InfoMagic Standards 1994 January/InfoMagic Standards - January 1994.iso/ccitt/1988/troff/6_7_08.tro

Text File | 1991-12-13 | 98KB | 3,676 lines

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.nr LL 40.5P
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.rs
\v | 5i'
.sp 2P
.LP
\fBRecommendation\ Q.705\fR
.RT
.sp 2P
.sp 1P
.ce 1000
\fBSIGNALLING\ NETWORK\ STRUCTURE\fR
.EF '%    Fascicle\ VI.7\ \(em\ Rec.\ Q.705''
.OF '''Fascicle\ VI.7\ \(em\ Rec.\ Q.705    %'
.ce 0
.sp 1P
.LP
\fB1\fR     \fBIntroduction\fR
.sp 1P
.RT
.PP
This Recommendation describes aspects which are pertinent to and
should be considered in the design of international signalling networks.
Some or all of these aspects may also be relevant to the design of national
networks. Some aspects are dealt with for both international and national
networks (e.g.\ availability), others are discussed in the context of the
international network only (e.g.\ number of \fIsignalling transfer points\fR
in a
signalling relation). A number of aspects require further study for national
networks. This Recommendation also gives in Annex\ A examples of how the
.PP
signalling network procedures may be applied to the mesh network
representation.
.PP
The national and international networks are considered to be
structurally independent and, although a particular \fIsignalling point\fR may
belong to both networks, signalling points are allocated \fIsignalling
point\fR
\fIcodes\fR according to the rules of each network.
.PP
The signalling network procedures are provided in order to effectively
operate a signalling network having different degrees of complexity. They
provide for reliable message transfer across the network and for
reconfiguration of the network in the case of failures.
.PP
The most elementary signalling network consists of \fIoriginating and\fR
\fIdestination signalling points\fR connected by a single \fIsignalling
link\fR . To
meet availability requirements this may be supplemented by additional links
in parallel which may share the signalling load between them. If, for all
signalling relations, the originating and destination signalling points are
directly connected in this way in a network then the network operates in the
\fIassociated mode\fR .
.PP
For technical or economic reasons a simple associated network may not be
suitable and a \fIquasi\(hyassociated network\fR may be implemented in
which the
information between originating and destination signalling points may be
transferred via a number of signalling transfer points. Such a network may
be represented by a \fImesh network\fR such as that given in Annex\ A, as other
networks are either a subset of the mesh network or are structured using
this network or its subsets as components.
.RT
.sp 2P
.LP
\fB2\fR     \fBNetwork components\fR
.sp 1P
.RT
.sp 1P
.LP
2.1
    \fISignalling links\fR
.sp 9p
.RT
.PP
Signalling links are basic components in a signalling network
connecting together signalling points. The signalling links encompass the
\fIlevel\ 2\fR functions which provide for message error control (detection and
subsequent correction). In addition, provision for maintaining the correct
message sequence is provided (see Recommendation\ Q.703).
.RT
.sp 1P
.LP
2.2
    \fISignalling points\fR
.sp 9p
.RT
.PP
Signalling links connect signalling points at which signalling
network functions such as message routing are provided at \fIlevel\ 3\fR and at
.PP
which the user functions may be provided at \fIlevel\ 4\fR if it is also an
originating or destination point (see Recommendation\ Q.704, \(sc\ 2.4).
.PP
A signalling point that only transfers messages from one signalling
link to another at level\ 3 serves as a signalling transfer point (STP).
.PP
The signalling links, signalling transfer points, and signalling
(originating or destination) points may be combined in many different ways
to form a \fIsignalling network\fR .
.RT
.sp 2P
.LP
\fB3\fR     \fBStructural independence of international and national
signalling networks\fR
.sp 1P
.RT
.PP
The worldwide signalling network is structured into two
functionally independent levels, namely the international and national
levels, as illustrated in Figure\ 1/Q.705. This structure makes possible
a clear
division of responsibility for signalling network management and allows
numbering plans of
signalling points
of the international network and the different national networks to be
independent of one another.
.bp
.RT
.LP
.rs
.sp 23P
.ad r
\fBFigure 1/Q.705. p.\fR
.sp 1P
.RT
.ad b
.RT
.PP
A signalling point (SP), including a signalling transfer point (STP), may
be assigned to one of three categories:
.LP
    \(em
     national signalling point (NSP) (signalling transfer point) which belongs
to the national signalling network only (e.g.\ NSP\d1\u) and is identified
by a signalling point code (OPC or DPC) according to the national numbering
plan of signalling points;
.LP
    \(em
    international signalling point (ISP) (signalling transfer
point) which belongs to the international signalling network only
(e.g.\ ISP\d3\u) and is identified by a signalling point code (OPC or DPC)
according to the international numbering plan of signalling points;
.LP
    \(em
    a node that functions both as an
international signalling   point
(signalling transfer point) and a
national signalling point
(signalling transfer point) and therefore belongs to both the international
signalling network and a national signalling network and accordingly is
identified by a specific signalling point code (OPC or DPC) in each of the
signalling networks.
.PP
If a discrimination between international and national signalling
point codes is necessary at a signalling point, the network indicator is
used (see Recommendation\ Q.704, \(sc\ 14.2).
.sp 2P
.LP
\fB4\fR     \fBConsiderations common to both international and national
signalling networks\fR
.sp 1P
.RT
.sp 1P
.LP
4.1
    \fIAvailability of the network\fR
.sp 9p
.RT
.PP
The signalling network structure must be selected to meet the most stringent
availability requirements of any User Part served by a specific
network. The availability of the individual components of the network
signalling links, (signalling points, and signalling transfer points) must
be considered in determining the network structure (see
Recommendation\ Q.709).
.RT
.sp 1P
.LP
4.2
    \fIMessage transfer delay\fR
.sp 9p
.RT
.PP
In order to take account of signalling message delay
considerations, regard should be given, in the structuring of a particular
signalling network, to the overall number of signalling links (where there
are a number of signalling relations in tandem) related to a particular
user
transaction (e.g.,\ to a specific call in the telephone application) (see
Recommendation\ Q.709).
.bp
.RT
.sp 1P
.LP
4.3
    \fIMessage sequence control\fR
.sp 9p
.RT
.PP
For all messages for the same transaction (e.g. a telephone call) the Message
Transfer Part will maintain the same routing provided that the same \fIsignalling
link selection\fR code is used in the absence of failure. However,
a transaction does not necessarily have to use the same signalling route for
both forward and backward messages.
.RT
.sp 1P
.LP
4.4
    \fINumber of signalling links used in load sharing\fR
.sp 9p
.RT
.PP
The number of signalling links used to share the load of a given
flow of signalling traffic typically depends on:
.RT
.LP
    \(em
    the total traffic load,
.LP
    \(em
    the availability of the links,
.LP
    \(em
    the required availability of the path between the two
signalling points concerned, and
.LP
    \(em
    the bit rate of the signalling links.
.PP
Load sharing requires at least two signalling links for all bit
rates, but more may be needed at lower bit rates.
.PP
When two links are used, each of them should be able to carry the
total signalling traffic in case of failure of the other link. When more than
.PP
two links are used, sufficient reserve link capacity should exist to satisfy
the availability requirements specified in Recommendation\ Q.706.
.RT
.sp 1P
.LP
4.5
    \fISatellite working\fR
.sp 9p
.RT
.PP
Due to the considerable increase in overall signalling delay, the use of
satellites in Signalling System No.\ 7 connections requires
consideration, and further study is required.
.PP
In international operation, when the network served by the signalling network
is routed on terrestrial circuits, only in exceptional circumstances
should a satellite circuit be employed for the supporting signalling
connection.
.RT
.sp 2P
.LP
\fB5\fR     \fBInternational signalling network\fR
.sp 1P
.RT
.sp 1P
.LP
5.1
    \fIGeneral\fR
.sp 9p
.RT
.PP
The international signalling network will use the procedures to be defined
in the Signalling System No.\ 7 Recommendations. The international
network structure to be defined can also serve as a model for the structure
of national networks.
.RT
.sp 1P
.LP
5.2
    \fINumber of signalling transfer points in signalling relations\fR
.sp 9p
.RT
.PP
In the international signalling network the number of signalling
transfer points between an originating and a destination signalling point
should not exceed two in a normal situation. In failure situations, this
number may become three or even four for a short period of time. This constraint
is
intended to limit the complexity of the administration of the international
signalling network.
.RT
.sp 1P
.LP
5.3
    \fINumbering of signalling points\fR
.sp 9p
.RT
.PP
A 14\(hybit code is used for the identification of signalling points. The
allocation scheme of international signalling point codes is defined in
Recommendation\ Q.708.
.RT
.sp 1P
.LP
5.4
    \fIRouting rules\fR
.sp 9p
.RT
.PP
5.4.1
In order to ensure full flexibility for the routing of
signalling in the System No.\ 7 international signalling network it appears
desirable that at least one signalling point in each country should provide
means for the international STP function. Such an approach should ease
the use of Signalling System\ No.\ 7 on small traffic routes.
.sp 9p
.RT
.sp 1P
.LP
5.4.2
    \fIOther routing rules\fR
.sp 9p
.RT
.PP
(For further study.)
.bp
.RT
.sp 1P
.LP
5.5
    \fIStructures\fR
.sp 9p
.RT
.PP
(Requires further study.)
.RT
.sp 1P
.LP
5.6
    \fIProcedures\fR
.sp 9p
.RT
.PP
(Requires further study.)
.RT
.sp 2P
.LP
\fB6\fR     \fBSignalling network for cross\(hyborder traffic\fR
.sp 1P
.RT
.sp 1P
.LP
6.1
    \fIGeneral\fR
.sp 9p
.RT
.PP
For cross\(hyborder traffic between signalling points, the need for a special
signalling network configuration is identified, because their common
interests are such as to generate a considerable volume of traffic between
them.
.PP
Two alternative arrangements of the signalling network for
cross\(hyborder traffic are provided so that Administrations may adopt either
alternative upon a bilateral agreement.
.RT
.sp 1P
.LP
6.2
    \fIUse of international hierarchical level\fR
.sp 9p
.RT
.PP
6.2.1
This arrangement could be applied in the case that there are
only a relatively small number of signalling points in a country which serve
for cross\(hyborder traffic.
.sp 9p
.RT
.PP
6.2.2
The signalling points and the signalling transfer points which are involved
in a signalling of cross\(hyborder traffic should belong to the
international hierarchical level described in \(sc\ 3. When those signalling
points or signalling transfer points are also involved in signalling of
national traffic, they should belong to their national hierarchical level as
well. Therefore the double numbering of signalling point codes based on both
the international and national numbering schemes should be required.
.PP
6.2.3
A discrimination between international and national point codes is made
by the network indicator in the service information octet (see
Recommendation\ Q.704, \(sc\ 14.2).
.PP
6.2.4
Signalling network management procedures in this network
arrangement require further study.
.sp 1P
.LP
6.3
    \fIIntegrated numbering of national signalling networks\fR
.sp 9p
.RT
.PP
6.3.1
By this arrangement the signalling points, which serve
cross\(hyborder traffic, should be identified by common national signalling
point codes.
.sp 9p
.RT
.PP
6.3.2
Common block of national signalling point codes is provided by
bilateral agreement (further study is required).
.sp 1P
.LP
6.4
    \fIInterworking of national signalling networks\fR
.sp 9p
.RT
.PP
At the cross\(hyborder signalling network interface, the international
specification of Signalling System No.\ 7 should be preferred without exclusion
of bilateral agreements.
.RT
.sp 2P
.LP
\fB7\fR     \fBNational signalling network\fR
.sp 1P
.RT
.PP
Any specific structures for national signalling networks are not
required to be included in the Recommendation, however, Administrations
should cater for requirements imposed on a national network for the protection
of
international services in terms of network related user requirements such as
availability and performance of the network perceived by users, (see
Recommendation\ Q.709).
.RT
.sp 2P
.LP
\fB8\fR     \fBProcedures to prevent unauthorized use of an STP\fR
(Optional)
.sp 1P
.RT
.sp 1P
.LP
8.1
    \fIGeneral\fR
.sp 9p
.RT
.PP
Administrations may make bilateral agreements to operate SS7
between their networks. These agreements may place restrictions on the SS7
messages authorized for one administration to send to the other. Restrictions
could be made, for example, in the interest of network security or as a
result of service restrictions. Unauthorized signalling traffic may be,
for example, STP traffic for calls set up via networks other than that
containing the STP, which has not been agreed bilaterally.
.bp
.PP
An Administration making an agreement with restrictions may wish to
identify and provide special treatment to unauthorized SS7 messages.
.PP
The measurements in Table\ 6/Q.791 provide some capability to identify
unauthorized SS7 messages. The procedures in this section for identifying
and responding to unauthorized traffic are additional options for use at
an STP
with signalling links to other networks.
.RT
.sp 1P
.LP
8.2
    \fIIdentifying unauthorized SS7 messages\fR
.sp 9p
.RT
.PP
In addition to the normal signalling message handling, procedures specified
in Recommendation\ Q.704, it shall be possible to inhibit/allow
.PP
messages destined for another signalling point (SP) based on any one or
combination  of the following options:
.RT
.LP
    i)
    to inhibit/allow STP access by a combination of designated
incoming link sets to designated DPCs;
.LP
    This combination of DPC/incoming link set shall effectively
operate in the form of a single matrix. This matrix shall
consist of a maximum of 128\ DPCs and a maximum of 64\ incoming
link sets.
