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All drawings appearing in this Fascicle have been done in Autocad.
Recommendation Q.705
SIGNALLING NETWORK STRUCTURE
1 Introduction
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 signalling transfer points 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 signalling network
procedures may be applied to the mesh network representation.
The national and international networks are considered to be structurally
independent and, although a particular signalling point may belong to both
networks, signalling points are allocated signalling point codes according to the
rules of each network.
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.
The most elementary signalling network consists of originating and
destination signalling points connected by a single signalling link. 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 associated
mode.
For technical or economic reasons a simple associated network may not be
suitable and a quasi-associated network 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 mesh network 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.
2 Network components
2.1 Signalling links
Signalling links are basic components in a signalling network connecting
together signalling points. The signalling links encompass the level 2 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).
2.2 Signalling points
Signalling links connect signalling points at which signalling network
functions such as message routing are provided at level 3 and at which the user
functions may be provided at level 4 if it is also an originating or destination
point (see Recommendation Q.704, S 2.4).
A signalling point that only transfers messages from one signalling link
to another at level 3 serves as a signalling transfer point (STP).
The signalling links, signalling transfer points, and signalling
(originating or destination) points may be combined in many different ways to
form a signalling network.
Fascicle VI.7 - Rec. Q.705 PAGE1
3 Structural independence of international and national
signalling networks
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 po of the international network and the
different national networks to be independent of one another.
Figure 1/Q.705 - CCITT 28811
A signalling point (SP), including a signalling transfer point (STP), may
be assigned to one of three categories:
- national signalling point (NSP) (signalling transfer point) which
belongs to the national signalling network only (e.g. NSP1) and is
identified by a signalling point code (OPC or DPC) according to the
national numbering plan of signalling points;
- international signalling point (ISP) (signalling transfer point) which
belongs to the international signalling network only (e.g. ISP3) and is
identified by a signalling point code (OPC or DPC) according to the
international numbering plan of signalling points;
- 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.
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, S 14.2).
4 Considerations common to both international and national signalling
networks
4.1 Availability of the network
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).
4.2 Message transfer delay
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).
4.3 Message sequence control
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
signalling link selection 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.
4.4 Number of signalling links used in load sharing
The number of signalling links used to share the load of a given flow of
signalling traffic typically depends on:
- the total traffic load,
- the availability of the links,
- the required availability of the path between the two signalling points
concerned, and
- the bit rate of the signalling links.
Load sharing requires at least two signalling links for all bit rates, but
more may be needed at lower bit rates.
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 two links
are used, sufficient reserve link capacity should exist to satisfy the
availability requirements specified in Recommendation Q.706.
4.5 Satellite working
PAGE20 Fascicle VI.7 - Rec. Q.705
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.
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.
5 International signalling network
5.1 General
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.
Fascicle VI.7 - Rec. Q.705 PAGE1
5.2 Number of signalling transfer points in signalling relations
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.
5.3 Numbering of signalling points
A 14-bit code is used for the identification of signalling points. The
allocation scheme of international signalling point codes is defined in
Recommendation Q.708.
5.4 Routing rules
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.
5.4.2 Other routing rules
(For further study.)
5.5 Structures
(Requires further study.)
5.6 Procedures
(Requires further study.)
6 Signalling network for cross-border traffic
6.1 General
For cross-border 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.
Two alternative arrangements of the signalling network for cross-border
traffic are provided so that Administrations may adopt either alternative upon a
bilateral agreement.
6.2 Use of international hierarchical level
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-border traffic.
6.2.2 The signalling points and the signalling transfer points which are
involved in a signalling of cross-border traffic should belong to the
international hierarchical level described in S 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.
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,
S 14.2).
6.2.4 Signalling network management procedures in this network arrangement
require further study.
PAGE20 Fascicle VI.7 - Rec. Q.705
6.3 Integrated numbering of national signalling networks
6.3.1 By this arrangement the signalling points, which serve cross-border
traffic, should be identified by common national signalling point codes.
6.3.2 Common block of national signalling point codes is provided by bilateral
agreement (further study is required).
