Signaling & Network Control NETW 704

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)

Message Transfer Part 3 (MTP 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

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.

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 theload across different linksets or different links within a linkset) 2- Originating PC (OPC).3- Destination PC (DPC).

Signaling Messages Handling

• It is mainly responsible for transferring the SUs to the correct destination.

Signaling Messages Handling

• It is mainly responsible for transferring the SUs to the correct destination.

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.

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).

Signaling Network Management

• It is responsible for network failures.

• It handle the failures in three processes: 1) Link Management 2) Route Management 3) Traffic Management

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.

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

1. Prohibited2. 2. Restricted3. 3. Allowed

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

STP2

SSP A

SSP B

TFP B

Unavailable Traffic Route

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

STP2

SSP A

SSP B

TFR B

Unavailable Traffic Route

TFR B

Alternate Route

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

STP2

SSP A

SSP B

TFA B

Restored Traffic Route

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.

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.

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.

STP1SSP A

SSP B

Unavailable Traffic Route

TFA A

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.

Traffic Management1. 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

STP2

SSP A

SSP B

1.COO

2.COA

Traffic Management2. 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

STP2

SSP A

SSP B

1.Link has become unavailable

2.Changeover after T1 expiration

Traffic Management4. 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

STP2

SSP A

SSP B

1.Newly available Link

2.Changeback after T3

expiration

Traffic Management6. 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

STP2

SSP A

SSP B

TFP B

Unavailable Traffic Route

Forced Reroute to STP2

Traffic Management7. 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

STP2

SSP A

SSP B

TFR B

Unavailable Traffic Route

Forced Reroute to STP2

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

STP2

SSP A

SSP B

TFA B

Unavailable Traffic Route

Forced Reroute to STP1

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

STP2

SSP A

SSP B

TFA B

Unavailable Traffic Route

Forced Reroute to STP1

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

STP2

SSP A

SSP B

1. LIN

Unavailable Traffic Route3.Controlled

Reroute to STP2

2. LIA