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Different Functionalities of MTP3 Layer

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  • Last Updated : 19 May, 2022
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In the field of computer networking, the network management functions are widely used to signal links or restore the failed links, activate the idle links, deactivate the aligned links, also, maintaining the services if the signal links and SPs are lost, forced routing, control routing and signaling the traffic flow control.

When it comes to the communication between devices, there is a need to establish a link between devices for reliable transmission, and this connection can be made using protocols. MTP or Message Transfer Protocol is a type of protocol that is part of the SS7, or signaling system7. In computer networking, it is a useful method for communication between devices. It is responsible for transporting SS7 messages between communication devices in a reliable, unduplicated, and in-sequence manner.

Here, in this article, we will cover the MTP protocol and its three layers, particularly the MTP3. We will be discussing its role in computer networking, as well as its features and functionalities, and the benefits it provides.

Now, talking about the MTP layer or Message Transfer Part of the SS7 protocol, it allows the safe routing and network interface, which support SCCP, TCAP, and ISUP. It is further divided into the 3 different layers:

1. MTP layer 1 is a physical Level, also known as a physical layer, which defines the physical, electrical, and functional features of the digital signaling links. 

2. MTP layer 2 provides a reliability feature for MTP. such as error monitoring and recovery. It is a signaling link, that is used in conjunction with MTP3 to provide reliable transmission of signaling messages between two signaling points, which are directly connected.

3. MTP Layer 3 is responsible for the links, route, and traffic control components in MTP. MTP 3 ensures that signaling messages are successfully transmitted even if signaling links and signaling transfer points fail. As a result, the protocol provides the functions and procedures, which are required for both to notify remote elements of the signaling network of the results of a failure and to properly adjust message routing via the signaling network.

To understand it properly you can see the diagram below:

MTP3 OSI Model in SS7 Protocol

MTP3 OSI Model in SS7 Protocol

The Features of the MTP3 layer:

The features that MTP3 supports are given down below: 

1. Firstly, it works as a signaling transfer point.

2. It can handle national or international signaling traffic.

3. It provides a message discrimination function. Where it determines the true destination point of the message, and If is the desired destination point, then the message will be delivered to the message distribution function. If it’s not, then the transfer capability is enabled, and the message is directed to the message routing function.

4. It has a message distribution function. Which helps deliver the message to the correct user part or MTP3 function.

5. It also supports the message routing function. Which is capable of Selecting the right signaling link in order to transmit the messages.  also performs the load sharing between links or linksets.

6. Another feature is that it uses signaling traffic management procedures to help divert the signaling traffic from signaling links or signaling routes. Along with that, there are some other features available, for instance:

  • Changeover
  • Chargeback
  • Forced rerouting
  • Controlled rerouting
  • Signaling point restart
  • Management inhibiting
  • Processor outage etc.

Functionalities of the MTP3 layer:

The MTP3 layer has been used as a message identifier as well as a message distributor. It determines the signaling messages and then, distributes them to an appropriate user component. Also, its work is to route the inbound messages back out with the proper link to the right network location. It works as an administrator, which conducts the features that create rules and procedures for opening, maintaining, and restoring the links. 

So, in this manner, it is divided into two parts, which are as follows:

  • SMH or the Signaling Message Handling 
  • SNM or the Signaling Network Management

1. The SHM or Signaling Message Handling

The Signaling Message Handling is responsible for routing messages to the desired network location. Each node here determines the message based on its DCP or Destination Point Code to check whether the message is destined for that node. Now, If the node, which is on the receiving end, is the destination, then, the incoming message will be delivered to the appropriate MTP3 user. But, If the receiving node is not the desired location, then it will be re-routed only if the message is capable to perform routing. 

When the message is generated by the node, the MTP3 is responsible for directing its route toward its location with the help of DCP. MTP3 can be further divided into three parts based on its processing. 

 

Signaling Message Handling

Signaling Message Handling

A. The Discrimination Part: In the message discrimination part, MTP3 determines whether, the inbound message is destined for the current processing node, here each node and its point code defines a particular network type.

Each node that receives a message needs to confirm if the message is in its destined location. Which is determined by the DCP in the routing label and also with point code. Here, if the point code is a match, the message will be received and sent to the distribution part for further processing. And if not, then it will be sent to a routing function if the node is capable of routing. 

B. The Distribution Part: Once, the discrimination function is done doing its part, next, it proceeds with the distribution process, by which it examines the service indicator, which is further a part of SIO in the routing label. Here, the service indicator is responsible to decide which MTP3 user to send the message to for further processing. The working of the service indicator is to direct the message to the next logical stage. 

C. The Routing Part: The routing part takes place when it is confirmed that the message is destined for another node. Only in two cases, it can be done, firstly, if the node generates a message that needs to be forwarded to the network. And secondly, when the STP has received a message that is desired to be sent to another node.

In other words, the routing function proceeds only if the discrimination function has declared that the received message is not for the STP. If a Signaling End Point, either SCP or SSP receives a message but the discrimination part declares that the message is not meant for that particular node, then the message will be automatically discarded, as these nodes don’t have any transfer capabilities. Instead, the UPU or A User Part Unavailable will be sent to the originating node as the indication that the message can’t be delivered. 

