the signalling points. The maximum
number of signalling points and STPs allowed are listed in Table 1/Q.709.
The unavailability of the overall international component of the
signalling network should not exceed the following totals per year for both the
50 and 95 percent cases.
- 20 minutes for large country to large country,
- 30 minutes for large country to average-sized country, and
- 40 minutes per year for average-sized country to average-sized country.
TABLE 1/Q.709
Maximum number of signalling points and STPs in international component
Country size (Note) Percent of Number of Number of
connections STPs signalling
points
Large mean 3
to 3
Large 95 4
Large mean 4
to 4
Average-sized 95 4
Average-sized mean 5
to 5
Average-sized 95 7
PAGE1 Fascicle VI.7 - Rec. Q.709
Note See Note 2 to S 3.1.
The maximum signalling transfer delay under normal conditions for the
international component of a connection should not be worse than the values
listed in Table 2/Q.709.
3.3 National components of hypothetical signalling reference connection
The national components of the hypothetical signalling reference
connection includes all national exchanges in the connection (but does not
include the international switching centre in the country) and all STPs carrying
signalling messages between the national exchanges and between the highest level
national exchange and the international switching centre. The maximum number of
signalling points and STPs allowed are listed in Table 3/Q.709.
TABLE 2/Q.709
Maximum signalling delays for international component
Delay (Note) (ms)
Country size Percent of Message type
connections
Simple Processing
(e.g. answer) intensive
(e.g. IAM)
Large mean 390 600
to
Large 95 410 620
Large mean 520 800
to
Average-sized 95 540 820
Average-sized mean 650 1000
to
Average-sized 95 690 1040
Note - Only the mean delay component from Table 4/Q.706, Table 3/Q.725 and
Table 1/Q.766 have been used in calculating the delay. Further study is
required, e.g. for the mean values as well as the inclusion of overload
and/or 95 percentile cases of each component value.
Fascicle VI.7 - Rec. Q.709 PAGE1
TABLE 3/Q.709
Maximum number of signalling points and STPs in national components
Country size (Note 1) Percent of Number of Number of
connections STPs signalling
points
mean 3 3
Large
95 4 4
mean 2 2
Average-sized
95 3 3
Note 1 - See Note 2 to S 3.1.
Note 2 - The values in this Table are provisional. (A higher number of
signalling points and/or STPs might be included in a national network,
e.g. in the case that a two-level hierarchical signalling network is
adopted. This matter is for further study.)
The unavailability of each of the overall national components of the
signalling network should not exceed the following totals per year:
- 20 minutes for mean case of average-sized countries,
- 30 minutes for 95 percent case of average-sized countries and mean case
PAGE1 Fascicle VI.7 - Rec. Q.709
of large countries, and
- 40 minutes for 95 percent case of large countries.
Note 1 - Although the signalling component of the international switching
centre in the country was not included in Table 3/Q.709, it is included in the
unavailability objectives.
Note 2 - The hypothetical signalling reference connection define a unique
path through the national and international networks, therefore when considering
the overall unavailability of each national component, no account is taken of any
standby path, if provided, in that national network. The values given are based
on those for each component route-set as specified in Recommendation Q.706, S
1.1. They are provisional and for further study.
The maximum signalling transfer delay under normal conditions for each of
the national components of a connection should not be worse than the values
listed in Table 4/Q.709.
TABLE 4/Q.709
Maximum signalling delays for each national component
Delay (Notes 1 and 2) (ms)
Country size Percent of Message type
connections
Simple Processing
(e.g. answer) intensive
(e.g. IAM)
mean 390 600
Large
95 520 800
mean 260 400
Average-sized
95 390 600
Note 1 - See Note to Table 2/Q.709.
Note 2 - The delay does not include any delay for the International
Switching Centre in the country, which is included in the international
component.
Fascicle VI.7 - Rec. Q.709 PAGE1
4 Overall signalling delay for link-by-link signalling
From the hypothetical signalling reference connection and the values of
message transfer times given for signalling point and STP, the overall signalling
delay due to signalling point, and STP delays can be determined from Tables 2 and
4 of this Recommendation, for a given load in a given network. Average delays and
95 percentile delays are given in Table 5/Q.709 for various combinations of large
and average-sized countries. Average signalling point and STP delays at normal
loading are assumed.
These values must be increased by the transmission propagation delays (see
Table 1/Q.41).