(These\ values are for guidance and may be
adjusted to satisfy the requirements of the
concerned
Operator/Administration.)
.LP
    ii)
    To inhibit/allow STP access by a combination of designated
outgoing link sets to designated DPCs.
.LP
    This combination of DPC/outgoing link set shall effectively
operate in the form of a single matrix. This matrix shall
consist of a maximum of 128 DPCs and a maximum of 64\ outgoing
link sets.
(These\ values are for guidance and may be
adjusted to satisfy the requirements of the
concerned
Operator/Administration.)
.LP
    iii)
    to inhibit/allow STP access by examination of OPC
and DPC combination in the incoming STP message.
.LP
    This combination of DPC/OPC shall effectively operate in the form of
a single matrix. This matrix shall consist of a maximum
of 128\ DPCs and a maximum of 128\ OPCs. (These values are
for guidance and may be adjusted to satisfy the requirements of the
concerned Operator/Administration.)
.sp 1P
.LP
8.3
    \fITreatment of unauthorized SS7 messages\fR
.sp 9p
.RT
.PP
An STP identifying unauthorized SS7 messages should be able, on a per link
set or per signalling point code basis, to:
.RT
.LP
    i)
    provide all unauthorized SS7 messages with the same handling
as authorized traffic, or
.LP
    ii)
    discard all unauthorized SS7 messages.
.PP
In addition, an STP should be able to:
.LP
    i)
    allow all STP messages outside the designated ranges as given  in \(sc\ 8.2,
.LP
    ii)
     bar (discard) all STP messages outside the designated ranges as given
in \(sc\ 8.2.
.sp 1P
.LP
8.4
    \fIMeasurements\fR
.sp 9p
.RT
.PP
An STP identifying unauthorized SS7 messages incoming from another
network should be able to count and record details of the unauthorized
messages on a per link set and/or signalling point code basis.
.RT
.sp 1P
.LP
8.5
    \fINotification to unauthorized user\fR
.sp 9p
.RT
.PP
An STP identifying unauthorized SS7 messages from another network may wish
to notify the Administration orginating the unauthorized message(s).
.PP
This notification should be undertaken by administrative means and not
involve any mechanism in Signalling System No.\ 7.
.PP
In addition, a violation fault report shall be issued giving the
unauthorized message content. It shall be possible to selectively restrict
the number of violation reports on a per link set and/or signalling point
code
basis.
.PP
It shall also be possible to inhibit the violation reporting mechanism
on a point code/link set basis, nodally, or on a message direction, i.e.\
if an inhibited message is destined for an RPOA then it shall be possible
to suppress the violation reports whilst allowing violation reports on
inhibited messages from the RPOA.
.bp
.RT
.ce 1000
ANNEX\ A
.ce 0
.ce 1000
(to Recommendation Q.705)
.sp 9p
.RT
.ce 0
.ce 1000
\fBMesh signalling network examples\fR
.sp 1P
.RT
.ce 0
.LP
A.1
    \fIGeneral\fR
.sp 1P
.RT
.PP
This Annex is provided to demonstrate the procedures defined in
Recommendation\ Q.704. While the example uses a specific \fImesh\fR network to
demonstrate the procedures, it is not the intent of this annex to recommend
either implicitly or explicitly the network described.
.PP
The \fImesh\fR network is used to demonstrate the Message Transfer Part
level 3 procedures because it is thought to be a possible international
network implementation as shown on it, or subsets of it, may be used to
construct other network structures.
.RT
.sp 1P
.LP
A.2
    \fIBasic network structures (example)\fR
.sp 9p
.RT
.PP
Figure A\(hy1/Q.705 shows the basic mesh network structure, while
three simplified versions derived from this basic network structure are
shown in Figure\ A\(hy2/Q.705. More complex signalling networks can be
built, using these as building components.
.PP
In the following, the basic mesh network Figure\ A\(hy1/Q.705 is taken
as an example to explain the procedures defined in
Recommendation\ Q.704.
.PP
In this network, each signalling point with level\ 4 functions is
connected by two link sets to two signalling transfer points. Each pair of
signalling transfer points is connected to each other pair by four link
sets. Moreover, there is a link set between the two signalling transfer
points of
each pair.
.PP
The simplified versions a), b) and c) of the basic signalling
network are obtained by deleting respectively:
.RT
.LP
    a)
    two out of four intersignalling transfer point link sets;
.LP
    b)
    link sets between signalling transfer points of the same
pair; and
.LP
    c)
    a) and b) together.
.PP
It should be noted that for a given signalling link availability, the more
signalling link sets removed from the basic signalling network
[e.g.\ in going from Figure\ A\(hy1/Q.705 to Figure\ A\(hy2c)/Q.705], the
lower the
avail
ability of the signalling network. However, an increase in the
availability of the simplified signalling networks may be attained by
adding one or more parallel signalling links to each of the remaining
signalling link sets.
.LP
.rs
.sp 19P
.ad r
\fBFigure A\(hy1/Q.705, p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.LP
.rs
.sp 47P
.ad r
\fBFigure A\(hy1/Q.705, p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.sp 2P
.LP
A.3
    \fIRouting\fR
.sp 1P
.RT
.sp 1P
.LP
A.3.1
    \fIGeneral\fR
.sp 9p
.RT
.PP
This section gives some routing examples in the basic mesh network in Figure\
A\(hy1/Q.705. Routing actions required to change message routes under
failure conditions are described in \(sc\ A.4. The following routing
principles are assumed for the examples in \(sc\ A.3:
.RT
.LP
    \(em
    Message routes should pass through a minimum number of
intermediate signalling transfer points.
.LP
    \(em
    Routing at each signalling point will not be affected by
message routes used up to the concerned signalling transfer points.
.LP
    \(em
    When more than one message route is available, signalling
traffic should be load\(hyshared by such message routes.
.LP
    \(em
     Messages relating to a given user transaction and sent in a given direction
will be routed over the same message route to ensure correct
message sequence.
.sp 1P
.LP
A.3.2
    \fIRouting in the absence of failures\fR
.sp 9p
.RT
.PP
Figure A\(hy3/Q.705 illustrates an example of routing in the absence of
failures for messages from signalling point\ A to signalling point\ F.
.RT
.LP
.rs
.sp 22P
.ad r
\fBFigure A\(hy3/Q705, p.\fR
.sp 1P
.RT
.ad b
.RT
.PP
The following points are worthy of note:
.LP
    a)
    In distributing traffic for load\(hysharing at the originating
signalling point and intermediate signalling transfer points,
care should be taken in the use of signalling link selection
.LP
(SLS) codes so that traffic will be distributed over four available routes
evenly. In the example, originating signalling point\ A uses the second least
significant bit of the signalling link selection code, and signalling transfer
points\ B and\ C the least significant bit.
.LP
    b)
    Other than that described above, the choice of a particular
link for a given signalling link selection code can be made at each signalling
point independently. As a result, message routes for a given user transaction
(e.g.\ SLS\ =\ 0010) in two directions may take different paths
(e.g.\ A\ \(ra\ C\ \(ra\ D\ \(ra\ F and F\ \(ra\ E\ \(ra\ B\ \(ra\ A).
.bp
.LP
    c)
     Links BC and DE are not used in the absence of failures. They will be
used in certain failure situations described in \(sc\ A.4.
.LP
    d)
    When the number of links in a link set is not a power of 2
(i.e.\ 1, 2, 4, 8), SLS load sharing does not achieve even distribution
of traffic across the individual links.
.sp 2P
.LP
A.3.3
    \fIRouting under failure conditions\fR
.sp 1P
.RT
.sp 1P
.LP
A.3.3.1
    \fIAlternative routing information\fR
.sp 9p
.RT
.PP
In order to cope with failure conditions that may arise, each
signalling point has alternative routing information which specifies, for
each normal link set, alternative link set(s) to be used when the former
become(s) unavailable (see Recommendation\ Q.704, \(sc\ 4.2).
.PP
Table A\(hy1/Q.705 gives, as an example, a list of alternative link
sets for all normal link sets at signalling point\ A and at signalling
transfer point\ B. In the basic mesh network, all link sets except those
between
signalling transfer points of the same pair are normal links which carry
signalling traffic in the absence of failures. In case a normal link set
becomes unavailable, signalling traffic formerly carried by that link set
should be diverted to the alternative link set with priority\ 1. Alternative
.PP
link sets with priority\ 2 (i.e.\ link sets between signalling transfer
points of the same pair) will be used only when both the normal link set
and alternative link set(s) with priority 1 become unavailable.
.PP
Paragraphs A.3.3.2 to A.3.3.5 present some typical examples of the
consequences of faults in signalling links and signalling points on the
routing of signalling traffic. For the sake of simplicity, link sets are
supposed to
consist of only one link each.
.RT
.LP
.sp 5
.ce
\fBH.T. [T1.705]\fR
.ce
TABLE\ A\(hy1/Q.705
.ce
\fBList of alternative link sets at signalling\fR
.ce
\fBpoints A and B\fR
.ps 9
.vs 11
.nr VS 11
.nr PS 9
.TS
center box;
lw(42p) | cw(42p) | cw(42p) | cw(30p) .
    Normal link set    Alternative link set    Priority | ua\d\u)\d
_
.T&
lw(42p) | cw(42p) | cw(42p) | cw(30p) .
Signalling    AB    AC    1
.T&
lw(42p) | cw(42p) | cw(42p) | cw(30p) .
point A    AC    AB    1
_
.T&
lw(42p) | cw(42p) | cw(42p) | cw(30p) .
Signalling transfer    BA    BC    2
.T&
lw(42p) | cw(42p) | cw(42p) | cw(30p) .
point B    BC    None
.T&
lw(42p) | cw(42p) | cw(42p) | cw(30p) .
    BE    BD    1
.T&
lw(42p) | cw(42p) | cw(42p) | cw(30p) .
        BC    2
.T&
lw(42p) | cw(42p) | cw(42p) | cw(30p) .
    BD    BE    1
.T&
lw(42p) | cw(42p) | cw(42p) | cw(30p) .
        BC
2
.TE
.LP

\ua\d\u)\d
\fIPriority\ 1\fR
\ \(em\ used with normal link set on load\(hysharing basis in the absence of failures.
.LP
\fIPriority\ 2\fR
\ \(em\ used only when all the link sets with priority 1
become unavailable.
.nr PS 9
.RT
.ad r
\fBTable [T1.705], p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.sp 1P
.LP
A.3.3.2
    \fISingle link failure examples\fR
.sp 9p
.RT
.PP
\fIExample\ 1:\fR \ Failure of a link between a signalling point and a
signalling transfer point (e.g. link\ AB) (see Figure\ A\(hy4/Q.705).
.RT
.LP
.rs
.sp 16P
.ad r
\fBFigure A\(hy4/Q.705, p.\fR
.sp 1P
.RT
.ad b
.RT
.PP
As indicated in Table A\(hy1/Q.705, A diverts traffic formerly
carried by link\ AB to link\ AC, while B diverts such traffic to link\ BC. It
should be noted that the number of signalling transfer points traversed by
signalling messages from F to\ A which passes through\ B is increased by
one and becomes three in this case.
.PP
The principle to minimize the number of intermediate signalling
transfer points in \(sc\ A.3.1 is applied in this case at signalling transfer
point\ B to get around the failure. In fact, the procedures defined in
Recommendation\ Q.704 assume that traffic is diverted at a signalling point
only in the case of a signalling link being unavailable on the route outgoing
from that signalling point. Therefore, the procedures do not provide for
sending an indication that traffic routed via signalling transfer point\
B will traverse a further signalling transfer point.
.PP
\fIExample\ 2:\fR \ Failure of an intersignalling transfer points link
(e.g.\ link\ BD) (see Figure\ A\(hy5/Q.705).
.PP
As indicated in Table A\(hy1/Q.705, B diverts traffic carried by
link\ BD to link\ BE. In the same sense, D\ diverts traffic carried by
link\ DB to link\ DC.
.RT
.LP
.rs
.sp 13P
.ad r
\fBFigure A\(hy5/Q.705, p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.PP
\fIExample\ 3:\fR \ Failure of a link between signalling transfer points
of the same pair (e.g. link\ BC) (see Figure\ A\(hy6/Q.705).
.PP
No routing change is required as a result of this kind of failure.
Only B and C take note that the link\ BC has become unavailable.
.RT
.LP
.rs
.sp 16P
.ad r
\fBFigure A\(hy6/Q.705, p.7\fR
.sp 1P
.RT
.ad b
.RT
.sp 1P
.LP
A.3.3.3
    \fIMultiple link failure examples\fR
.sp 9p
.RT
.PP
As there are a variety of cases in which more than one link set
becomes unavailable, only some typical cases are given as examples in the
following.
.PP
\fIExample\ 1:\fR \ Failure of a link between a signalling point and a
signalling transfer point, and of the link between that signalling transfer
point and that of the same pair (e.g.\ links DF,\ DE) (see Figure\ A\(hy7/Q.705).
.PP
B diverts traffic destined to F from link BD to link BE, because
destination F becomes inaccessible via\ D. It should be noted that only the
traffic destined to\ F is diverted from link\ BD to link\ BE, and not all the
traffic on link\ BD. The same applies to C, which diverts traffic destined
to\ F from link\ CD to link\ CE. F\ diverts all the traffic formerly carried
by link\ FD to link\ FE in the same way as the single link failure example in
\(sc\ A.3.3.2.
