SS7 Network Test Possible Error Cases

in Layer 3 that automatically detect error situations and are able to try to cor- rect them autonomously. Errors can be separated into one of three categories:
•
Overload on a single SS7 link;
•
Outagebringing into service an SPSTP;
•
Outagebringing into service an SS7 link between SPs or STPs. SS7 network management has to be able to detect each error situation and to
allow for a fix with the appropriate actions. Those actions consist mainly in rerouting signaling data, as well as blocking and unblocking routes. The details
of how that is performed are described next.

8.6.1 SS7 Network Test

ITU recommendations deal separately with SS7 network test and SS7 network management. However, they serve similar tasks and thus will be presented
together. Table 8.2 shows how SS7 distinguishes network test and network management by assigning two different SIs. The SI is part of the SIO.
Signaling System Number 7
139
STP STP
STP SP
SP SP
SP SP
SP
Figure 8.12 Example of an SS7 network.
Table 8.2
Service Indicators for SS7 Network Management and Network Test
SI Binary User Part
00 Network management
01 Test and maintenance

8.6.2 Possible Error Cases

The messages in the following descriptions frequently are abbreviated. Sections 8.6.5 and 8.6.6 explain the abbreviations
8.6.2.1 Behavior in an Overload Situation Figure 8.13 illustrates a situation in which an overload situation occurs on the
link between the STP and the SPSTP. In that case, both STPs inform their direct neighbors about the limited availability of that connection. The informa-
tion is sent in TFC messages and TFR messages. Alternative routes will be used after the neighbors are informed about the problem. The changeover procedure
sending of a COO message is used to actually implement rerouting. When the overload situation has ceased to exist, the neighbors are informed in TFA
messages that the link is available again. To actually utilize that link, the chan- geback procedure has to be executed sending of a CBD message. In the time
period between when the TRCTFR messages and the TFA messages are sent, the neighbor SPsSTPs periodically check the overload situation by sending
RSR or RCT messages. Which of the messages is actually used depends on whether the sender is an SP or an STP. See the tables at the end of the chapter
for explanations of the messages.
8.6.2.2 Behavior at OutageBringing SPSTP Into Service Figure 8.14 shows the same SS7 network, but this time with the failure of an
STP. As with the overload situation, all neighbors have to be informed immedi- ately about the problem. In the case of an outage, that is performed by the
sending of TFP messages to all affected SPs and STPs. The rerouting process, as in the overload situation, is done via the changeover procedure. When
the failed STP comes back into service, the neighbors first recognize that when the links toward that STP synchronize again. All the neighbor nodes periodi-
cally send RST messages during the outage to check the state of the STP. All
140 GSM Networks: Protocols, Terminology, and Implementation
STP STP
STP SP
SP SP
SP SP
SP
Figure 8.13 SS7 network with overload dashed link.
the neighbors send TRA messages to the STP when Layer 2 is established again. The STP that got back into service also sends TRA messages to all neighbors
an SP would not send those messages. The TRA messages have the purpose of informing all neighbors that the respective routes are available again. Finally, a
changeback procedure is used to cancel all the established detours.
8.6.2.3 Behavior When SS7 Link FailsIs Established Another possible scenario is the total loss of an SS7 link between an SP and
an STP Figure 8.15. In that situation, the STP has to inform all neighbors about the loss of the connection. That is done with a TFP message. Establish-
ing alternative routes to and from the affected SP is performed by means of the changeover procedure. Both the SP and the STP periodically test the link by
sending RST messages during the time period when the link is not available.
The reverse process is executed when the link is brought back into service. The STP sends TFA messages to all affected neighbors, and the alternative
routes are canceled by means of the changeback procedure.
Signaling System Number 7
141
STP STP
STP SP
SP SP
SP SP
SP
Figure 8.14 SS7 network with STP outage.
STP STP
STP SP
SP SP
SP SP
SP
Figure 8.15 SS7 network in which a link has failed.
8.6.2.4 More Error Cases Other situations may occur, and appropriate measures have to be taken. Such
situations include loss of single user parts in an SP, intentional shutdown of an SS7 link by network management, and automatic addition of SS7 links in the
case of increasing load or link failures. The tables in the next sections describe the corresponding messages to transmit that information.

8.6.3 Format of SS7 Management Messages and Test Messages