6.4 Interworking of national signalling networks
At the cross-border signalling network interface, the international
specification of Signalling System No. 7 should be preferred without exclusion of
bilateral agreements.
7 National signalling network
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).
8 Procedures to prevent unauthorized use of an STP (Optional)
8.1 General
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.
An Administration making an agreement with restrictions may wish to
identify and provide special treatment to unauthorized SS7 messages.
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.
8.2 Identifying unauthorized SS7 messages
In addition to the normal signalling message handling, procedures
specified in Recommendation Q.704, it shall be possible to inhibit/allow messages
destined for another signalling point (SP) based on any one or combination of the
following options:
i) to inhibit/allow STP access by a combination of designated incoming
link sets to designated DPCs;
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.)
ii) To inhibit/allow STP access by a combination of designated outgoing
link sets to designated DPCs.
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.)
Fascicle VI.7 - Rec. Q.705 PAGE1
iii) to inhibit/allow STP access by examination of OPC and DPC
combination in the incoming STP message.
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.)
8.3 Treatment of unauthorized SS7 messages
An STP identifying unauthorized SS7 messages should be able, on a per link
set or per signalling point code basis, to:
i) provide all unauthorized SS7 messages with the same handling as
authorized traffic, or
ii) discard all unauthorized SS7 messages.
In addition, an STP should be able to:
i) allow all STP messages outside the designated ranges as given in S 8.2,
ii) bar (discard) all STP messages outside the designated ranges as given
in S 8.2.
8.4 Measurements
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.
8.5 Notification to unauthorized user
An STP identifying unauthorized SS7 messages from another network may wish
to notify the Administration orginating the unauthorized message(s).
This notification should be undertaken by administrative means and not
involve any mechanism in Signalling System No. 7.
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.
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.
ANNEX A
(to Recommendation Q.705)
Mesh signalling network examples
A.1 General
This Annex is provided to demonstrate the procedures defined in
Recommendation Q.704. While the example uses a specific mesh network to
demonstrate the procedures, it is not the intent of this annex to recommend
either implicitly or explicitly the network described.
The mesh 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.
A.2 Basic network structures (example)
Figure A-1/Q.705 shows the basic mesh network structure, while three
simplified versions derived from this basic network structure are shown in Figure
A-2/Q.705. More complex signalling networks can be built, using these as building
components.
In the following, the basic mesh network Figure A-1/Q.705 is taken as an
example to explain the procedures defined in Recommendation Q.704.
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.
The simplified versions a), b) and c) of the basic signalling network are
obtained by deleting respectively:
a) two out of four intersignalling transfer point link sets;
b) link sets between signalling transfer points of the same pair; and
c) a) and b) together.
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-1/Q.705 to Figure A-2c)/Q.705], the lower the availability of the
signalling network. However, an increase in the availability of the simplified
PAGE20 Fascicle VI.7 - Rec. Q.705
signalling networks may be attained by adding one or more parallel signalling
links to each of the remaining signalling link sets.
Figure A-1/Q.705 - CCITT 35310
Figure A-1/Q.705 - CCITT 35320
Fascicle VI.7 - Rec. Q.705 PAGE1
A.3 Routing
A.3.1 General
This section gives some routing examples in the basic mesh network in
Figure A-1/Q.705. Routing actions required to change message routes under failure
conditions are described in S A.4. The following routing principles are assumed
for the examples in S A.3:
- Message routes should pass through a minimum number of intermediate
signalling transfer points.
- Routing at each signalling point will not be affected by message routes
used up to the concerned signalling transfer points.
- When more than one message route is available, signalling traffic
should be load-shared by such message routes.
- 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.
A.3.2 Routing in the absence of failures
Figure A-3/Q.705 illustrates an example of routing in the absence of
failures for messages from signalling point A to signalling point F.
Figure A-3/Q705 - CCITT 35330
The following points are worthy of note:
a) In distributing traffic for load-sharing at the originating signalling
point and intermediate signalling transfer points, care should be taken
in the use of signalling link selection (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.