.RT
.LP
.rs
.sp 16P
.ad r
\fBFigure A\(hy7/Q.705, p.8\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.PP
\fIExample\ 2:\fR \ Failure of two intersignalling transfer point links
(e.g.\ links BD, BE) (see Figure\ A\(hy8/Q.705).
.PP
B diverts traffic formerly carried by link BD to link BC, because its alternative
link set with priority\ 1, i.e.\ link\ BE, is also unavailable. The
same applies to traffic formerly carried by link\ BE, and B diverts it to
link\ BC. D\ and E divert traffic formerly carried by links\ DB and\ EB
respectively to links DC and\ EC in the same way as the single link failure
example in \(sc\ A.3.3.2.
.RT
.LP
.rs
.sp 15P
.ad r
\fBFigure A\(hy8/Q.705, p.9\fR
.sp 1P
.RT
.ad b
.RT
.PP
\fIExample\ 3:\fR \ Failure of a link between a signalling point and a
signalling transfer point, and of an intersignalling transfer point link
(e.g.\ links DF and\ BD) (see Figure\ A\(hy9/Q.705).
.PP
This example is a combination of Examples 1 and 2 in \(sc\ A.3.3.2.
D diverts traffic formerly carried by link\ DF to link\ DE, while F diverts
it to link\ FE. Moreover D diverts traffic formerly carried by link\ DB
to link\ DC
(this traffic will be that generated by signalling points other than F
connected to\ D). In the same sense, B\ diverts traffic carried by link\ BD to
link\ BE.
.PP
It should be noted that in this case only the portion of traffic sent by
C to F via D traverses three signalling transfer points (C, D and\ E),
while all the other portions continue to traverse two.
.RT
.LP
.rs
.sp 15P
.ad r
\fBFigure A\(hy9/Q.705, p.10\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.PP
\fIExample\ 4:\fR \ Failure of the two links between a signalling point
and its signalling transfer points (e.g.\ DF and\ EF) (see Figure\ A\(hy10/Q.705).
.PP
In this case the signalling relations between F and any other
signalling point of the network are blocked. Therefore F stops all outgoing
signalling traffic, while A stops only traffic destined to\ F.
.RT
.LP
.rs
.sp 16P
.ad r
\fBFigure A\(hy10/Q.705, p.\fR
.sp 1P
.RT
.ad b
.RT
.sp 1P
.LP
A.3.3.4
    \fISingle signalling point failure examples\fR
.sp 9p
.RT
.PP
\fIExample\ 1:\fR \ Failure of a signalling transfer point (e.g.\ D) (see
Figure\ A\(hy11/Q.705).
.PP
B diverts all the traffic formerly carried by link BD to link\ BE. The
same applies to C which diverts all the traffic carried by link\ CD to
link\ CE. Originating point F diverts all the traffic carried by link\
FD to link\ FE as in the case of the link\ FD failure (see Example\ 1 in
\(sc\ A.3.3.2).
.RT
.LP
.rs
.sp 16P
.ad r
\fBFigure A\(hy11/Q.705, p.\fR
.sp 1P
.RT
.ad b
.RT
.PP
Attention is drawn to the difference to Example 1 in \(sc\ A.3.3.3
where only a part of the traffic previously carried by links BD and CD was
diverted.
.bp
.PP
\fIExample\ 2:\fR \ Failure of a destination point (e.g.\ F) (see
Figure\ A\(hy12/Q.705).
.PP
In this case A stops all the traffic to F formerly carried on links AB and\ AC.
.RT
.LP
.rs
.sp 15P
.ad r
\fBFigure A\(hy12/Q.705, p.\fR
.sp 1P
.RT
.ad b
.RT
.sp 1P
.LP
A.3.3.5
    \fIMultiple signalling transfer point failure examples\fR
.sp 9p
.RT
.PP
Two typical cases of two signalling transfer points failing
together are presented in the following examples.
.PP
\fIExample\ 1:\fR \ Failure of two signalling transfer points not pertaining
to the same pair (e.g. B and\ D) (see Figure\ A\(hy13/Q.705).
.PP
As a result of the failure of B, A diverts traffic formerly carried by
link AB to link\ AC, while E diverts traffic formerly carried by link\
EB to
link\ EC. Similarly as a result of the failure of D, F diverts traffic
formerly carried by link\ FD to link\ FE, while C diverts traffic formerly
carried by
link\ CD to link\ CE.
.PP
It should be noted that, in this example, all the traffic between A
and F is concentrated on only one intersignalling transfer point link, since
failure of a signalling transfer point has an effect similar to a simultaneous
failure of all the signalling links connected to it.
.RT
.LP
.rs
.sp 15P
.ad r
\fBFigure A\(hy13/Q.705, p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.PP
\fIExample\ 2:\fR \ Failure of two signalling transfer points pertaining
to the same pairs (e.g.\ D and\ E) (see Figure\ A\(hy14/Q.705).
.PP
This example is equivalent to Example 4 in \(sc\ A.3.3.3 as far as
the inaccessibility of\ F is concerned, but in this case any other signalling
point connected by its links to D and\ E also becomes inaccessible. In
this case A stops signalling traffic destined to\ F, while F stops all
outgoing signalling traffic.
.RT
.LP
.rs
.sp 16P
.ad r
\fBFigure A\(hy14/Q.705, p.\fR
.sp 1P
.RT
.ad b
.RT
.sp 1P
.LP
A.4
    \fIActions relating to failure conditions\fR
.sp 9p
.RT
.PP
In the following, four typical examples of the application of
signalling network management procedures to the failure cases illustrated in
\(sc\ A.3.3 are shown. In the case of multiple failures, an arbitrary failure
(and restoration) sequence is assumed for illustrative purpose.
.RT
.sp 1P
.LP
A.4.1
    \fIExample\ 1:\ Failure of a link between a signalling point and a\fR
\fIsignalling transfer point (e.g.\ link\ AB)\fR (see Figure\ A\(hy15/Q.705)
.sp 9p
.RT
.PP
(Same as \(sc\ A.3.3.2, Example\ 1.)
.RT
.LP
.rs
.sp 16P
.ad r
\fBFigure A\(hy15/Q.705, p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.sp 1P
.LP
A.4.1.1
    \fIFailure of link AB\fR
.sp 9p
.RT
.LP
    a)
    When the failure of link AB is detected in A and in B, they
initiate the changeover procedure, by exchanging changeover
messages via\ C. Once buffer updating is completed, A\ restarts
the traffic originally carried by the failed link on link\ AC;
similarly, B\ restarts traffic destined to\ A on link\ BC.
.LP
    b)
    In addition, B sends a transfer\(hyprohibited message to C
referred to destination\ A (according to the criterion indicated in
Recommendation\ Q.704, \(sc\ 13.2.2).
.LP
    c)
     On the reception of the transfer\(hyprohibited message, C starts the
periodic sending of signalling\(hyroute\(hyset\(hytest messages, referred
to\ A, to\ B (see Recommendation\ Q.704, \(sc\ 13.5.2).
.sp 1P
.LP
A.4.1.2
    \fIRestoration of link AB\fR
.sp 9p
.RT
.PP
When the restoration of link AB is completed, the following
applies:
.RT
.LP
    a)
     B initiates the changeback procedure, by sending a changeback declaration
to A via\ C. Once it has received the changeback acknowledgement, it restarts
traffic on the restored link. Moreover, it sends to\ C a
transfer\(hyallowed message, referred to destination\ A (see Recommendation\
Q.704, \(sc\ 13.3.2). When C receives the transfer\(hyallowed message,
it stops sending
signalling\(hyroute\(hyset\(hytest messages to\ B.
.LP
    b)
     A initiates the changeback procedure, by sending a changeback declaration
to B via\ C; once it has received the changeback acknowledgement, it restarts
traffic on the normal link. The only traffic to be diverted is that
for which link\ AB is the normal link set according to the load sharing rule
(see \(sc\ A.3.3.1). It
must be pointed out, however, that if there is load sharing on parallel
links between B and\ C, there is the possibility of missequencing.
Concerning\ b), for example, the changeback declaration sent from\ A to\ B
via\ C might overrun messages still buffered at signalling point\ C
(due to e.g.\ retransmissions on the parallel link\ CB).
.sp 1P
.LP
A.4.2
    \fIExample\ 2:\ Failure of signalling transfer point D\fR  | (see
Figure\ A\(hy16/Q.705)
.sp 9p
.RT
.PP
(Same as \(sc\ A.3.3.4, Example 1.)
.RT
.LP
.rs
.sp 13P
.ad r
\fBFigure A\(hy16/Q.705, p.\fR
.sp 1P
.RT
.ad b
.RT
.sp 1P
.LP
A.4.2.1
    \fIFailure of signalling transfer point D\fR
.sp 9p
.RT
.LP
    a)
     Changeover is initiated at signalling points B, C and F from blocked
links BD, CD and\ FD to the first priority alternative links\ BE, CE
and\ FE respectively. Due to the failure of\ D, the concerned signalling
points will receive no changeover acknowledgement message in response,
and therefore they will restart traffic on alternative links at the expiry
of the time\ T2
(see Recommendation\ Q.704, \(sc\ 5.7.2). In addition E\ will send to B,
C and\ F
transfer\(hyprohibited messages referred to destination\ D. These signalling
points (B, C and\ F) will thus start periodic sending to\ E of signalling\(hyroute\(hyset\(hytest
messages referred to\ D.
.bp
.LP
    b)
     When B receives a transfer\(hyprohibited message from E referred to\
D, it updates its routing information so that traffic to\ D will be diverted
to\ C, thus sending a transfer\(hyprohibited message to\ C referred to\
D. The same applies to\ C, and C sends a transfer\(hyprohibited message
to\ B.
.LP
    c)
     So, when B receives a transfer\(hyprohibited message from C, it finds
that destination\ D has become inaccessible and sends a
transfer\(hyprohibited message to\ A. The same applies to\ C and thus C
also sends a transfer\(hyprohibited message to\ A. Having received transfer\(hyprohibited
messages from both B and\ C, A\ recognizes that D has become inaccessible
and stops
traffic to\ D.
.LP
    d)
    In the same manner, i.e. link\(hyby\(hylink transmission of
transfer\(hyprohibited messages referred to\ D, other signalling points B, C, E
and\ F will finally recognize that destination\ D has become inaccessible.
Each signalling point will, therefore, start periodic sending of
signalling\(hyroute\(hyset\(hytest messages referred to\ D to their respective
adjacent signalling points.
.sp 1P
.LP
A.4.2.2
    \fIRecovery of signalling transfer point D\fR
.sp 9p
.RT
.LP
    a)
    Signalling points B, C, E send traffic restart allowed
messages to signalling point\ D, as soon as signalling point\ D becomes
accessible.
.LP
    b)
     Signalling transfer point D broadcasts traffic restart allowed messages,
after T20 (see Recommendation\ Q.704, \(sc\ 16.8) has stopped or expired,
to all adjacent SPs.
.LP
    c)
    Changeback at signalling points B, C and F from the
alternative to their normal links is performed. In all the three cases
changeback includes the time\(hycontrolled diversion procedure (see
Recommendation\ Q.704, \(sc\ 6.4), since D\ is still inaccessible via\
E at\ B, C and\ F (as a result of previous reception of transfer\(hyprohibited
message
from\ E).
.LP
    d)
    E sends to B, C and F transfer\(hyallowed messages referred to
destination\ D. These signalling points will thus send transfer allowed
messages to their respective adjacent signalling points. Thus, the link\(hyby\(hylink
transmission of transfer\(hyallowed messages will declare to all signalling
points that destination\ D has become accessible.
.LP
    e)
    On reception of a transfer\(hyallowed message, each signalling
point stops periodic sending of signalling\(hyroute\(hyset\(hytest messages
to their
respective adjacent signalling points.
.LP
    f
)
    On recovery of the previously unavailable links BD, CD   and FD,
signalling points\ B, C and\ F will restart all the traffic normally routed
via signalling transfer point\ D after T21 (see Recommendation\ Q.704,
\(sc\ 16.8) has
stopped or expired. (They would restart any traffic terminating at\ D,
if D had an endpoint function as well as being an STP, immediately D becomes
accessible, that is after successful signalling link tests to\ D.)
.sp 1P
.LP
A.4.3
    \fIExample\ 3:\ Failure of link between a signalling point and a\fR
\fIsignalling transfer point, and of the link between that signalling\fR
\fItransfer point and that of the same pair (e.g.\ links DF,\ DE)\fR (see
Figure\ A\(hy17/Q.705)
.sp 9p
.RT
.PP
(Same as \(sc\ A.3.3.3, Example 1.)
.RT
.LP
.rs
.sp 12P
.ad r
\fBFigure A\(hy17/Q.705, p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.sp 1P
.LP
A.4.3.1
    \fIFailure of link DE\fR
.sp 9p
.RT
.PP
On failure of link DE, this link is marked unavailable at both
signalling transfer points D and\ E. Since in the absence of failures,
link\ DE does not carry signalling traffic, no change in message routing
takes place at this time.
.PP
However, D and E send to signalling points B, C and F
transfer\(hyprohibited messages referred to destination E or\ D respectively.
These signalling points will thus start periodic sending of signalling\(hyroute\(hyset\(hytest
messages, referred to D or\ E, to E and\ D respectively.
.RT
.sp 1P
.LP
A.4.3.2
    \fIFailure of link DF in the presence of failure of link DE\fR
.sp 9p
.RT
.LP
    a)
    On failure of link DF the following actions occur:
.LP
    i)
    Signalling point D which no longer has access to signalling
point F indicates this condition to signalling transfer points B and C by
sending transfer\(hyprohibited messages.  B and C will thus start the periodic
sending of signalling\(hyroute\(hyset\(hytest messages referred to F, to\ D.