2. The SNM or Signaling Network Management

MTP3 Network Management is a set of messages and procedures, which are used to keep the signaling transport system in a reliable condition. This includes automatically triggering actions in response to network events like connection or route failures, as well as reporting network status to other nodes.

Signaling Network Management is further divided into three parts:

  1. The traffic management
  2. The route management
  3. The link management

A. The Traffic management Part: This part is responsible for signaling traffic management. Which initially are the messages created by the MTP3 users, such as SCCP and ISUP. The purpose of this function is to keep the traffic going toward the destined location, despite the network failures and congestion. Sometimes, it can include the re-routing of traffic to an alternative network path, or in some cases, it may require message re-transmissions.

Traffic management is entirely dependent on the data received from the link management and route management to direct user traffic. For this purpose following functions are responsible to complete the task:

  • Changeover
  • Emergency changeover
  • Time-controlled changeover
  • Chargeback
  • Time-controlled diversion
  • Forced rerouting
  • Controlled rerouting
  • MTP restart
  • Management inhibiting

B. The Route Management: Route management involves the exchange of information on the routing states between the nodes. When a situation occurs, which can affect the availability of the route, it sends a warning message to notify other nodes about the change in the routing state. Route management sends the information to the traffic management, so that, it can adjust the traffic patterns and flow according to the situation.

This function uses the following messages to notify others about the routing status of other network nodes:

  • Transfer Restricted: This condition implies that messages can only be routed in a restricted number of ways. It indicates that the primary route is unavailable and that another route, if available, should be taken.
  • Transfer Prohibited: This state indicates a total failure to route messages to the correct location. If one exists in this situation, another route must be picked for routing. If no route exists, traffic management is advised that messages cannot be routed to their destination. There are two methods available for sending the TFP status:
    • Broadcast method
    • Response method
  •  Transfer Allowed: The transfer allowed state shows that a route is open for traffic. This is the normal status for routes that are in service. When a route is restored to full routing capacity after being restricted or barred, the route’s status is changed to transfer allowed. 
  • Transfer Controlled: The Transfer Controlled message is used to communicate that a route to a specific destination is congested. The TFC message signifies “transmit” congestion, as opposed to MTP2’s “receive” buffer congestion.

C. The Link Management: The MTP3 users are provided with a link, once they are proved to be capable of transporting messages. When a link fails, it directly affects the two nodes, which are connected through the link. Here, the work of link management is to detect and strive to restore any communication loss in the process. In an attempt to re-establish the connection, both nodes linked to the link engage restoration processes. Here, the link management process can be divided into three parts:

  • Activation is a process, which makes the link available to carry MTP3 user traffic. It is usually done by maintenance personnel using commands from an OAM interface to request that the link be activated for use.
  • Deactivation often refers to the removal of a link from service or the inability to carry traffic across it. This process begins by executing commands from an OAM interface, exactly as it does with activation. When a link is deactivated, it is declared as not available for further process by traffic management.
  • Restoration is an automatic procedure, where after a link failure, an automated attempt is made to return the link to service, making it available for traffic control. 

Some other Functionalities:

Some other functionalities of the MTP3 layer are briefly explained below:

1. ANSI Network and Cluster Routing: Routing is typically performed hierarchically. Messages on ANSI networks can be routed by matching only a part of the DPC. The match is performed on a subset of the DPC’s Most Significant Bits, allowing messages to be routed with fewer entries in the routing tables. This function is especially helpful when dealing with traffic headed for the network of another operator.

The example of ANSI Cluster routing can be seen in the diagram below:

ANSI Cluster Routing

ANSI Cluster Routing

2. Load Sharing: To create network reliability, a well-designed SS7 network uses different message pathways. To maintain a balanced load on network infrastructure, user traffic is often load-shared across many paths. Because each line is carrying traffic, load sharing ensures that faults on each path are recognized immediately. 

There are mainly two types of Message load sharing, which are as follows:

  • Load-sharing across linksets in a combined linkset
  • Load-sharing across links within a linkset

3. SLS in ITU-T Networks: The four-bit SLS value is used in ITU-T networks. The SLS values remain constant as the message travels via the network. If the combined linkset is used, then a one-bit SLS code will be used to select the linkset for each node. And the remaining bits will be used to choose a link from the linkset. However, if the combined linkset is not used, all bits can be used to pick a link inside the linkset.

4. SLS in ANSI Networks: To begin with, ANSI networks employ an eight-bit SLS code. If the message is routed across a combined linkset, the least significant bit of the SLS is used for linkset selection, and the remaining bits are used for link selection. When routing over a single linkset, all bits are used to select the link.

Benefits:

1. Reliability: MTP3 ensures that signaling traffic between SS7 nodes is delivered reliably. the Point Codes are used in signaling message handling, which ensures that the messages are sent to the correct destination. and also, discriminates the inbound messages to determine whether they have reached their correct destination.

2. Provides network failure and traffic management solutions: The Signaling Network Management process in SS7 provides a reliable method for managing the network failures and traffic with the least amount of message loss, duplication, or is-sequencing.

3. Ensures Network integrity: Timers keep track of the SNM process and messages to ensure that the correct action is taken to keep the network integrity.


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