Signaling & Network Control NETW 704 MTP 3

1. Signaling & Network Control NETW 704MTP 3

2. Message Transfer Part 3 (MTP 3) • Primary purpose is to route messages between SS7 network nodes in a reliable manner. It is equivalent to Layer 3 (Network) in the OSI model. • It is divided into 2 categories • 1) Signaling Messages Handling (SMH) • 2) Signaling Network Management (SNM)

3. Point Codes • Each node is identified by a Point Code (PC) • National Point Code in national networks. • International Point Code in international networks. • Each MSU contains the Originating Point Code & Destination Point Code. • International PC is based on a hierarchical structure. • Zone • Network • Signaling Point • National PC is also based on a hierarchical structure. • Network • Cluster • Member

4. Message Format • MTP3 portion of an SS7 msg consists of 2 fields: SIO & SIF. • SIO • Subservice Field: Network Indicator & Priority. • Service Indicator Field: MTP3 user.

5. Message Format • MTP3 portion of an SS7 msg consists of 2 fields: SIO & SIF. • SIF • User Data Field: Data sent by MTP3 user. • Routing Label Field: • 1- Signaling Link Selector (which runs a code to share the load across different linksets or different links within a linkset) • 2- Originating PC (OPC). • 3- Destination PC (DPC).

6. Signaling Messages Handling • It is mainly responsible for transferring the SUs to the correct destination.

7. Signaling Messages Handling • It is mainly responsible for transferring the SUs to the correct destination.

8. Signaling Messages Handling • 1) Discrimination • Determines whether this destination is destined to this node or any other node. • 2) Distribution • Distributes the data according to the MTP 3 User indicated in the service indicator field. • 3) Routing • Either msg coming from upper layer or msg coming from another SP. • Routing forwards according to the route found in DPC in Routing Table.

9. Signaling Messages Handling • Alias Point Code Routing • Another PC other than the primary PC, used for Mated Pair STPs. It identifies both & allow load sharing as the unique PC cannot be used (it uniquely identifies 1 PC).

10. Signaling Network Management • It is responsible for network failures. • It handle the failures in three processes: • Link Management • Route Management • Traffic Management

11. Link Management • Handles failures at the network’s links. 1. Activation: The process of making a link available, after the link is aligned at level 2 & has passed the proving period. 2. Deactivation: The process of removing a link from service. 3. Restoration: The process of restoring a link to service after failure. After the link is aligned at level 2 and has passed the proving period, a Signaling Link Test is performed at level 3.

12. Route Management • Provides means to communicate route changes & route availability using SNM msgs. • Routing Status Messages 1. Transfer Prohibited (TFP) 2. Transfer Restricted (TFR) 3. Transfer Allowed (TFA) 4. Transfer Controlled (TFC) • Route States • Prohibited • 2. Restricted • 3. Allowed

13. Route Management 1. Transfer Prohibited (TFP) • When the prohibited state is reached, TFP msg is sent. Prohibited state is when there is complete inability to route msgs to the affected destination. • Traffic Management is notified by TFP. • TFP could be broadcasted or sent in response of a msg. • On reception of TFP, Traffic Management performs FORCED REROUTING(Explained later). STP1 TFP B SSP A SSP B STP2 Unavailable Traffic Route

14. Route Management 2. Transfer Restricted (TFR) • When the restricted state is reached, TFR msg is sent. Restricted state is when there is limited ability to route msgs to the affected destination. • Traffic Management is notified by TFR. • On reception of TFR, Traffic Management performs CONTROLLED REROUTING (Explained Later). STP1 TFR B SSP A SSP B TFR B STP2 Alternate Route Unavailable Traffic Route

15. Route Management 3. Transfer Allowed (TFA) • When the allowed state is reached, TFA msg is sent. Allowed state is reached when a route has been in restricted/prohibited stat & routing capability has been restored. • Traffic Management is notified by TFA. STP1 TFA B SSP A SSP B STP2 Restored Traffic Route

16. Route Management 4. Transfer Controlled (TFC) • When there are many msgs destined to the same SP thus the buffer is full and the link is congested. • TFC is sent to all SPs trying to send to this destination. • Note:- There are multiple congestion levels that indicate the severity of the congestion.

17. Route Management • Routeset Test • While the status is prohibited or restricted, routeset test is run to check and ensure that the status at one SP is the same as at the other SP. • On reception of TFP/TFR, RSP/RSR msgs are sent at periodic intervals T10. • The status is compared with the current status if they match, msg is discarded, else if they do not match a msg with the new status is sent. • On reception of TFA, Routeset test is stopped. • Why it is important? • Consider STP sent TFA msg to SSP for a previously prohibited routeset and SSP failed to receive this message, then STP would have a routset marked as TFA and SSP marked as TFP.