.LP
    ii)
     Emergency changeover from link FD to link FE is initiated at signalling
point\ F, since D becomes inaccessible to\ F due also to the previous failure.
.LP
    b)
     On receiving the transfer\(hyprohibited messages forced rerouting is
initiated at points\ B and\ C. This causes traffic destined to\ F to be
diverted from links terminating on\ D to links terminating on\ E. Forced
rerouting thus permits recovery from a failure condition caused by a fault
in a remote part of the network.
.sp 1P
.LP
A.4.3.3
    \fIRestoration of link FD in the presence of failure of\fR
\fIlink DE\fR
.sp 9p
.RT
.LP
    a)
    On recovery of link FD the following actions occur:
.LP
    i)
     Signalling point D sends a transfer\(hyallowed message to B and C to
indicate that D once again has access to\ F. B\ and\ C will thus stop the
sending of signalling\(hyroute\(hyset\(hytest messages referred to F to\ D.
.LP
    ii)
    F initiates changeback with time controlled diversion from
link FE to link\ FD. This procedure permits changeback to be executed at
one end of a link, when it is impossible to notify the other end of the
link (in this example, because
.LP
link\ DE is unavailable). Traffic in this case is not diverted from the
alternative link until a time interval has
elapsed, in order to minimize the danger of mis\(hysequencing of messages (see
Recommendation\ Q.704, \(sc\ 6.4).
.LP
    b)
    On receiving the transfer\(hyallowed message, controlled
rerouting of traffic from the alternative routes (BEF, CEF) to the normal
routes (BDF, CDF) is initiated at points\ B and\ C. Controlled rerouting
involves diversion of traffic to a route which has become available after
a time
interval (see Recommendation\ Q.704, \(sc\ 8.2.1), provisionally set at
one second to minimize the danger of mis\(hysequencing messages.
.sp 1P
.LP
A.4.3.4
    \fIRestoration of link DE\fR
.sp 9p
.RT
.PP
On recovery of link DE it is marked available at signalling
transfer points D and E. Signalling points\ D and\ E send to B, C and\ F
transfer\(hyallowed messages referred to destination\ E or\ D respectively.
These
signalling transfer points will thus stop sending of signalling\(hyroute\(hyset\(hytest
messages.
.RT
.sp 1P
.LP
A.4.4
    \fIExample\ 4:\ Failure of links DF and EF\fR  | (see Figure A\(hy18/Q.705)
.sp 9p
.RT
.LP
.rs
.sp 11P
.ad r
\fBFigure A\(hy18/Q.705, p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.sp 1P
.LP
A.4.4.1
    \fIFailure of link DF\fR
.sp 9p
.RT
.PP
When the failure of link DF is detected, D and F perform the
changeover procedure; D diverts traffic, destined to\ F, to link\ DE, while F
concentrates all the outgoing traffic on link\ FE.
.PP
In addition, D sends to E a transfer\(hyprohibited message, referred to
destination F; E will thus start sending of signalling\(hyroute\(hyset\(hytest
messages, referred to\ F, towards\ D (see also \(sc\ A.4.1.1).
.RT
.sp 1P
.LP
A.4.4.2
    \fIFailure of link EF in the presence of failure of link DF\fR
.sp 9p
.RT
.LP
    a)
    When the failure of link EF is detected, the following
applies:
.LP
    i)
     Since all destinations become inaccessible F stops sending all signalling
traffic.
.LP
    ii)
     E sends to B, C and D a transfer\(hyprohibited message, referred to destination\
F. B, C and\ D start periodic sending of
signalling\(hyroute\(hyset\(hytest messages referred to F to\ E.
.LP
    b)
     When D receives the transfer\(hyprohibited message, it sends to B and
C a transfer\(hyprohibited message, referred to destination\ F (see
Recommendation\ Q.704, \(sc\ 13.2.2 | i)). B\ and\ C start periodic sending
of test
messages referred to F to\ D.
.LP
    c)
     When B receives the transfer\(hyprohibited messages from D and E, it
sends a transfer\(hyprohibited message to\ C; the same applies for\ C (it
sends the message to\ B). As soon as B and\ C have received the transfer\(hyprohibited
messages from all the three possible routes (BD, BE and\ BC, or\ CD, CE and\ CB
respectively), they send a transfer\(hyprohibited message to\ A.
.LP
    \fINote\fR \ \(em\ Depending on the sequence of reception of
transfer\(hyprohibited messages at B or C, they may start a forced rerouting
procedure on a route not yet declared to be unavailable; such procedure
is then aborted as soon as a transfer\(hyprohibited message is received
also from that
route.
.LP
    d)
     As soon as A receives the transfer\(hyprohibited messages from B and
C, it declares destination\ F inaccessible and stops sending traffic towards
it. Moreover, it starts the periodic sending of signalling\(hyroute\(hyset\(hytest
messages, referred to F, to B and\ C.
.sp 1P
.LP
A.4.4.3
    \fIRestoration of link EF in the presence of failure\fR
\fIon link DF\fR
.sp 9p
.RT
.LP
    a)
    When restoration of link EF is completed, the following
applies:
.LP
    i)
    Signalling point F restarts traffic on link\ EF.
.LP
    ii)
     E sends a transfer\(hyallowed message, referred to destination F, to
B, C and\ D; moreover it restarts traffic on the restored link.
.LP
    b)
     When B and C receive the transfer\(hyallowed message, they send a transfer\(hyallowed
message to A and C or\ A and\ B, respectively and they stop
sending signalling\(hyroute\(hyset\(hytest messages to E; moreover, they
restart the
concerned traffic on link BE or\ CE respectively.
.LP
    c)
     When D receives the transfer\(hyallowed message from E, it sends transfer\(hyallowed
messages to B and\ C and stops sending
signalling\(hyroute\(hyset\(hytest messages to\ E; moreover, it starts
the concerned
traffic on link\ DE. On receipt of the transfer\(hyallowed message, B and\
C will
divert to links\ BD and\ CD, by means of a controlled rerouting procedure,
traffic carried by links BE and CE for which they are the normal links (see
\(sc\ A.3.3). Moreover, they will stop sending signalling\(hyroute\(hyset\(hytest
messages to\ D.
.LP
    \fINote\fR \ \(em\ According to the rules stated in Recommendation\ Q.704,
\(sc\ 13.3.2, on receipt of transfer\(hyallowed messages from E [phase\
b) above], B
and\ C should send transfer\(hyallowed messages also to D and\ E. However,
this is not appropriate in the network configurations such as the one here
considered, taking into account that:
.LP
    \(em
     there is no route, for example, from D (or E) to F via B (or C) and therefore
the transfer\(hyallowed messages would be ignored by D and\ E;
.LP
    \(em
     on restarting traffic to F on links BD, BE, CD and CE it would anyway
be necessary that B and\ C send transfer\(hyprohibited messages to D and\
E, which would contradict the previous transfer\(hyallowed messages.
.LP
    d)
     As soon as A receives a transfer\(hyallowed message from B or C, it restarts
signalling traffic to B and\ C. If traffic has already been
restarted on one link when the transfer\(hyallowed message is received on the
other link, a changeback procedure is performed to establish the normal
routing situation on both links (i.e.\ to divert part of the traffic on
the latter
link).
.bp
.sp 1P
.LP
A.4.4.4
    \fIRestoration of link DF\fR
.sp 9p
.RT
.PP
When the restoration of link DF is completed, the following
applies:
.RT
.LP
    a)
     D initiates the changeback procedure to link DF; moreover, it sends to
E a transfer\(hyallowed message, referred to destination\ F,
.LP
    b)
     F sends signalling\(hyroute\(hyset\(hytest message to D referred to the
destination points it normally accesses via\ D. It initiates the changeback
procedure to link\ DF; this procedure refers only to the traffic for which
link DF is the normal one, according to the routing rules.
.sp 1P
.LP
A.5
    \fIExplanatory note from the implementors forum for clarification\fR
\fIof load sharing\fR
.sp 9p
.RT
.PP
A.5.1
In general, to improve the distribution of traffic, load sharing at a particular
signalling point (amongst link sets to a given destination)
will be on the basis of a part of the signalling link selection field which
is different than that part used for load sharing amongst signalling links
within a selected link set. In the example represented in Figure\ 5/Q.704,
if link set DF contains more than one signalling link, then the least significant
bit of
the signalling link selection field is not used in sharing traffic within
link set DF amongst the signalling links. Similar considerations can apply
to link set\ DE.
.sp 9p
.RT
.PP
A.5.2
At an originating signalling point it is assumed that for a given
signalling relation, signalling link selection field values are evenly
distributed and traffic is shared over the appropriate link sets and signalling
links within each link set on this basis. In general, to achieve this a
different oad sharing rule is needed for each number of link sets, and each
number of signalling links within a link set, over which traffic is to be
shared. The intention is to attain, for a given signalling relation, as
een as possible a traffic balance over the link sets and the signalling
links within each link set, based on the signalling link selection field
and the numbers of link sets and signalling links within each ink set;
such an even traffic
balance may result if the fixed part of the signalling link selection field
is not excluded from consideration by the load sharing rules.
.PP
A.5.3
At a signalling transfer point, for a given signalling relation, signalling
link selection field values may not be evenly distributed (see
Figure\ 5/Q.704, signalling transfer point\ E). A different set of load
sharing rules to those for originating signalling points may be provided
to deal with this possibility. These are again based on the signalling
link selection field and the numbers of link sets and signalling links
within each link set, but
assume that a particular part of the signalling link selection field is
fixed. The fixed part of the signalling link selection field may be different
at
different signalling transfer points. Where signalling messages for different
signalling relations arriving at a particular signalling transfer point
do not have the same part of the signalling link selection field fixed,
an uneven
sharing of traffic for a particular signalling relation amongst the relevant
link sets and signalling links within each link set may result.
\v'1P'
.sp 2P
.LP
\fBRecommendation\ Q.706\fR
.RT
.sp 2P
.sp 1P
.ce 1000
\fBMESSAGE\ TRANSFER\ PART\ SIGNALLING\ PERFORMANCE\fR
.EF '%    Fascicle\ VI.7\ \(em\ Rec.\ Q.706''
.OF '''Fascicle\ VI.7\ \(em\ Rec.\ Q.706    %'
.ce 0
.sp 1P
.PP
The message transfer part of Signalling System No.\ 7 is
designed as a joint transport system for the messages of different users.
The requirements of the different users have to be met by the Message Transfer
Part. These requirements are not necessarily the same and may differ in
importance and stringency.
.sp 1P
.RT
.PP
In order to satisfy the individual requirements of each user the Message
Transfer Part of Signalling System No.\ 7 is designed in such a way that
it meets the most stringent User Part requirements envisaged at the time
of
specification. To this end, the requirements of the telephone service, the
data transmission service and the signalling network management, in particular,
were investigated. It is assumed that a signalling performance which satisfies
the requirements mentioned above will also meet those of future users.
.bp
.PP
In the light of the above, signalling system performance is understood
to be the capability of the Message Transfer Part to transfer messages
of
variable length for different users in a defined manner. In order to achieve
a proper signalling performance, three groups of parameters have to be
taken into account:
.RT
.LP
    \(em
    The first group covers the objectives derived from the
requirements of the different users. The aims are limitation of
message delay, protection against all kinds of failures and
guarantee of availability.
.LP
    \(em
    The second group covers the features of the signalling
traffic, such as the loading potential and the structure of the
signalling traffic.
.LP
    \(em
    The third group covers the given environmental influences,
such as the characteristics (e.g. error rate and proneness to
burst) of the transmission media.
.PP
The three groups of parameters are considered in the specification of the
procedures to enable the Message Transfer Part to transfer the messages
in such a way that the signalling requirements of all users are met and
that a uniform and satisfactory overall signalling system performance is
achieved.
.sp 2P
.LP
\fB1\fR     \fBBasic parameters related to Message Transfer Part signalling
performance\fR
.sp 1P
.RT
.PP
Signalling performance is defined by a great number of different
parameters. In order to ensure a proper signalling performance for all
users to be served by the common Message Transfer Part, the following design
objectives are established for the Message Transfer Part.
.RT
.sp 1P
.LP
1.1
    \fIUnavailability of a signalling route set\fR
.sp 9p
.RT
.PP
The unavailability of a signalling route set is determined by the unavailability
of the individual components of the signalling network
(signalling links and the signalling points) and by the structure of a
signalling network.
.PP
The unavailability of a signalling route set should not exceed a total
of 10\ minutes per year.
.PP
The unavailability of a signalling route set within a signalling
network may be improved by replication of signalling links, signalling paths
and signalling routes.
.RT
.sp 1P
.LP
1.2
    \fIUnavoidable message transfer part malfunction\fR
.sp 9p
.RT
.PP
The Message Transfer Part of Signalling System No. 7 is designed to transport
messages in a correct sequence. In addition, the messages are
protected against transmission errors. However, a protection against
transmission errors cannot be absolute. Furthermore, mis\(hysequencing
and loss of messages in the Message Transfer Part cannot be excluded in
extreme cases.
.PP
For all User Parts, the following conditions are guaranteed by the
Message Transfer Part:
.RT
.LP
    a)
    \fIUndetected errors\fR
.LP
    On a signalling link employing a signalling data link
which has the error rate characteristic as described in
Recommendation\ Q.702 not more than one in\ 10\u1\d\u0\d
of all signal unit errors will be undetected by the message
Transfer Part.
