MTP3 (Message Transfer Part3)

There is some questioner on MTP3 protocol.

1.) What are the responsibilities of MTP3.
2.) Define the ITU and ANSI Point codes.
3.) How MTP3 routed the SU to end point.
4.) Define the Routing label.
5.) How the Load sharing achieved on MTP3.
6.) compare the MTP 3 to IP Protocol.
7.) What type of Signaling  Network Management on MTP3.
8.) define the link management, SLTA/SLTM.
9.) Define the route management.
10.) Define the Traffic management.
11.) What Inhibiting and Uninhibited.
12.)  How many maximum links are possible in one linkset and why.
13.) What is alias point code routing.

Answers:

 1.) This responsibility is divided into two categories:
• Signaling Message Handling (SMH) - Signaling Message Handling is concerned with routing messages to the appropriate network destination. Each node analyzes the incoming message based on its Destination Point Code (DPC) to determine whether the message is destined for that node.

From SS7 Network           Is it for Me?         Yes
-------------------->  Discrimination ------------>   Distribution ------>SNM, ISUP,SCCP, BISUP, etc
                                         |
                                         | No
                                         |-------------------------> Routing  ----- >To SS7 Network

Routing can be done on STP, or the originating nodes in case of SSP. if the message does not belong to SSP, it replies to originating node User Part Unavailable (UPU).
• Signaling Network Management (SNM) - Signaling Network Management is a set of messages and procedures whose purpose is to handle network failures in a manner that allows messages to continue to reach their destination whenever possible.

2.) Point codes uniquely identifies the origination (OPC) and destination (DPC) Nodes in SS7 Network.

ITU Point codes (14 bits) contains following three fields:
- Zone (3 bit)
- Area/Network (8 bit)
- Signaling Point (3 bit)
ANSI Point codes (24 bits) contains following three fields:
- Network (8 bit)
- Cluster (8 bit)
- Cluster Member (8 bit)

3.) Message is routed to end point using the following field of MSU:
- OPC from the Routing label of SIF field of MSU
- DPC from the Routing label of SIF field of MSU
- SLS of Routing label of SIF field of MSU
- network indicator (NI use for national/international) of SSF (Subsystem field) of SIO (service Information octet)
- SI (Service Indicator to define the MTP3 user) of SIO (service Information octet)

4.) Routing Lable: SLS + OPC +DPC

5.) Two type of Load sharing in SS7 network:

- Load Sharing across linkset in a combined Linksets. - SLS most significant bit used to loadsharing between the linksets.

- Load Sharing across the links in a linkset. SLS four bit used to load share across the link in linkset. bit rotation. For a particular call, all messages uses same SLS value to insure the in - sequence delivery of SUs, all subsequent nodes uses same SLS to ensure the in-sequence delivery. Of-cource in case of failure, alternate path to be used which increase the chance of out-of-sequence delivery.

Messages generated by MTP3 ( SNM, SNT, and SNTS) replace the SLS field with the SLC (signaling link code) and no load sharing perform for these messages.

SLS is 4 bit in size for ITU Network and 8 bit in ANSI network.

6.) Comparing the IP to MTP3:

IP                                                                                     SS7
===========================================================
Source IP Address                                                Originating Point Code
Destination IP Address                                       Destination Point Code
Protocol                                                                    Service Indicator
Precedence (part of TOS field)                         Priority
Data                                                                            User Data

7.)Signaling Network Management is divided into three processes:
• Traffic management - responsible for dealing with signaling traffic, keep traffic moving towards its destination.
• Route management - exchanges information about routing status between nodes
• Link management - activates, deactivates, and restores signaling links.

All Network Management messages contains routing label and H0/H1 identifiers. Ho/H1 identifiers used to identify the message type. Ho for family and H1 for member (specific message type) of that family.

| MTP2 |                                SIF                             |        SIO       |  MTP2   |
                           |H0|H1| Routing Label|

8.)  Link Management: Link management can be divided into three processes:
• Activation - process of making a link available to carry MTP3 user traffic.
• Deactivation - removes a link from service, making it unavailable for carrying traffic.
• Restoration - automated attempt to restore the link to service after a failure.

signaling Link Test Control (SLTC) - The SLTC ensures that the two connected nodes can communicate at level 3 before placing a link into service for user traffic.
when link is activated, it undergo the intial alignment at MTP2. And then link perform the signaling link test initiated by the SLTC function. In which, MTP3 sent SLTM (Signaling link test Message) over the link with the node's DPC at far node of the linkset. SLC (Signaling Link Code) identifies the links on which the test to perform, that's why SLC should be same on both end. The receiving node Replies with the SLTA (signaling link Test Ack) contains the same test pattern received in SLTM.

9.) Route Managements: Signaling route management communicates the availability of routes between SS7 nodes. Route management uses the following messages to convey routing status to other network nodes:
Route Managements: Signaling route management communicates the availability of routes between SS7 nodes. Route management uses the following messages to convey routing status to other network nodes:
• Transfer Prohibited (TFP) - The Transfer Prohibited state indicates a complete inability to route messages to the affected destination.

• Transfer Restricted (TFR) - The restricted state indicates a limited ability to route messages. This status signifies that the primary route is unavailable and that another route should be chosen, if it exists. If the restricted route is the last available route in a routeset, it is still used for routing. provided that indirect route toward the affected destination.

• Transfer Allowed (TFA) - The transfer allowed state indicates that a route is available for carrying traffic.

• Transfer Controlled (TFC) - The Transfer Controlled message is used to indicate congestion for a route to a particular
destination.

10 and 11.) Traffic management: Traffic is diverted away from unavailable links and linksets, stopped in the case of unavailable routesets, and reduced where congestion exists. Traffic management to accomplish such tasks:

• Changeover - Changeover is the process of diverting traffic to a new link when a link becomes unavailable.If there are no other available links in the linkset and another linkset is available, traffic is diverted to the alternate linkset.A Changeover Acknowledgement (COA) is sent in response to a Changeover order (COO).

• Emergency changeover - It is possible that a node cannot determine the last acknowledged message when a link fails.In this case, an Emergency Changeover (ECO) is sent to the far end to initiate a changeover.

• Time-controlled changeover - There are times when a link might fail and no alternate path exists between the nodes at
each end of the link.

• Changeback - Changeback is the process of diverting traffic from an alternative signaling link back to the link that is usually used.
• Time-controlled diversion - There are situations where a changeback should occur, but there is no way to signal the changeback to the other end of the signaling link.

• Forced rerouting - Forced rerouting is used to divert traffic away from an unavailable route immediately.

• Controlled rerouting - Controlled rerouting is used in response to TFR and TFA messages. This procedure is more “controlled” than forced rerouting in the sense that traffic is sent over an available route and is shifted to another available route.

• MTP restart -
• Management 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.

12.)  16 links are possible in one linkset because SLS is four bit only to select the link. 2 is to power 4=16.

13.) Alias Point code Routing /compatibility Point code: An Alias Point code is a secondary point code used, when multiple nodes (usually two) share the Alias PC. this allows messages to route to either nodes using the PC for the load sharing.