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Introduction : SS7 Structure

Message Transfer Part (MTP)

The message transfer part represent the transport of messages between users. The term “user” is referring to the functional units which use the transport capability of the message transfer part. Each user part compose the functions, protocols, coding of the signaling in SS7 for a specific user type.
MTP transports messages in signal units of varying lengths. A signal unit is by the functions of level 2. In addition it also contains control information for the message exchange.
The MTP in SS7 is divided into 4 levels.
Level 1 Defines the physical layer
Level 2 Defines the function and procedure for correct exchange of user messages

    • delimitation of signal units by flags
    • error detection using check bits and error correction by retransmitting signal units
    • restoration of fault-free operation

Level 3 Defines the interworking functions of signaling links

    • message routing
    • message distribution
    • signaling network management

Signal Units

Three different kinds of signal units

  • message signal units (MSU)
  • link status signal units (LSSU)
  • Fill-in signal units (FISU)

Using the message signal units the messages transfer part transfers user messages, i.e. messages from user parts (level 4) and messages from the signaling network management (level 3). The link status signal units contain information for the operation of the signaling link (e.g. alignment) and the fill-in signal units are used to maintain the acknowledgement cycle when no user messages are to be sent in one of the two directions of the signaling link.

Service Information Octets

The service information octet in the message signaling unit contains the service indicator and the network indicator.
The service indicator is used for addressing. This means the level 3 function distributes the messages, with the help of service indicator, to the corresponding user parts. As a rule, the ISUP or SCCP user parts are used here. If the link status is sent, this is also displayed via the service indicator.
The network indicator cites the corresponding network in which the transmitter and receiver of the message are located. There are four possibilities : NAT0, NAT1, INAT0 or INAT1 (not used yet).

Signal Units

Three different kinds of signal units

  • message signal units (MSU)
  • link status signal units (LSSU)
  • Fill-in signal units (FISU)

Using the message signal units the messages transfer part transfers user messages, i.e. messages from user parts (level 4) and messages from the signaling network management (level 3). The link status signal units contain information for the operation of the signaling link (e.g. alignment) and the fill-in signal units are used to maintain the acknowledgement cycle when no user messages are to be sent in one of the two directions of the signaling link.

Service Information Octets

The service information octet in the message signaling unit contains the service indicator and the network indicator.
The service indicator is used for addressing. This means the level 3 function distributes the messages, with the help of service indicator, to the corresponding user parts. As a rule, the ISUP or SCCP user parts are used here. If the link status is sent, this is also displayed via the service indicator.
The network indicator cites the corresponding network in which the transmitter and receiver of the message are located. There are four possibilities : NAT0, NAT1, INAT0 or INAT1 (not used yet).

Addressing Signal Units

The routing label in a message signal unit consists of the following

  • destination point code (DPC)
  • originating point code (OPC)
  • signaling link selection (SLS) field

A code is assigned to each signaling point in the signaling network according to a numbering plan. The message transfer part uses the code for message routing. The destination point code in a message is to be transferred. The originating point code specifies the signaling point from which the message originates. The contents of the signaling link selection field determine the signaling route along which the message is to be transmitted. In this way, the signaling link selection field is used for load-sharing on the signaling links. The service information octet (SIO) contains additional address information. Using the service indicator, the destination message transfer part identifies the user part for which the message is intended. The network indicator, for example, enables a message to be identified as being for national or international traffic. Link status signal units and fill-in signal units require no routing label as they are only exchanged between level 2 adjacent message transfer parts.

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