.LP
    b)
    \fILoss of messages\fR
.LP
    Not more than one in 10\u7\d messages will be lost due to
failure in the message transfer part.
.LP
    c)
    \fIMessages out\(hyof\(hysequence\fR
.LP
    Not more than one in 10\u1\d\u0\d messages will be delivered
out\(hyof\(hysequence to the User Parts due to failure in the message
transfer part. This value also includes duplication of messages.
.sp 1P
.LP
1.3
    \fIMessage transfer times\fR
.sp 9p
.RT
.PP
This parameter includes:
.RT
.LP
    \(em
    handling times at the signalling points (see \(sc\ 4.3);
.LP
    \(em
    queueing delays including retransmission delays (see
\(sc\ 4.2);
.LP
    \(em
    signalling data link propagation times.
.sp 1P
.LP
1.4
    \fISignalling traffic throughput capability\fR
.sp 9p
.RT
.PP
Needs further study (see \(sc\ 2.2).
.bp
.RT
.sp 2P
.LP
\fB2\fR     \fBSignalling traffic characteristics\fR
.sp 1P
.RT
.sp 1P
.LP
2.1
    \fILabelling potential\fR
.sp 9p
.RT
.PP
The design of Signalling System No.\ 7 provides the potential for
labels to identify 16 | 84 signalling points. For each of the 16\ different
User Parts a number of user transactions may be identified, e.g.\ in the
case of the telephone service up to 4096 speech circuits.
.RT
.sp 1P
.LP
2.2
    \fILoading potential\fR
.sp 9p
.RT
.PP
Considering that the load per signalling channel will vary
according to the traffic characteristics of the service, to the user
transactions served and to the number of signals in use, it is not practicable
to specify a general maximum limit of user transactions that a signalling
.PP
channel can handle. The maximum number of user transactions to be served
must be determined for each situation, taking into account the traffic
characteristics applied so that the total signalling load is held to a level
which is acceptable from different points of view.
.PP
When determining the normal load of the signalling channel, account
must be taken of the need to ensure a sufficient margin for peak traffic
loads.
.PP
The loading of a signalling channel is restricted by several factors which
are itemized below.
.RT
.sp 1P
.LP
2.2.1
    \fIQueueing delay\fR
.sp 9p
.RT
.PP
The queueing delay in absence of disturbances is considerably
influenced by the distribution of the message length and the signalling
traffic load (see \(sc\ 4.2).
.RT
.sp 1P
.LP
2.2.2
    \fISecurity requirements\fR
.sp 9p
.RT
.PP
The most important security arrangement is redundancy in
conjunction with changeover. As load sharing is applied in normal operation,
the load on the individual signalling channels has to be restricted so
that, in the case of changeover, the queueing delays do not exceed a reasonable
limit. This requirement has to be met not only in the case of changeover
to one
predetermined link but also in the case of load distribution to the remaining
links.
.RT
.sp 1P
.LP
2.2.3
    \fICapacity of sequence numbering\fR
.sp 9p
.RT
.PP
The use of 7 bits for sequence numbering finally limits the number of signal
units sent but not yet acknowledged to the value of\ 127.
.PP
In practice this will not impose a limitation on the loading
potential.
.RT
.sp 1P
.LP
2.2.4
    \fISignalling channels using lower bit rates\fR
.sp 9p
.RT
.PP
A loading value for a signalling channel using bit rates of less
than 64\ kbit/s will result in greater queueing delays than the same loading
value for a 64\(hykbit/s signalling channel.
.RT
.sp 1P
.LP
2.3
    \fIStructure of signalling traffic\fR
.sp 9p
.RT
.PP
The Message Transfer Part of Signalling System No.\ 7 serves
different User Parts as a joint transport system for messages. As a result,
the structure of the signalling traffic largely depends on the types of
User Parts served. It can be assumed that at least in the near future the
telephone
service will represent the main part of the signalling traffic also in
integrated networks.
.PP
It cannot be foreseen yet how the signalling traffic is influenced by the
integration of existing and future services. The traffic models given in
\(sc\ 4.2.4 have been introduced in order to consider as far as possible the
characteristics and features of different services within an integrated
network. If new or more stringent requirements are imposed on signalling
(e.g.\ shorter delays) as a consequence of future services, they should
be met by appropriate dimensioning of the load or by improving the structure
of the
signalling network.
.bp
.RT
.sp 2P
.LP
\fB3\fR     \fBParameters related to transmission characteristics\fR
.sp 1P
.RT
.PP
No special transmission requirements are envisaged for the
signalling links of Signalling System\ No.\ 7. Therefore, System\ No.\
7 provides appropriate means in order to cope with the given transmission
characteristics of ordinary links. The following items indicate the actual
characteristics to be expected \(em\ as determined by the responsible Study
Groups\ \(em and their
consequences on the specifications of the Signalling System\ No.\ 7 Message
Transfer Part.
.RT
.sp 1P
.LP
3.1
    \fIApplication of Signalling System No. 7 to 64\(hykbit/s links\fR
.sp 9p
.RT
.PP
The Message Transfer Part is designed to operate satisfactorily
with the following transmission charac
teristics:
.RT
.LP
    a)
    a long\(hyterm bit error rate of the signalling data link of
less than\ 10\uD\dlF261\u6\d\ [1];
.LP
    b)
    a medium\(hyterm bit error rate of less than\ 10\uD\dlF261\u4\d;
.LP
    c)
     random errors and error bursts including long bursts which might occur
in the digital link due to, for instance, loss of frame alignment or octet
slips in the digital link. The maximum tolerable interruption period is
specified for the signal unit error rate monitor (see Recommendation\ Q.703,
\(sc\ 10.2).
.sp 1P
.LP
3.2
    \fIApplication of Signalling System No. 7 to links using lower bit\fR
\fIrates\fR
.sp 9p
.RT
.PP
(Needs further study.)
.RT
.sp 2P
.LP
\fB4\fR     \fBParameters of influence on signalling performance\fR
.sp 1P
.RT
.sp 1P
.LP
4.1
    \fISignalling network\fR
.sp 9p
.RT
.PP
Signalling System No.\ 7 is designed for both associated and
nonassociated applications. The reference section in such applications
is the signalling route set, irrespective of whether it is served in the
associated
or quasi\(hyassociated mode of operation.
.PP
For every signalling route set in a signalling network, the
unavailability limit indicated in \(sc\ 1.1 has to be observed irrespective
of the number of signalling links in tandem of which it is composed.
.RT
.sp 1P
.LP
4.1.1
    \fIInternational signalling network\fR
.sp 9p
.RT
.PP
(Needs further study.)
.RT
.sp 1P
.LP
4.1.2
    \fINational signalling network\fR
.sp 9p
.RT
.PP
(Needs further study.)
.RT
.sp 1P
.LP
4.2
    \fIQueueing delays\fR
.sp 9p
.RT
.PP
The Message Transfer Part handles messages from different User
Parts on a time\(hyshared basis. With time\(hysharing, signalling delay
occurs when it is necessary to process more than one message in a given
interval of time. When this occurs, a queue is built up from which messages
are transmitted in
order of their times of arrival.
.PP
There are two different types of queueing delays: queueing delay in
the absence of disturbances and total queueing delay.
.RT
.sp 1P
.LP
4.2.1
    \fIAssumptions for derivation of the formulas\fR
.sp 9p
.RT
.PP
The queueing delay formulas are basically derived from the
\fIM\fR /\fIG\fR /1 queue with priority assignment. The assumptions for
the derivation of the formulas in the absence of disturbances are as
follows:
.RT
.LP
    a)
    the interarrival time distribution is exponential
(\fIM\fR );
.LP
    b)
    the service time distribution is general (\fIG\fR );
.LP
    c)
    the number of server is one (1);
.LP
    d)
    the service priority refers to the transmission priority
within level\ 2 (see Recommendation\ Q.703, \(sc\ 11.2); however, the link
status
signal unit and the independent flag are not considered;
.bp
.LP
    e)
    the signalling link loop propagation time is constant
including the process time in signalling terminals; and
.LP
    f
)
    the forced retransmission case of the preventive
cyclic retransmission method is not considered.
.PP
In addition, for the formulas in the presence of disturbances, the assumptions
are as follows:
.LP
    g)
    the transmission error of the message signal unit is
random;
.LP
    h)
    the errors are statistically independent of each other;
.LP
    i)
    the additional delay caused by the retransmission of the
erroneous signal unit is considered as a part of the waiting time of the
concerned signal unit; and
.LP
    j
)
     in case of the preventive cyclic retransmission method, after the error
occurs, the retransmitted signal units of second priority are accepted
at the receiving end until the sequence number of the last sent new
signal unit is caught up by that of the last retransmitted signal unit.
.PP
Furthermore, the formula of the proportion of messages delayed
more than a given time is derived from the assumption that the probability
density function of the queueing delay distribution may be exponentially
decreasing where the delay time is relatively large.
.sp 1P
.LP
4.2.2
    \fIFactors and parameters\fR
.sp 9p
.RT
.LP
    a)
    The notations and factors required for calculation of the
queueing delays are as follows:
.LP
    \fIQ\fR\d\fIa\fR\u
    mean queueing delay in the absence of
disturbances
.LP
    \(*s
\s6\fIa\fR 2
.PS 10
    variance of
queueing delay in the absence of disturbances
.RT
.LP
    \fIQ\fR\d\fIt\fR\u    mean total queueing delay
.LP
    \(*s
\s62
\fI\fIt\fR .PS 10
    variance of
total queueing delay
.RT
.LP
    \fIP\fR (\fIT\fR )\fR     proportion of messages delayed more  than \fIT\fR
.LP
    \fIa\fR     traffic loading by message signal units (MSU)
(excluding retransmission)
.LP
    \fIT\fR\d\fIm\fR\u    mean emission time of message signal units
.LP
    \fIT\fR\d\fIf\fR\u    emission time of fill\(hyin signal units
.LP
    \fIT\fR\d\fIL\fR\u    signalling loop propagation time including
processing time in signalling terminal
.LP
    \fIP\fR\d\fIu\fR\u    error probability of message signal units
\v'6p'
.LP
    \fIk\fR \d1\u =
@ { nd~moment~of~message~signal~units~emission~time } over { fIT~\dm\u\fR~\u2\d } @
.LP
.sp 1
    \fIk\fR \d2\u =
@ { rd~moment~of~message~signal~units~emission~time } over { fIT~\dm\u\fR~\u3\d } @
.LP
.sp 1
    \fIk\fR \d3\u =
@ { th~moment~of~message~signal~units~emission~time } over { fIT~\dm\u\fR~\u4\d } @
.LP
.sp 1
.LP
    \fINote\fR \ \(em\ As a consequence of zero insertion at level 2 (see
Recommendation\ Q.703, \(sc\ 3.2), the length of the emitted signal unit
will be
increased by approximately 1.6\ percent on average. However, this increase
has negligible effect on the calculation.
.LP
    b)
    The parameters used in the formulas are as follows:
.sp 1P
.ce 1000
\fIt\fR\d\fIf\fR\u\ =\ \fIT\fR\d\fIf\fR\u/\fIT\fR\d\fIm\fR\u
.ce 0
.sp 1P
.ce 1000
\fIt\fR\d\fIL\fR\u\ =\ \fIT\fR\d\fIL\fR\u/\fIT\fR\d\fIm\fR\u
.ce 0
.sp 1P
.LP
    for the basic method,
\v'6p'
.sp 1P
.ce 1000
\fIE\fR \d1\u = 1 + \fIP
\du\u\fR \fIt
\dL\u\fR \v'6p'
.ce 0
.sp 1P
.ce 1000
\fIE\fR \d2\u = \fIk\fR \d1\u + \fIP
\du\u\fR \fIt
\dL\u\fR (\fIt
\dL\u\fR + 2)
\v'6p'
.ce 0
.sp 1P
.ce 1000
\fIE\fR \d3\u = \fIk\fR \d2\u + \fIP
\du\u\fR \fIt
\dL\u\fR (\fIt
\dL\u\fR \u2\d
+ 3\fIt
\dL\u\fR + 3\fIk\fR \d1\u)
.ce 0
.sp 1P
.LP
.sp 1
.bp
.LP
    for the preventive cyclic retransmission (PCR) method,
.LP
    \fIa\fR\d3\u\ =\ exp
(\(em\fIat\fI\d\fIL\fR\u):\ traffic loading caused by
fill\(hyin signal units.