TABLE 5/Q.709
Maximum overall signalling delays
Delay (Note) (ms)
Country size Percent of Message type
connections
Simple Processing
(e.g. answer) intensive
(e.g. IAM)
Large mean 1170 1800
to
Large 95 1450 2220
Large mean 1170 1800
to
Average-sized 95 1450 2220
Average mean 1170 1800
to
Average-sized 95 1470 2240
Note - See Note to Table 2/Q.709.
5 Hypothetical signalling reference connection (HSRC) components
for end-to-end signalling
5.1 General
The components of a hypothetical signalling reference connection are
signalling end points (SEP), signalling points with SCCP relay function (SPR) and
STPs which are connected in series by signalling data links to produce an
end-to-end signalling connection (Note 1). The number of the various signalling
nodes depends on the size of the network. Two limits are prescribed to cover mean
or 95% cases. Separate cases are allowed for large countries and average-sized
countries (Note 2). This section outlines the considerations involved in
formulating a hypothetical signalling reference connection and details the number
of hypothetical signalling reference connection components and the delays they
produce.
Note 1 - a) Signalling End Point (SEP) - This includes processing in
UP/AP (User part/application part), SCCP (Signalling connection
control part), MTP (Message transfer part) and also
MTP-SCCP-UP/AP
b) Signalling Point with SCCP relay function (SPR) - This includes
only processing in MTP-SCCP-MTP
c) Signalling Transfer Point - This includes processing in MTP
exclusively.
Note 2 - When the maximum distance between an international switching
centre and a subscriber who can be reached from it does not exceed 1000 km or,
exceptionally, 1500 km, and when the country has less than n x 107 subscribers,
the country is considered as of average size. A country with a larger distance
between an international switching centre and a subscriber, or with more than n x
107 subscribers is considered as of large size. (The value of n is for further
study.)
5.1.1 Number of signalling nodes in the end-to-end HSRC
PAGE1 Fascicle VI.7 - Rec. Q.709
The same signalling network is used for end-to-end messages and
link-by-link messages. This means that the maximum number of signalling nodes is
equal in both cases. The maximum number of signalling nodes from the originating
node to the destination node is 18 in 50 percent of the connections and 23 in 95
percent of the connections except for average-sized to average-sized country. In
that case the value is 24.
In general a fast transfer of end-to-end signalling messages has to be
required. For such messages a route with a minimum number of signalling transfer
and relay points is highly desirable.
It is desirable to use the message routing of the MTP (STP functions) as
far as possible and trying in this way to avoid processing in higher layers (SCCP
or user functions).
5.1.2 Signalling network availability
The availability of a signalling connection is an important network
parameter. It is necessary for the availability to be significantly better than
the availability of the component being controlled (e.g. a circuit). A figure of
ten minutes down time per year maximum unavailability is recommended for any
particular signalling route set (Recommendation Q.706, S 1.1).
This corresponds to an availability of 0.99998, which can be achieved by
the use of suitable network redundancies.
5.1.3 Signalling message transfer delay
Signalling message transfer delay is another important network parameter.
It affects call set up delay and also affects network response time to service
requests made during a call.
The use of signalling points with SCCP relay functions (SPR) should be
kept to a minimum. In an SPR additional processing is performed which causes an
additional delay, for example address translation for CR or UDT message types
(processing intensive messages) or a local reference message mapping for CC or DT
messages (processing simple message types). The cross office transit time for SPR
is defined in Q.716. The cross-office transit time for an SEP is equal to Tcu in
Q.766 or Q.725 and for an STP is equal to Tcs in Q.706.
5.2 International component of hypothetical signalling reference connection
The international component of the hypothetical signalling reference
connection includes all international signalling nodes (e.g. SPR and STP) in the
connection. The maximum number of SPRs and STPs allowed are listed in Table
6/Q.709.
TABLE 6/Q.709
Maximum number of SPRs and STPs in international component
Country size Percent of Number of Number of
connections STPs SPRs
Large mean 4 2
to
Large 95 4 3
Large mean 6 2
to
Average-sized 95 6 3
Average-sized mean 8 2
to
Average-sized 95 8 4
The unavailability of the overall international component of the
signalling network should not exceed the following totals per year for both the
50 and 95 percent cases:
- 20 minutes for large country to large country;
- 30 minutes for large country to average-sized country, and
- 40 minutes per year for average-sized country to average-sized country.