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 -> C -> D -> F and F -> E -> B -> A).
PAGE20 Fascicle VI.7 - Rec. Q.705
c) Links BC and DE are not used in the absence of failures. They will be
used in certain failure situations described in S A.4.
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.
A.3.3 Routing under failure conditions
A.3.3.1 Alternative routing information
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, S 4.2).
Table A-1/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 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.
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.
TABLE A-1/Q.705
List of alternative link sets at signalling points A and B
Normal link Alternative Priority a)
set link set
Signalling AB AC 1
point A AC AB
Signalling BA BC 2
transfer BC Nome
point B BE BD 1
BC 2
BD BE 1
BC 2
a) Priority 1 - used with normal link set on load-sharing basis
in the absence of failures.
Priority 2 - used only when all the link sets with priority
1 become unavailable.
Fascicle VI.7 - Rec. Q.705 PAGE1
A.3.3.2 Single link failure examples
Example 1: Failure of a link between a signalling point and a signalling
transfer point (e.g. link AB) (see Figure A-4/Q.705).
Figure A-4/Q.705 - CCITT 35340
As indicated in Table A-1/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.
The principle to minimize the number of intermediate signalling transfer
points in S 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.
Example 2: Failure of an intersignalling transfer points link (e.g. link
BD) (see Figure A-5/Q.705).
As indicated in Table A-1/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.
Figure A-5/Q.705 - CCITT 35350
PAGE20 Fascicle VI.7 - Rec. Q.705
Example 3: Failure of a link between signalling transfer points of the
same pair (e.g. link BC) (see Figure A-6/Q.705).
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.
Figure A-6/Q.705 - CCITT 35360
A.3.3.3 Multiple link failure examples
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.
Example 1: 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-7/Q.705).
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 S A.3.3.2.
Figure A-7/Q.705 - CCITT 35370
Fascicle VI.7 - Rec. Q.705 PAGE1
Example 2: Failure of two intersignalling transfer point links (e.g. links
BD, BE) (see Figure A-8/Q.705).
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 S A.3.3.2.
Figure A-8/Q.705 - CCITT 35380
Example 3: 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-9/Q.705).
This example is a combination of Examples 1 and 2 in S 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.
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.
Figure A-9/Q.705 - CCITT 35390
PAGE20 Fascicle VI.7 - Rec. Q.705
Example 4: Failure of the two links between a signalling point and its
signalling transfer points (e.g. DF and EF) (see Figure A-10/Q.705).
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.
Figure A-10/Q.705 - CCITT 35400
A.3.3.4 Single signalling point failure examples
Example 1: Failure of a signalling transfer point (e.g. D) (see Figure
A-11/Q.705).
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 S A.3.3.2).
Figure A-11/Q.705 - CCITT 35410
Attention is drawn to the difference to Example 1 in S A.3.3.3 where only
a part of the traffic previously carried by links BD and CD was diverted.
Fascicle VI.7 - Rec. Q.705 PAGE1
Example 2: Failure of a destination point (e.g. F) (see Figure
A-12/Q.705).
In this case A stops all the traffic to F formerly carried on links AB and
AC.
Figure A-12/Q.705 - CCITT 35420
A.3.3.5 Multiple signalling transfer point failure examples
Two typical cases of two signalling transfer points failing together are
presented in the following examples.
Example 1: Failure of two signalling transfer points not pertaining to the
same pair (e.g. B and D) (see Figure A-13/Q.705).
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.
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.
Figure A-13/Q.705 - CCITT 35430
PAGE20 Fascicle VI.7 - Rec. Q.705
Example 2: Failure of two signalling transfer points pertaining to the
same pairs (e.g. D and E) (see Figure A-14/Q.705).
This example is equivalent to Example 4 in S 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.
Figure A-14/Q.705 - CCITT 35440
A.4 Actions relating to failure conditions
In the following, four typical examples of the application of signalling
network management procedures to the failure cases illustrated in S A.3.3 are
shown. In the case of multiple failures, an arbitrary failure (and restoration)
sequence is assumed for illustrative purpose.