\v'6p'
.LP
    \fIa
\dz\u\fR = 1 \(em \fIa\fR \(em \fIa\fR \d3\u
\v'6p'
.LP
    \fIH\fR \d1\u = \fIat
\dL\u\fR \v'6p'
.LP
    \fIH\fR \d2\u = \fIat
\dL\u\fR (\fIk\fR \d1\u + \fIat
\dL\u\fR )
\v'6p'
.LP
    \fIH\fR \d3\u = \fIat
\dL\u\fR (\fIk\fR \d2\u + 3\fIat
\dL\uk\fR \d1\u +
\fIa\fR \u2\d\fIt
\dL\u\fR \u2\d
)
\v'6p'
.LP
.sp 1
.LP
    \fIF\fR \d1\u = \fIat
\dL\u\fR /2
\v'6p'
.LP
    \fIF\fR \d2\u = \fIat
\dL\u\fR (\fIk\fR \d1\u
/2 + \fIat
\dL\u\fR /3)
\v'6p'
.LP
    \fIF\fR \d3\u = \fIat
\dL\u\fR (\fIk\fR \d2\u
/2 + \fIat
\dL\uk\fR \d1\u +
\fIa\fR \u2\d\fIt
\dL\u\fR \u2\d
/4)
\v'6p'
.LP
    \fIq
\da\u\fR =
@ { fIk\fR~\d1\u (\fIa\fR~+~\fIa~\dz\u\fR ) +~\fIa\fR~\d3\u\fIt~\df~\u\fR } over {  (1~\(em~\fIa\fR ) } @
\v'6p'
.LP
    \fIs
\da\u\fR =
@ { fIak\fR~\d1\u } over { ~\(em~\fIa\fR } @  \fIq
\da\u\fR +
@ { fIk\fR~\d2\u (\fIa\fR~+~\fIa~\dz\u\fR ) +~\fIa\fR~\d3\u\fIt\fR~\fI~\df~\u\fR~\u2\d } over {  (1~\(em~\fIa\fR ) } @
\v'6p'
.LP
    \fIt
\da\u\fR =
@ { \fIak\fR~\d1\u\fIs~\da\u\fR~+~2\fIak\fR~\d2\u\fIq~\da\u\fR } over {  (1~\(em~\fIa\fR ) } @  +
@ { \fIa\fR~+~\fIa~\dz\u\fR )\fIk\fR~\d3\u~+~\fIa\fR~\d3\u\fIt\fR~\fI~\df~\u\fR~\u3\d } over {  (1~\(em~\fIa\fR ) } @
\v'6p'
.LP
    \fIZ\fR \d1\u = 2 + \fIP
\du\u\fR (1 + \fIH\fR \d1\u)
\v'6p'
.LP
    \fIZ\fR \d2\u = 4\fIK\fR \d1\u + \fIP
\du\u\fR (5\fIk\fR \d1\u + 3\fIH\fR \d1\u +
\fIH\fR \d2\u)
\v'6p'
.LP
    \fIZ\fR \d3\u = 8\fIk\fR \d2\u + \fIP
\du\u\fR (19\fIk\fR \d2\u +
27\fIk\fR \d1\u
\fIH\fR \d1\u + 9\fIH\fR \d2\u + \fIH\fR \d3\u)
\v'6p'
.LP
    \fIY\fR \d2\u = \fIs
\da\u\fR + 4\fIk\fR \d1\u + \fIF\fR \d2\u +  { fIq
\da\u\fR (2 +
\fIF\fR \d1\u) + 2\fIF\fR \d1\ }
\v'6p'
.LP
    \fIY\fR \d3\u = \fIt
\da\u\fR + 8\fIk\fR \d2\u + \fIF\fR \d3\u +  { fIs
\da\u\fR (2 +
\fIF\fR \d1\u) + \fIq
\da\u\fR (4\fIk\fR \d1\u + \fIF\fR \d2\u) + 2
\fIF\fR + 2 +
4\fIk\fR \d1\u
\fIF\fR \d1\ } + 12\fIq
\da\uF\fR \d1\u
\v'6p'
.LP
    \(*a =
@ { ~\(em~\fIa\fR { 2~+~\fIP~\du\u\fR (1~+~\fIat~\dL\u\fR~ } } over { ~+~~~\fIq~\da\u\fR~+~\fIat~\dL\u\fR~/2 } @
\v'6p'
.LP
    \fIq
\dd
\u\fR =
@ { fIaZ\fR~\d2\u~+~\(*a\fIY\fR~\d2\u } over {  (1~\(em~\fIaZ\fR~\d1\u) } @
\v'6p'
.LP
    \fIs
\dd
\u\fR =
@ { fIaZ\fR~\d2\u } over { ~\(em~\fIaZ\fR~\d1\u } @  \fIq
\dd
\u\fR +
@ { fIaZ\fR~\d3\u~+~\(*a\fIY\fR~\d3\u } over {  (1~\(em~\fIaZ\fR~\d1\u) } @
\v'6p'
.LP
    \fIq
\db\u\fR =
@ { fIq~\da\u\fR~+~1~+~\fIF\fR~\d1\u } over { ~\(em~\fIa\fR } @
\v'6p'
.LP
    \fIs
\db\u\fR =
@ { fIs~\da\u\fR~+~\fIk\fR~\d1\u~+~\fIF\fR~\d2\u } over { 1~\(em~\fIa\fR ) \u3\d } @  +
@ { { fIq~\da~\u\fR (1~+~\fIF\fR~\d1\u) +~\fIF\fR~\d1\ } } over { 1~\(em~\fIa\fR ) \u2\d } @
\v'6p'
.LP
    \fIq
\dc\u\fR =
@ { fIq~\dd~\u\fR~+~1~+~\fIP~\du\u\fR (1~+~\fIH\fR~\d1\u) } over { ~\(em~\fIa\fR } @
\v'6p'
.LP
    \fIs
\dc\u\fR =
@ { fIs~\dd~\u\fR~+~\fIk\fR~\d1\u~+~\fIP~\du\u\fR (3\fIk\fR~\d1\u~+~\fIH\fR~\d2\u) } over { 1~\(em~\fIa\fR ) \u3\d } @  + 2
@ { fIq~\dd~\u\fR~+~\fIP~\du\u\fR { fIq~\dd~\u\fR (1~+~\fIH\fR~\d1\u) +~2\fIH\fR~\d1\ } } over { 1~\(em~\fIa\fR ) \u2\d } @
\v'6p'
.LP
    \fIP
\dV\u\fR = \fIP
\du\ua\fR
@ { fIq~\da\u\fR~+~2~+~\fIat~\dL\u\fR~/2 } over { ~\(em~2\fIa\fR } @
@ left ( 1~+~\fIP~\du\u\fR~ { fIa\fR~+~\fIa\fR~\u2\d\fIt~\dL\u\fR } over { ~\(em~2\fIa\fR } right ) @
.bp
.sp 1P
.LP
4.2.3
    \fIFormulas\fR
.sp 9p
.RT
.PP
The formulas of the mean and the variance of the queueing delays
are described in Table\ 1/Q.706. The proportion of messages delayed more
than a given time \fIT\fR\d\fIx\fR\uis:
\v'6p'
.RT
.sp 1P
.ce 1000
\fIP\fR (
\fIT
\dx\u\fR ) \( = exp
@ left ( \(em~ { fIT~\dx\u\fR~\(em~\fIQ~\dx\u\fR~+~\(*s~\fI~\dx\u\fR } over { (*s~\fI~\dx\u\fR }  right ) @
.ce 0
.sp 1P
.LP
.sp 1
where \fIQ\fR\d\fIx\fR\u | and \(*s\fI\fI\d\fIx\fR\u | denote the mean
and the standard deviation of
queueing delay, respectively. This approximation is better suited in absence
of disturbances. In the presence of disturbances the actual distribution
may be
deviated further. Relation between \fIP\fR (\fIT\fR\d\fIx\fR\u) and\ \fIT\fR\d\fIx\fR\uis
shown in Figure\ 1/Q.706.
.sp 1P
.LP
4.2.4
    \fIExamples\fR
.sp 9p
.RT
.PP
Assuming the traffic models given in Table 2/Q.706, examples of
queueing delays are calculated as listed in Table\ 3/Q.706.
.PP
\fINote\fR \ \(em\ The values in the table were determined based on TUP
messages. With the increase of the effective message length, using ISUP
and TC, these values may be expected to be increased during the course
of further
study.
.RT
.LP
.rs
.sp 34P
.ad r
\fBFigure 1/Q.706, p.21\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.ce
\fBH.T. [T1.706]\fR
.ce
TABLE\ 1/Q.706
.ce
\fBQueueing delay formula\fR
.ps 9
.vs 11
.nr VS 11
.nr PS 9
.TS
center box;
cw(30p) | cw(30p) | cw(84p) | cw(84p) , ^  | l | l | l.
Error correction method    Disturbance    Mean \fIQ\fR    Variance \(*s\u2\d

_
.T&
cw(30p) | cw(30p) | cw(84p) | cw(84p) , ^  | c | c | c.
Basic    Absence     {
@ { fIQ~\da\u\fR } over { fIT~\dm\u\fR } @ \fR
=
[Unable to Convert Formula]
+
@ { fIak\fR~\d1\u\fR } over { (1 | (em | fIa\fR ) } @ \fR
 }     {
@ { \(*s~$$Ei:2:\fIa\fR~_\fR } over { fIT\fR~$$Ei:~2:\fIm\fR~_ } @ \fR
=
@ { \fIt\fR~$$Ei:~2:\fIf\fR~_\fR } over { 2 } @
+
@ { \fIa\fR~[4\fIk\fR~\d2\u~\(em (4\fIk\fR~\d2\u~\(em~3\fIk\fR~$$Ei:~2:1_ )\fIa\fR~] } over { 2 (1 | (em | fIa\fR ) \u2\d } @
 }
    Presence     {
@ { fIQ~\dt\u\fR } over { fIT~\dm\u\fR } @ \fR
=
[Unable to Convert Formula]
+
@ { fIaE\fR~\d2\u\fR } over { (1 | (em | fIaE\fR~\d1\u) } @ + \fIE\fR
\d1\u\fR
\(em 1
 }     {
@ { \(*s~$$Ei:2:\fIt\fR~_\fR } over { fIT\fR~$$Ei:~2:\fIm\fR~_ } @ \fR
=
@ { \fIt\fR~$$Ei:~2:\fIf\fR~_\fR } over { 2 } @
+
@ { \fIa\fR~[4\fIE\fR~\d3\u~\(em (4\fIE\fR~\d1\u\fIE\fR~\d3\u~\(em~3\fIE\fR~$$Ei:~2:~2_ )\fIa\fR~] } over { 2(1 | (em | fIaE\fR~\d1\u) \u2\d } @
+ \fIP
\du\u\fR
(1 | (em | fIP
\du\u\fR
)
\fIt\fR
$$Ei:
2:\fIL\fR
_
$$Be
 }
_
.T&
cw(30p) | cw(30p) | cw(84p) | cw(84p) , ^  | c | c | c.
 {
Preventive cyclic retransmission
 }    Absence     {
$$Bo\fIQ
\da\u\fR
} over { fIT
\dm\u\fR
$$Be\fR
= \fIq
\da
\u\fR
 }     {
$$Bo\(*s
$$Ei:2:\fIt\fR
_\fR
} over { fIT\fR
$$Ei:
2:\fIm\fR
_
$$Be\fR
= \fIs\fI
\(em \fIq\fR
$$Ei:
2:\fIa\fR
_\fR
 }
    Presence     {
$$Bo\fIQ
\dt\u\fR
} over { fIT
\dm\u\fR
$$Be\fR
= (1 | (em | fIP\fI
 | (em | fIP\fI
) \fIq\fI
 |  | fIP\fI
\fIq\fI
 |  | fIP\fI
\fIq\fI
 }     {
$$Bo\(*s
$$Ei:2:\fIt\fR
_\fR
} over { fIT\fR
$$Ei:
2:\fIm\fR
_
$$Be\fR
= (1 | (em | fIP\fI
 | (em | fIP\fI
) \fIs\fI
 |
+ | fIP\fI
\fIs\fI
 |  | fIP\fI
\fIs\fI
 | (em |
$$Bo\fIQ\fR
$$Ei:2:\fIt\fR
_
} over { fIT\fR
$$Ei:
2:\fIm\fR
_
$$Be
 }
_
.TE
.nr PS 9
.RT
.ad r
\fBTableau 1/Q.706 [T1.706], p.22\fR
.sp 1P
.RT
.ad b
.RT
.LP
.sp 5
.ce
\fBH.T. [T2.706]\fR
.ce
TABLE\ 2/Q.706
.ce
\fBTraffic model\fR
.ps 9
.vs 11
.nr VS 11
.nr PS 9
.TS
center box;
lw(66p) | cw(18p) | cw(36p) .
Model    A    B
_
.T&
lw(66p) | cw(18p) | cw(18p) | cw(18p) .
Message length (bits)    120    104    304
_
.T&
lw(66p) | cw(18p) | cw(18p) | cw(18p) .
Percent    100    \ 92    \ \ 8
_
.T&
lw(66p) | cw(18p) | cw(36p) .
Mean message length (bits)    120    120
_
.T&
lw(66p) | cw(18p) | cw(36p) .
\fIk\fR 1    1.0    1.2
_
.T&
lw(66p) | cw(18p) | cw(36p) .
\fIk\fR 2    1.0    1.9
_
.T&
lw(66p) | cw(18p) | cw(36p) .
\fIk\fR 3    1.0    3.8
_
.TE
.nr PS 9
.RT
.ad r
\fBTableau 2/Q.706 [T2.706], p.23\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.ce
\fBH.T. [T3.706]\fR
.ce
TABLE\ 3/Q.706
.ce
\fBList of examples\fR
.ps 9
.vs 11
.nr VS 11
.nr PS 9
.TS
center box;
cw(24p) | cw(36p) | cw(48p) | cw(36p) | cw(36p) .
Figure    Error control    Queueing delay    Disturbance    Model
_
.T&
lw(24p) | lw(36p) | lw(48p) | lw(36p) | lw(36p) .
2/Q.706    Basic/PCR    Mean    Absence    A and B
_
.T&
lw(24p) | lw(36p) | lw(48p) | lw(36p) | lw(36p) .
3/Q.706    Basic/PCR    Standard deviation    Absence    A and B
_
.T&
lw(24p) | lw(36p) | lw(48p) | lw(36p) | lw(36p) .