The maximum delay at the signalling nodes under normal conditions for the
international component of a connection should not be worse than the values
listed in Table 7/Q.709.
TABLE 7/Q.709
Maximum delay at the signalling nodes for international component
Delay (ms)
Country size Percent of Message type
connections
Fascicle VI.7 - Rec. Q.709 PAGE1
Processing Processing
simple intensive
Large mean 300 440
to
Large 95 410 620
Large mean 340 480
to
Average-sized 95 450 660
Average-sized mean 380 520
to
Average-sized 95 600 880
Note 1 - The maximum signalling nodes delay is the sum of all cross-office
delays involved.
Note 2 - All values are provisional.
5.3 National components of hypothetical signalling reference connections
The national components of the hypothetical signalling reference
connection includes all national signalling nodes (e.g., SEP, SPR, STP) in the
connection (but does not include the international switching centre in the
country). The maximum number of SEPs, SPRs and STPs allowed are listed in Table
8/Q.709.
The unavailability of each of the overall national components of the
signalling network should not exceed the following totals per year:
- 20 minutes for mean case of average-sized countries;
- 30 minutes for 95 percent case of average-sized countries and mean case
of large countries, and
- 40 minutes for 95 percent case of large countries.
TABLE 8/Q.709
Maximum number of SEPs, SPRs and STPs in national component
Country size Percent of Number of Number pf Number of
connections STPs SPRs SEPs
mean 4 1 1
Large
95 5 2 1
PAGE1 Fascicle VI.7 - Rec. Q.709
Average-sized mean 2 1 1
95 4 1 1
Note 1 - Although the signalling component of the international switching
centre in the country is not included in Table 8/Q.709, it is included in the
unavailability objectives.
Note 2 - The hypothetical signalling reference connection defines a unique
path through the national and international networks, therefore when considering
the overall unavailability of each national component, no account is taken of any
standby path, if provided, in that national network. The values given are based
on those for each component route-set as specified in Recommendation Q.706, S
1.1.
The maximum delay at the signalling nodes under normal conditions for each
of the national components of a connection should not be worse than the values
listed in Table 9/Q.709.
TABLE 9/Q.709
Maximum delay at the signalling nodes for each national component
Delay (ms)
Country size Percent of Message type
connections
Processing Processing
simple intensive
mean 300 440
Large
95 430 640
mean 260 400
Average-sized
95 300 440
Note 1 - The maximum signalling nodes delay is the sum of all cross-office delays
involved.
Note 2 - All values are provisional.
6 Overall signalling delay for end-to-end signalling
The link-by-link signalling delay is applicable where messages are
processed by each signalling point (e.g. during call establishment). The use of
end-to-end signalling intended to reduce the overall signalling delay.
From the hypothetical signalling reference connection and the values of
message transfer times given for SEPs, SPRs and STPs, the overall signalling
delay due to the node delays can be determined from Tables 7 and 9 of this
Recommendation, for a given load in a given network. Average delays and 95
percentile delays are given in Table 10/Q.709 for various combinations of large
and average-sized countries. Average signalling node delays at normal loading are
assumed.
TABLE 10/Q.709
Maximum overall delay at the signalling nodes
Delay (ms)
Country size
Fascicle VI.7 - Rec. Q.709 PAGE1
Percent of Message type
connections
Processing Processing
simple intensive
Large mean 900 1320
to
Large 95 1270 1900
Large mean 900 1320
to
Average-sized 95 1180 1740
Average-sized mean 9000 1320
to
Average-sized 95 1200 1760
Note 1 - The maximum signalling nodes delay is the sum of all cross-office delays
involved.
Note 2 - All values are provisional.
7 Remarks
7.1 The above values for signalling delays assumes a message length
distribution as given in Table 2/Q.706 and Table 2/Q.725, with a mean message
length of 15 octets. However, a message length of e.g. 128 octets for SCCP user
data in CR and CC messages and 255 octets for SCCP user data in DT messages are
permissible. For such a message length the transmission time at 64 kbit/s is, in
each signalling node, about 15 ms (128 octets) to 30 ms (255 octets) longer.
7.2 When defining an overall signalling delay the propagation delay must be
included. This delay cannot be completely neglected due to the geographical size
of the HSRC (see Table 1/Q.41).
PAGE1 Fascicle VI.7 - Rec. Q.709