A.4.1 Example 1: Failure of a link between a signalling point and a signalling
transfer point (e.g. link AB) (see Figure A-15/Q.705)
(Same as S A.3.3.2, Example 1.)
Figure A-15/Q.705 - CCITT 35450
Fascicle VI.7 - Rec. Q.705 PAGE1
A.4.1.1 Failure of link AB
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.
b) In addition, B sends a transfer-prohibited message to C referred to
destination A (according to the criterion indicated in Recommendation
Q.704, S 13.2.2).
c) On the reception of the transfer-prohibited message, C starts the
periodic sending of signalling-route-set-test messages, referred to A,
to B (see Recommendation Q.704, S 13.5.2).
A.4.1.2 Restoration of link AB
When the restoration of link AB is completed, the following applies:
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-allowed message, referred to destination A (see
Recommendation Q.704, S 13.3.2). When C receives the transfer-allowed
message, it stops sending signalling-route-set-test messages to B.
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 S 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).
A.4.2 Example 2: Failure of signalling transfer point D (see Figure A-16/Q.705)
(Same as S A.3.3.4, Example 1.)
Figure A-16/Q.705 - CCITT 35460
A.4.2.1 Failure of signalling transfer point D
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, S 5.7.2). In addition
E will send to B, C and F transfer-prohibited messages referred to
destination D. These signalling points (B, C and F) will thus start
periodic sending to E of signalling-route-set-test messages referred to
D.
PAGE20 Fascicle VI.7 - Rec. Q.705
b) When B receives a transfer-prohibited 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-prohibited message to C referred to D.
The same applies to C, and C sends a transfer-prohibited message to B.
c) So, when B receives a transfer-prohibited message from C, it finds that
destination D has become inaccessible and sends a transfer-prohibited
message to A. The same applies to C and thus C also sends a
transfer-prohibited message to A. Having received transfer-prohibited
messages from both B and C, A recognizes that D has become inaccessible
and stops traffic to D.
d) In the same manner, i.e. link-by-link transmission of
transfer-prohibited 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-route-set-test messages referred to D to their
respective adjacent signalling points.
A.4.2.2 Recovery of signalling transfer point D
a) Signalling points B, C, E send traffic restart allowed messages to
signalling point D, as soon as signalling point D becomes accessible.
b) Signalling transfer point D broadcasts traffic restart allowed
messages, after T20 (see Recommendation Q.704, S 16.8) has stopped or
expired, to all adjacent SPs.
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-controlled diversion procedure (see
Recommendation Q.704, S 6.4), since D is still inaccessible via E at B,
C and F (as a result of previous reception of transfer-prohibited
message from E).
d) E sends to B, C and F transfer-allowed messages referred to destination
D. These signalling points will thus send transfer allowed messages to
their respective adjacent signalling points. Thus, the link-by-link
transmission of transfer-allowed messages will declare to all
signalling points that destination D has become accessible.
e) On reception of a transfer-allowed message, each signalling point stops
periodic sending of signalling-route-set-test messages to their
respective adjacent signalling points.
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, S 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.)
A.4.3 Example 3: Failure of 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-17/Q.705)
(Same as S A.3.3.3, Example 1.)
Figure A-17/Q.705 - CCITT 35470
Fascicle VI.7 - Rec. Q.705 PAGE1
A.4.3.1 Failure of link DE
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.
However, D and E send to signalling points B, C and F transfer-prohibited
messages referred to destination E or D respectively. These signalling points
will thus start periodic sending of signalling-route-set-test messages, referred
to D or E, to E and D respectively.
A.4.3.2 Failure of link DF in the presence of failure of link DE
a) On failure of link DF the following actions occur:
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-prohibited messages. B and C will thus start the
periodic sending of signalling-route-set-test messages referred to
F, to D.
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.
b) On receiving the transfer-prohibited 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.
A.4.3.3 Restoration of link FD in the presence of failure of link DE
a) On recovery of link FD the following actions occur:
i) Signalling point D sends a transfer-allowed 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-route-set-test messages referred to F to
D.
