SS7 NETWORK MANAGEMENT (Cont.)
The table below describes each Signaling Route Management procedure in terms of the type of signaling point that may conduct each procedure, its purpose, and the likely cause(s) that may initiate it.
| Transfer Prohibited (TFP) | STP | Notify other points that the STP can no longer be used to reach specific points. | Link failure |
| Transfer Restricted (TFR) | STP | Notify other points that they should not, if possible, use the STP to reach specific points. | Link congestion or instability |
| Transfer Allowed (TFA) | STP | Notify other points that they may use the STP as needed to reach points accessible by it. | Link restoration or new link brought into service |
| Transfer Controlled (TFC) | STP, SSP, SCP | Notify other points of the congestion status at a specific point along a route. | Dependent upon the network |
The Signaling Route Management function also performs 2 tests: Signaling Route Set Test and Signaling Route Set Congestion Test. In the table below, each test is described in terms of the type of signaling point that may conduct it, its purpose, and the likely cause that would initiate it.
| Signaling Route Set Test | STP, SCP, SSP | Queries an adjacent STP to determine whether it can be used to reach a specified point. | Receipt of a TFP or TFR message from a point along a primary route. |
| Signaling Route Set Congestion Test | STP, SCP, SSP | Determines how much congestion exists along an alternative route. | Discovery of an alternative route to a specific point. |
Signaling Traffic Management
The Signaling Traffic Management function allows MTP Level 3 to redirect signaling unit traffic around links or routes that have become unavailable or are congested. This function also allows a signaling point to restore original patterns when congested or unavailable conditions have ceased to exist or to create new patterns as new links become available.
Signaling Traffic Management uses the following procedures:
- Signaling Point Restart
- Management Inhibiting
- Signaling Traffic Flow Control
- Forced Rerouting
- Controlled Rerouting
- Changeover
- Changeback
The table below provides a brief description of each Signaling Traffic Management procedure, including its basic purpose and the condition that will cause procedure execution.
| Signaling Point Restart | Restarts signaling unit traffic over a link from a signaling point destined for an end point. | A signaling point becomes available and establishes a link with an end point. |
| Management Inhibiting | Prohibits signal unit traffic from the user parts (i.e., TCAP, SCCP, ISUP) from being sent over a specific link. | Signaling link instability that requires maintenance and testing. |
| Signaling Traffic Flow Control | Restricts signaling unit traffic at the local point. | Link failures or congestion. |
| Forced Rerouting | On the first alternative route found, redirects all signaling unit traffic destined for a specific signaling point. | Route becomes unavailable. |
| Controlled Rerouting | On the optimal alternative route found, redirects all signaling unit traffic destined for a specific signaling point. | Route becomes unavailable. |
| Changeover | Maintains signaling unit integrity while traffic is redirected over alternate links/routes. | Links/Routes become unavailable. |
| Changeback | aintains signaling unit integrity while traffic is redirected to primary links/routes. | Links/Routes come back into service. |
MTP Restart
MTP's internal tables will change as links, applications, or other signaling points become available or unavailable. When a node fails, its MTP should be given enough time to verify the accessibility of the other network points and applications with which it was communicating prior to the failure. Otherwise, it will assume they are available and accessible, which may not be the case. This can create confusion and waste resources throughout the network because MTP has not properly "checked in" with the network.
The purpose of the SS7 MTP Restart procedure is to prevent such network confusion and inefficiency by isolating a node while its MTP is being restarted. It prevents a node from resuming normal message traffic until sufficient links are restored and communication with adjacent points is completed. MTP Restart is described in the following standards documents*:
- ITU-T Recommendation Q.704 (07/96), Chapter 9
- ANSI T1.111-1996, Chapter T1.111.4
*Note: The recommendations for MTP Restart in the ITU-T and ANSI specifications are very similar. For technical details regarding subtle implementation differences, timers, and message exchanges, please refer to the appropriate standards document.
The MTP Restart procedure's main purpose is to ensure MTP enough time to reactivate signaling links to handle expected message traffic, and to exchange current routing information with its adjacent signaling points before message routing resumes. In addition to the signaling route management messages (e.g., TFP, TFR) used by MTP, the following additional messages are used during the MTP Restart procedure:
- Traffic Re-start Waiting (TRW) (ANSI only)
- Traffic Re-start Allowed (TRA)
The key event that signifies the successful completion of MTP Restart is the exchange of TRA messages. When an adjacent signaling point has sent all its updated routing information to the restarting MTP, it will then send a TRA message. When the restarting MTP receives enough TRA messages from all adjacent signaling points (and sufficient time has passed), it will send a TRA message to all adjacent signaling points and normal message routing resumes.
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