4/Q.706    Basic    Mean    Presence    A
_
.T&
lw(24p) | lw(36p) | lw(48p) | lw(36p) | lw(36p) .
5/Q.706    Basic    Standard deviation    Presence    A
_
.T&
lw(24p) | lw(36p) | lw(48p) | lw(36p) | lw(36p) .
6/Q.706    PCR    Mean    Presence    A
_
.T&
lw(24p) | lw(36p) | lw(48p) | lw(36p) | lw(36p) .
7/Q.706    PCR    Standard deviation    Presence    A
_
.TE
.nr PS 9
.RT
.ad r
\fBTableau 3/Q.706 [T3.706], p.24\fR
.sp 1P
.RT
.ad b
.RT
.LP
.rs
.sp 31P
.ad r
\fBFigure 2/Q.706, p.25\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.LP
.rs
.sp 24P
.ad r
\fBFigure 3/Q.706, p.26\fR
.sp 1P
.RT
.ad b
.RT
.LP
.rs
.sp 24P
.ad r
\fBFigure 4/Q.706, p.27\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.LP
.rs
.sp 47P
.ad r
\fBFigure 5/Q.706, p.28\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.LP
.rs
.sp 26P
.ad r
\fBFigure 6/Q.706, p.29\fR
.sp 1P
.RT
.ad b
.RT
.LP
.rs
.sp 23P
.ad r
\fBFigure 7/Q.706, p.30\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.sp 1P
.LP
4.3
    \fIMessage transfer times\fR
.sp 9p
.RT
.PP
Within a signalling relation, the Message Transfer Part transports messages
from the originating User Part to the User Part of destination, using several
signalling paths. The overall message transfer time needed depends on the
message transfer time components (a) to (e) involved in each signalling
path.
.RT
.sp 1P
.LP
4.3.1
    \fIMessage transfer time components and functional reference points\fR
.sp 9p
.RT
.PP
A signalling path may include the following functional signalling network
components and transfer time components.
.RT
.LP
    a)
    Message Transfer Part sending function at the point
of origin (see Figure\ 8/Q.706).
.LP
    b)
    Signalling transfer point function (see Figure\ 9/Q.706).
.LP
    c)
    Message Transfer Part receiving function at the point
of destination (see Figure\ 10/Q.706).
.LP
    d)
    Signalling data link propagation time (see
Figure\ 11/Q.706).
.LP
    e)
    Queueing delay.
.LP
    An additional increase of the overall message transfer times is
caused by the queueing delays. These are described in \(sc\ 4.2.
.LP
.rs
.sp 15P
.ad r
\fBFigure 8/Q.706, p.31\fR
.sp 1P
.RT
.ad b
.RT
.LP
.rs
.sp 15P
.ad r
\fBFigure 9/Q.706, p.32\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.LP
.rs
.sp 12P
.ad r
\fBFigure 10/Q.706, p.33\fR
.sp 1P
.RT
.ad b
.RT
.LP
.rs
.sp 12P
.ad r
\fBFigure 11/Q.706, p.34\fR
.sp 1P
.RT
.ad b
.RT
.sp 2P
.LP
4.3.2
    \fIDefinitions\fR
.sp 1P
.RT
.sp 1P
.LP
4.3.2.1
    \fBmessage transfer part sending time T\fR\(da\fBm\fR\(da\fBs\fR
.sp 9p
.RT
.LP
    \fIF:\ temps d'\*'emission du Sous\(hysyst\*`eme Transport de\fR
\fIMessages\ T\fI\d\fIm\fR\\d\fIs\fR\u
.LP
    \fIS:\ tiempo de emisi\*'on de la parte de transferencia de\fR
\fImensajes\ T\fI\d\fIm\fR\\d\fIs\fR\u
.PP
\fIT\fR\d\fIm\fR\\d\fIs\fR\u | is the period which starts when the last
bit of the
message has left the User Part and ends when the last bit of the signal unit
enters the signalling data link for the first time. It includes the queueing
delay in the absence of disturbances, the transfer time from level\ 4 to
level\ 3, the handling time at level\ 3, the transfer time from level\ 3 to
level\ 2, and the handling time in level\ 2.
.RT
.sp 1P
.LP
4.3.2.2
    \fBmessage transfer time at signalling transfer\fR
\fBpoints\ T\fR\(da\fBc\fR\(da\fBs\fR
.sp 9p
.RT
.LP
    \fIF:\ temps de transfert des messages aux points de\fR
\fItransfert s\*'emaphore T\fI\d\fIc\fR\\d\fIs\fR\u
.LP
    \fIS:\ tiempo de transferencia de mensajes en los puntos\fR
\fIde transferencia de la se\o"n~"alizaci\*'on T\fI\d\fIc\fR\\d\fIs\fR\u
.PP
\fIT\fR\d\fIc\fR\\d\fIs\fR\u | 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 also includes the queueing delay in the absence of disturbances
but not the additional queueing delay caused by retransmission.
.bp
.RT
.sp 1P
.LP
4.3.2.3
    \fBmessage transfer part receiving time T\fR\(da\fBm\fR\(da\fBr\fR
.sp 9p
.RT
.LP
    \fIF:\ temps de r\*'eception du Sous\(hysyst\*`eme Transport de\fR
\fIMessages\ T\fI\d\fIm\fR\\d\fIr\fR\u
.LP
     \fIS:\ tiempo de recepci\*'on de la parte de transferencia de\fR \fImensajes
T\fI\d\fIm\fR\\d\fIr\fR\u
.PP
\fIT\fR\d\fIm\fR\\d\fIr\fR\u | is the period which starts when the last
bit of the
signal unit leaves the signalling data link and ends when the last bit
of the message has entered the User Part. It includes the handling time
in level\ 2,
the transfer time from level\ 2 to level\ 3, the handling time in level\
3 and the transfer time from level\ 3 to level\ 4.
.RT
.sp 1P
.LP
4.3.2.4
    \fBdata channel propagation time T\fR\(da\fBp\fR
.sp 9p
.RT
.LP
    \fIF:\ temps de propagation sur la voie de donn\*'ees T\fI\d\fIp\fR\u
.LP
    \fIS:\ tiempo de propagaci\*'on del canal de datos T\fI\d\fIp\fR\u
.PP
\fIT\fR\d\fIp\fR\uis the period which starts when the last bit of the signal
unit has entered the data channel at the sending side and ends when the
last
bit of the signal unit leaves the data channel at the receiving end
irrespective of whether the signal unit is disturbed or not.
.RT
.sp 1P
.LP
4.3.3
    \fIOverall message transfer times\fR
.sp 9p
.RT
.PP
The overall message transfer time \fIT\fR\d\fIo\fR\uis referred to the
signalling relation. \fIT\fR\d\fIo\fR\u\ starts when the message has left
the user part
(level\ 4) at the point of origin and ends when the message has entered
the user part (level\ 4) at the point of destination.
.PP
The definition of the overall message transfer time and the
definitions of the individual message transfer time components give rise
to the following relationships:
.RT
.LP
    a)
    In the absence of disturbances
\v'6p'
.sp 1P
.ce 1000
\fIT\fR \d\fIoa\fR \u = \fIT\fR \d\fIms\fR \u +
$$SO\fIn\fR +1
above sum above \fIi\fR =1
$$SE
\fIT
\dpi
\u\fR +
$$SO\fIn\fR above sum above \fIi\fR =1
$$SE \fIT
\dcsi
\u\fR +
\fIT
\dmr
\u\fR
.ce 0
.sp 1P
.LP
.sp 1
    b)
    In the presence of disturbances
\v'6p'
.sp 1P
.ce 1000
\fIT
\do\u\fR = \fIT
\doa
\u\fR +
$$So
above sum above
$$Se(\fIQ
\dt\u\fR \(em \fIQ
\da\u\fR )
.ce 0
.sp 1P
.LP
.sp 1
.LP
    Here
.LP
    \fIT\fR\d\fIo\fR\\d\fIa\fR\u    overall message transfer time in the absence of
disturbances
.LP
    \fIT\fR\d\fIm\fR\\d\fIs\fR\u    Message Transfer Part sending time
.LP
    \fIT\fR\d\fIm\fR\\d\fIr\fR\u    Message Transfer Part receiving time
.LP
     \fIT\fR\d\fIc\fR\\d\fIs\fR\u Message transfer time at signalling transfer
points
.LP
    \fIn\fR     number of STPs involved
.LP
    \fIT\fR\d\fIp\fR\u    data channel propagation time
.LP
    \fIT\fR\d\fIo\fR\u    overall message transfer time in the presence of
disturbances
.LP
    \fIQ\fR\d\fIt\fR\u    total queueing delay (see \(sc\ 4.2)
.LP
    \fIQ\fR\d\fIa\fR\u    queueing delay in the absence of disturbances (see
\(sc\ 4.2)
.PP
\fINote\fR \ \(em\ For \(*s"(\fIQ\fR\d\fIt\fR\u\ \(em\ \fIQ\fR\d\fIa\fR\u),
all signalling points
in the signalling relation must be taken into account.
.sp 1P
.LP
4.3.4
    \fIEstimates for message transfer times\fR
.sp 9p
.RT
.PP
(Needs further study.)
.PP
The estimates must take account of:
.RT
.LP
    \(em
    the length of the signal unit,
.LP
    \(em
    the signalling traffic load,
.LP
    \(em
    the signalling bit rate.
.bp
.PP
The estimates for \fIT\fR\d\fIm\fR\\d\fIr\fR\u, \fIT\fR\d\fIm\fR\\d\fIs\fR\u |
and \fIT\fR\d\fIc\fR\\d\fIs\fR\u | will be presented in the form of:
.LP
    \(em
    mean values,
.LP
    \(em
    95% level values.
.PP
The estimates for \fIT\fR\d\fIc\fR\\d\fIs\fR\u | for a signalling transfer
point are given in Table\ 4/Q.706.
.LP
.rs
.sp 11P
.ad r
\fBTABLE [T4.706], p.\fR
.sp 1P
.RT
.ad b
.RT
.PP
.sp 2
\fINote\fR \ \(em\ the values in the table were determined based on TUP
messages. With the increase of the effective message length, using ISUP
and TC, these values may be expected to be increased during the course
of further
study.
.PP
These figures are related to 64\(hykbit/s signalling bit rate. The normal
signalling traffic load is that load for which the signalling transfer
point is engineered. A mean value of 0.2\ Erlang per signalling link is
assumed. The
message length distribution is as given in Table\ 2/Q.706.
.RT
.sp 1P
.LP
4.4
    \fIError control\fR
.sp 9p
.RT
.PP
During transmission, the signal units are subject to disturbances which
lead to a falsification of the signalling information. The error control
reduces the effects of these disturbances to an acceptable value.
.PP
Error control is based on error detection by redundant coding and on error
correction by retransmission. Redundant coding is performed by generation
of 16\ check bits per signal unit based on the polynomial described in
Recommendation\ Q.703, \(sc\ 4.2. Moreover, the error control does not
introduce loss, duplication or mis\(hysequencing of messages on an individual
signalling link.
.PP
However, abnormal situations may occur in a signalling relation, which
are caused by failures, so that the error control for the signalling link
involved cannot ensure the correct message sequence.
.RT
.sp 1P
.LP
4.5
    \fISecurity arrangements\fR
.sp 9p
.RT
.PP
The security arrangements have an essential influence on the
observance of the availability requirements listed in \(sc\ 1.1 for a
signalling relation.
.PP
In the case of Signalling System No.\ 7, the security arrangements are
mainly formed by redundancy in conjunction with changeover.
.RT
.sp 1P
.LP
4.5.1
    \fITypes of security arrangements\fR
.sp 9p
.RT
.PP
In general, a distinction has to be made between security
arrangements for the individual components of the signalling network and
security arrangements for the signalling relation. Within a signalling
network, any security arrangement may be used, but it must be ensured that
the
availability requirements are met.
.bp
.RT
.sp 1P
.LP
4.5.1.1
    \fISecurity arrangements for the components of the signalling\fR
\fInetwork\fR
.sp 9p
.RT
.PP
Network components, which form a signalling path when being
interconnected, either have constructional security arrangements which exist
from the very beginning (e.g.\ replication of the controls at the exchanges
and signalling transfer points) or can be replicated, if need be (e.g.\
signalling data links). For security reasons, however, replication of signalling
data
links is effected only if the replicated links are independent of one another
(e.g.\ multipath routing). In the case of availability calculations for
a
signalling path set, special care has to be taken that the individual
signalling links are independent of one another.
.RT
.sp 1P
.LP
4.5.1.2
    \fISecurity arrangements for signalling relations\fR
.sp 9p
.RT
.PP
In quasi\(hyassociated signalling networks where several signalling
links in tandem serve one signalling relation, the security arrangements for
the network components, as a rule, do not ensure sufficient availability
of the signalling relation. Appropriate security arrangements must therefore
be made for the signalling relations by the provision of redundant signalling
path
sets, which have likewise to be independent of one another.
.RT
.sp 1P
.LP
4.5.2
    \fISecurity requirements\fR
.sp 9p
.RT
.PP
In the case of 64\(hykbit/s signalling links, a signalling network has
to be provided with sufficient redundancy so that the quality of the signalling
traffic handled is still satisfactory. (Application of the above to signalling
links using lower bit rates needs further study.)
.RT
.sp 1P
.LP
4.5.3
    \fITime to initiate changeover\fR
.sp 9p
.RT
.PP
If individual signalling data links fail, due to excessive error
rates, changeover is initiated by signal unit error monitoring (see
Recommendation\ Q.703, \(sc\ 8). With signal unit error monitoring, the
time between the occurrence of the failure and the initiation of changeover
is dependent on the message error rate (a complete interruption will result
in an error rate
equal to\ 1).