18. Route Management • Routeset Test • While the status is prohibited or restricted, routeset test is run to check and ensure that the status at one SP is the same as at the other SP. • On reception of TFP/TFR, RSP/RSR msgs are sent at periodic intervals T10. • The status is compared with the current status if they match, msg is discarded, else if they do not match a msg with the new status is sent. • On reception of TFA, Routeset test is stopped. TFA A SSP A STP1 SSP B Unavailable Traffic Route

19. Traffic Management • Deals with signaling traffic, the actual msgs generated by MTP3 users. • Procedures 1. Changeover. 2. Emergency Changeover. 3. Time-Controlled Changeover. 4. Changeback. 5. Time-Controlled Diversion. 6. Forced Rerouting. 7. Controlled Rerouting. 8. Link Inhibiting.

20. Traffic Management • Changeover • When a link becomes unavailable and there are other links in the linkset that can be used. • Traffic is changed over to one of the other links or to an alternate linkset. • Change Over Order (COO) msg is sent with the last correctly received SU in the FSN. • Change Over ACK (COA) is sent back with last correctly received SU by the other end in the FSN. • SSP A with one link in each linkset to STP 1 and STP 2. When the link to STP 2 fails, SSP A detects the failure and performs a changeover to the STP 1 linkset. The changeover is made to a new linkset because no other links are available in the same linkset. If more links were available in the STP 2 linkset, the changeover would be to a new link in the same linkset. STP1 1.COO SSP A SSP B 2.COA STP2

21. Traffic Management 2. Emergency Changeover • It is possible that a node cannot determine the last acknowledged message when a link fails. An example is the failure of the signaling terminal hardware. Typically, the signaling terminal hardware contains the receive buffers and keeps track of the FSN for incoming signaling units. There is no way to determine where the request for retransmission should start if this information is lost. In this case, an Emergency Changeover (ECO) is sent to the far end to initiate a changeover. The ECO does not contain the last accepted FSN field because the last good message cannot be determined. 3. Time-Controlled Changeover • When there is no route to send the COO & COA. • A timer T1 is set and when it times out, the system changes over automatically. STP1 SSP A SSP B 2.Changeover after T1 expiration STP2 1.Link has become unavailable

22. Traffic Management 4. Changeback • When COO & COA have been sent and the link that has failed was restored. • Change back and send SUs on the restored link. 5. Time-Controlled Changeback (Diversion) • When there is no route to send change back msg. • A timer T3 is set and when it times out, the system changes back automatically. • As shown, the SSP A – STP 2 linkset that was unavailable has been restored. Assuming that SSP A set its routing table to load share between STP 1 and STP 2 for traffic destined to SSP B, the MSUs previously diverted to STP 1 should now be sent to STP 2. STP1 SSP A SSP B 2.Changeback after T3 expiration STP2 1.Newly available Link

23. Traffic Management 6. Forced Rerouting • When Traffic Management is forced to move traffic away from an unavailable route. • It is sent in response to a TFP msg sent by Route Management. STP1 TFP B SSP A SSP B STP2 Forced Reroute to STP2 Unavailable Traffic Route

24. Traffic Management 7. Controlled Rerouting • When Traffic Management moves traffic away from an unavailable route in a controlled way. • It is sent in response to a TFR msg sent by Route Management. • SSP A receives a TFR from STP 1 for SSP B. SSP A has a routeset for destination SSP B with two routes in the routeset. SSP A performs controlled rerouting of traffic from STP 1 to STP 2. STP1 TFR B SSP A SSP B STP2 Forced Reroute to STP2 Unavailable Traffic Route

25. Traffic Management 7. Controlled Rerouting (Cont) • When the route from STP1 to SSP B is restored, STP 1 sends a TFA to indicate that full routing capability toward SSP B has been restored. SSP A performs controlled rerouting again, this time shifting traffic from the STP 2 route to the STP 1 route using the same basic procedure. STP1 TFA B SSP A SSP B STP2 Forced Reroute to STP1 Unavailable Traffic Route

26. Traffic Management 8. Link Inhibiting • Signaling link management inhibiting is used to prevent user traffic on the links while leaving the links themselves in service. This process is useful for isolating links for testing purposes. The inhibit procedure uses the Link Inhibit (LIN) and Link Inhibit Acknowledgement (LIA) messages to communicate between the two nodes concerning the linkset being inhibited. STP1 TFA B SSP A SSP B STP2 Forced Reroute to STP1 Unavailable Traffic Route

27. Traffic Management 8. Link Inhibiting (Cont) • A maintenance engineer at STP 1 must perform testing on a link that has had intermittent problems. The engineer issues the command at a maintenance terminal to place the link in an inhibited state so it is not used by normal user traffic. STP 1 sends a LIN message to SSP A. Because SSP A has other links available for routing, it determines that it can safely remove the link from traffic service and respond with an LIA back to STP 1 in acknowledgement. Because SSP A has only 1 per linkset, it performs a controlled reroute of traffic to STP 2 linkset. STP1 1. LIN 2. LIA SSP A SSP B STP2 3.Controlled Reroute to STP2 Unavailable Traffic Route