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 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-sequencing of messages (see Recommendation Q.704, S 6.4).
b) On receiving the transfer-allowed 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, S 8.2.1),
provisionally set at one second to minimize the danger of
mis-sequencing messages.
A.4.3.4 Restoration of link DE
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-allowed
messages referred to destination E or D respectively. These signalling transfer
points will thus stop sending of signalling-route-set-test messages.
A.4.4 Example 4: Failure of links DF and EF (see Figure A-18/Q.705)
Figure A-18/Q.705 - CCITT 35480
A.4.4.1 Failure of link DF
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.
In addition, D sends to E a transfer-prohibited message, referred to
destination F; E will thus start sending of signalling-route-set-test messages,
referred to F, towards D (see also S A.4.1.1).
A.4.4.2 Failure of link EF in the presence of failure of link DF
a) When the failure of link EF is detected, the following applies:
i) Since all destinations become inaccessible F stops sending all
signalling traffic.
ii) E sends to B, C and D a transfer-prohibited message, referred to
destination F. B, C and D start periodic sending of
signalling-route-set-test messages referred to F to E.
PAGE20 Fascicle VI.7 - Rec. Q.705
b) When D receives the transfer-prohibited message, it sends to B and C a
transfer-prohibited message, referred to destination F (see
Recommendation Q.704, S 13.2.2 ii)). B and C start periodic sending of
test messages referred to F to D.
c) When B receives the transfer-prohibited messages from D and E, it sends
a transfer-prohibited message to C; the same applies for C (it sends
the message to B). As soon as B and C have received the
transfer-prohibited messages from all the three possible routes (BD, BE
and BC, or CD, CE and CB respectively), they send a transfer-prohibited
message to A.
Note - Depending on the sequence of reception of transfer-prohibited
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-prohibited message is received also from
that route.
d) As soon as A receives the transfer-prohibited messages from B and C, it
declares destination F inaccessible and stops sending traffic towards
it. Moreover, it starts the periodic sending of
signalling-route-set-test messages, referred to F, to B and C.
A.4.4.3 Restoration of link EF in the presence of failure on link DF
a) When restoration of link EF is completed, the following applies:
i) Signalling point F restarts traffic on link EF.
ii) E sends a transfer-allowed message, referred to destination F, to
B, C and D; moreover it restarts traffic on the restored link.
Fascicle VI.7 - Rec. Q.705 PAGE1
b) When B and C receive the transfer-allowed message, they send a
transfer-allowed message to A and C or A and B, respectively and they
stop sending signalling-route-set-test messages to E; moreover, they
restart the concerned traffic on link BE or CE respectively.
c) When D receives the transfer-allowed message from E, it sends
transfer-allowed messages to B and C and stops sending
signalling-route-set-test messages to E; moreover, it starts the
concerned traffic on link DE. On receipt of the transfer-allowed
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 S A.3.3). Moreover, they will stop
sending signalling-route-set-test messages to D.
Note - According to the rules stated in Recommendation Q.704, S 13.3.2,
on receipt of transfer-allowed messages from E [phase b) above], B and
C should send transfer-allowed 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:
- there is no route, for example, from D (or E) to F via B (or C) and
therefore the transfer-allowed messages would be ignored by D and
E;
- on restarting traffic to F on links BD, BE, CD and CE it would
anyway be necessary that B and C send transfer-prohibited messages
to D and E, which would contradict the previous transfer-allowed
messages.
d) As soon as A receives a transfer-allowed 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-allowed 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).
A.4.4.4 Restoration of link DF
When the restoration of link DF is completed, the following applies:
a) D initiates the changeback procedure to link DF; moreover, it sends to
E a transfer-allowed message, referred to destination F,
b) F sends signalling-route-set-test 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.
A.5 Explanatory note from the implementors forum for clarification of load
sharing
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.
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.
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
PAGE20 Fascicle VI.7 - Rec. Q.705
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.
Fascicle VI.7 - Rec. Q.705 PAGE1