.PP
Changeover leads to substantial additional queueing delays. To keep
the latter as short as possible, the signalling traffic affected by an
outage is reduced to a minimum by the use of load sharing on all existing
signalling links.
.RT
.sp 1P
.LP
4.5.4
    \fIChangeover performance times\fR
.sp 9p
.RT
.PP
There are two performance times associated with link changeover.
Both times are maximum time values (not normal values). They are defined
to be the point at which 95% of the events occur within the recommended
performance time at a signalling point traffic load that is 30% above normal.
.PP
The performance times are measured from outside the signalling
point.
.RT
.sp 1P
.LP
4.5.4.1
    \fIFailure response time\fR
.sp 9p
.RT
.PP
This time describes the time taken by a signalling point to
recognize that a changeover is needed for a signalling link. This time
begins when the signalling link is unavailable, and ends when the signalling
point
sends a changeover (or emergency changeover) order to the remote signalling
point. A link is unavailable when a signalling unit with status indication
out of service (SIOS) or processor outage (SIPO) is sent or received on
the link.
.PP
Failure response time (maximum permissible): 500 ms.
.RT
.sp 1P
.LP
4.5.4.2
    \fIAnswer time to changeover order\fR
.sp 9p
.RT
.PP
This time describes the time taken by a signalling point to answer a changeover
(or emergency changeover) order. This time begins when the
signalling point receives a changeover (or emergency changeover) order
message, and ends when the signalling point sends a changeover (or emergency
changeover) acknowledgement message.
.PP
Answer time to changeover order (maximum permissible):
300 ms.
.bp
.RT
.sp 2P
.LP
4.6
    \fIFailures\fR
.sp 1P
.RT
.sp 1P
.LP
4.6.1
    \fILink failures\fR
.sp 9p
.RT
.PP
During transmission, the messages may be subject to disturbances. A measure
of the quality of the signalling data link is its signal unit error
rate.
.PP
Signal unit error monitoring initiates the changeover at a signal unit
error rate of about\ 4 | (mu | 0\uD\dlF261\u3\d.
.PP
The error rate, which Signalling System No.\ 7 has to cope with,
represents a parameter of decisive influence on its efficiency.
.PP
As a result of error correction by retransmission, a high error rate causes
frequent retransmission of the message signal units and thus long
queueing delays.
.RT
.sp 1P
.LP
4.6.2
    \fIFailures in signalling points\fR
.sp 9p
.RT
.PP
(Needs further study.)
.RT
.sp 1P
.LP
4.7
    \fIPriorities\fR
.sp 9p
.RT
.PP
Priorities resulting from the meaning of the individual signals are not
envisaged. Basically, the principle \*Qfirst\(hyin \(em\ first\(hyout\*U
applies.
.PP
Although the service indicator offers the possibility of determining different
priorities on a user basis, such user priorities are not yet
foreseen.
.PP
Transmission priorities are determined by Message Transfer Part
functions. They are solely dependent on the present state of the Message
Transfer Part and completely independent of the meaning of the signals (see
Recommendation\ Q.703, \(sc\ 11).
.RT
.sp 2P
.LP
\fB5\fR     \fBPerformance under adverse conditions\fR
.sp 1P
.RT
.sp 1P
.LP
5.1
    \fIAdverse conditions\fR
.sp 9p
.RT
.PP
(Needs further study.)
.RT
.sp 1P
.LP
5.2
    \fIInfluence of adverse conditions\fR
.sp 9p
.RT
.PP
(Needs further study.)
.RT
.sp 2P
.LP
    \fBReference\fR
.sp 1P
.RT
.LP
[1]
    CCITT Recommendation \fIError performance on an international digital\fR
\fIconnection forming part of an integrated services digital network\fR ,
Vol.\ III, Rec.\ G.821.
\v'6p'
.sp 2P
.LP
\fBRecommendation\ Q.707\fR
.RT
.sp 2P
.sp 1P
.ce 1000
\fBTESTING\ AND\ MAINTENANCE\fR
.EF '%    Fascicle\ VI.7\ \(em\ Rec.\ Q.707''
.OF '''Fascicle\ VI.7\ \(em\ Rec.\ Q.707    %'
.ce 0
.sp 1P
.LP
\fB1\fR     \fBGeneral\fR
.sp 1P
.RT
.PP
In order to realize the performance requirements described in
Recommendation\ Q.706, means and procedures for signalling network
testing and maintenance are required in addition to the means defined in
Recommendations\ Q.703 and\ Q.704.
.RT
.sp 2P
.LP
\fB2\fR     \fBTesting\fR
.sp 1P
.RT
.sp 1P
.LP
2.1
    \fISignalling data link test\fR
.sp 9p
.RT
.PP
As defined in Recommendation Q.702, \(sc\ 1, the signalling
data link is a bidirectional transmission path for signalling. Testing and
maintenance functions can be initiated independently at either end.
.bp
.PP
The signalling data link and the constituent parts of the digital and analogue
versions are described in Recommendation\ Q.702, \(sc\ 1.
.PP
They must be tested before being put into service to ensure that they meet
the requirements of Recommendation\ Q.702, \(sc\ 3.
.PP
Since interruptions of the signalling data link will affect many
transactions, they must be treated with the utmost care. Appropriate special
measures should be taken to prevent unauthorized maintenance access which
could result in interruptions to service. These special measures may include
marking or flagging the equipment and indications on distribution frames
or test bays where access is possible (see Recommendation\ M.1050\ [1]).
.PP
The signal unit error rate monitor and the alignment error rate
monitor described in Recommendation\ Q.703, \(sc\ 10, also provide means
for detecting deterioration of a signalling data link.
.PP
Further studies are required with reference to
Recommendation\ V.51\ [2].
.RT
.sp 1P
.LP
2.2
    \fISignalling link test\fR
.sp 9p
.RT
.PP
As defined in Recommendation Q.703, \(sc\ 1.1.1 and illustrated in
Figure\ 1/Q.701, the signalling link comprises a signalling data link with
signalling link functions at either end.
.PP
In the following, an on\(hyline signalling link test procedure is
specified which involves communication between the two ends of the concerned
signalling link. This procedure is to be used when a signalling link
is
activated or restored (see Recommendation\ Q.704, \(sc\ 12). The signalling
link
becomes available only if the test is successful. This procedure is intended
for use while the signalling link is in service. In addition, local failure
detection procedures should be performed at either end; these are not specified
in this Recommendation.
.PP
In case the signalling link test (SLT) is applied while the
signalling link is in service the signalling link test message is sent at
regular intervals\ T2 (see \(sc\ 5.5). The testing of a signalling link
is performed independently from each end.
.PP
The ability to send a signalling test acknowledgement message, defined
below, must always be provided at a signalling point.
.PP
The signalling point initiating the tests transmits a signalling link test
message on the signalling link to be tested. This message includes a test
pattern which is chosen at the discretion of the end initiating the test.
After receiving a signalling link test message, a signalling point responds
with a
signalling link test acknowledgement message on the signalling link identified
by the SLS contained in the signalling link test message. The test pattern
included in the signalling
link test acknowledgement message is identical to the test pattern received.
.PP
The signalling link test will be considered successful only if the
received signalling link test acknowledgement message fulfills the following
criteria:
.RT
.LP
    a)
    the SLC identifies the physical signalling link on which the
SLTA was received.
.LP
    b)
    the OPC identifies the signalling point at the other end of
the link.
.LP
    c)
    the test pattern is correct.
.PP
In the case when the criteria given above are not met or a
signalling link test acknowledgement message is not received on the link
being tested within\ T1 (see \(sc\ 5.5) after the signalling link test
message has been
sent, the test is considered to have failed and is repeated once. In the
case when also the repeated test fails, the following actions have to be
taken:
.LP
    \(em
    SLT applied on activation/restoration,
the link is put out of service, restoration is attempted and
a management system must be informed.
.LP
    \(em
    SLT applied periodically,
for further study.
.PP
The formats and codes of signalling link test and signalling link test
acknowledgement messages used for signalling link testing are specified
in \(sc\ 5.4.
.sp 2P
.LP
\fB3\fR     \fBFault location\fR
.sp 1P
.RT
.PP
Fault location operations, employing particular manual or automatic internal
test equipment are left to the discretion of the individual signalling
points.
.PP
Tests requiring provision of messages are for further study.
See\ [3].
.bp
.RT
.sp 2P
.LP
\fB4\fR     \fBSignalling network monitoring\fR
.sp 1P
.RT
.PP
In order to obtain information on the status of the signalling
network, monitoring of the signalling activity must be provided (for example
measures of the signalling load on the signalling data link). The specification
of such means and procedures is contained in Recommendations\ Q.791 and
Q.795.
.RT
.sp 2P
.LP
\fB5\fR     \fBFormats and codes of signalling network testing and\fR
\fBmaintenance messages\fR
.sp 1P
.RT
.sp 1P
.LP
5.1
    \fIGeneral\fR
.sp 9p
.RT
.PP
The signalling network testing and maintenance messages are carried on
the signalling channel in message signal units, the format of which is
described in Recommendation\ Q.703, \(sc\ 2. As indicated in Recommendation\
Q.704, \(sc\ 14.2.1, these messages are distinguished by the configuration\
0001 of the
service indicator (SI). The Sub Service Field (SSF) of signalling network
testing and maintenance messages is used in accordance with
Recommendation\ Q.704, \(sc\ 14.2.2.
.PP
The Signalling Information Field (SIF) consists of an integral
number of octets and contains the label, the heading code and one or more
signals and indications.
.RT
.sp 1P
.LP
5.2
    \fILabel\fR
.sp 9p
.RT
.PP
For signalling network testing and maintenance messages, the label has
the same structure as the label of signalling network management messages
(see Recommendation\ Q.704, \(sc\ 15.2).
.RT
.sp 1P
.LP
5.3
    \fIHeading code H0\fR
.sp 9p
.RT
.PP
The heading code H0 is the 4\(hybit field following the label and
identifies the message group. The different heading codes are allocated as
follows:
.RT
.LP
    0000 Spare
.LP
    0001 Test messages
.PP
The remaining codes are spare.
.sp 1P
.LP
5.4
    \fISignalling link test messages\fR
.sp 9p
.RT
.PP
The format of the signalling link test messages is shown in
Figure\ 1/Q.707.
.RT
.LP
.rs
.sp 8P
.ad r
\fBFigure 1/Q.707, p.\fR
.sp 1P
.RT
.ad b
.RT
.PP
The signalling link test messages, are made up of the following
fields:
.LP
    \(em
    Label: (32 bits), see \(sc\ 5.2
.LP
    \(em
    Heading code H0: (4 bits)
.LP
    \(em
    Heading code H1: (4 bits)
.LP
    \(em
    Spare bits: (4 bits)
.LP
    \(em
    Length indicator: (4 bits)
.LP
    \(em
    Test pattern: (\fIn\fR \ \(mu\ 8\ bits, 1\ \(=\ \fIn\fR \ \(=\ 15).
.PP
In the label, the signalling link code identifies the signalling link on
which the test message is sent.
.bp
.PP
The heading code H1 contains signal codes as follows:
.RT
.LP
    bits
    D
    C
    B
    A
.LP
    0
    0
    0
    1\ signalling link test message (SLTM)
.LP
    0
    0
    1
    0\ signalling link test acknowledgement
message (SLTA)
.PP
The length indicator gives the number of octets which the test
pattern comprises.
.PP
The test pattern is an integral number of octets and is chosen at the discretion
of the originating point.
.RT
.sp 1P
.LP
5.5
    \fITime\(hyout values and tolerances\fR
.sp 9p
.RT
.LP
.sp 1
.rs
.sp 10P
.ad r
\fBTable [T1.707], p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.sp 2
.sp 2P
.LP
\fB6\fR     \fBState transition diagrams\fR
.sp 1P
.RT
.PP
The state transition diagram is intended to show precisely the
behaviour of the signalling system under normal and abnormal conditions as
viewed from a remote location. It must be emphasized that the functional
partitioning shown in the following diagram is used only to facilitate
understanding of the system behaviour and is not intended to specify the
functional partitioning to be adopted in a practical implementation of the
signalling system.
.RT
.LP
.rs
.sp 19P
.ad r
Blanc
.ad b
.RT
.LP
.bp
.LP
.rs
.sp 47P
.ad r
\fBFigure\ 2/Q.707 (feuillet 1 sur 2), p.38\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.LP
.rs
.sp 47P
.ad r
\fBFigure\ 2/Q.707 (feuillet 2 sur 2), p.39\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.sp 2P
.LP
    \fBReferences\fR
.sp 1P
.RT
.LP
[1]
    CCITT Recommendation \fILining up an international point\(hyto\(hypoint\fR
\fIleased circuit\fR , Vol.\ IV, Rec.\ M.1050.
.LP
[2]
    CCITT Recommendation \fIOrganization of the maintenance of\fR
\fIinternational telephone\(hytype circuits used for data transmission\fR ,
Vol.\ VIII, Rec.\ V.51.
.LP
[3]
    \fIIbid.\fR , \(sc\ 5.
.LP
.rs
.sp 48P
.sp 2P
.LP
\fBMONTAGE: RECOMMANDATION Q.708 SUR LE RESTE DE CETTE PAGE\fR
.sp 1P
.RT
.LP
.bp