From TUP, ISUP, and SCCP points of view, MTP is only a message transfer point that is used to transfer the messages of user parts to the user parts of destination signaling points.
In brief, SS7 can be regarded as a TCP/IP network
MTP Functions
Message Transfer Part (MTP) provides services for transferring the messages of user parts (such as TUP, ISUP, and SCCP) to destinations through SS7. If SS7 experiences a fault, the messages can be transmitted correctly through the signaling network management function. The overall objectives of the MTP are provided the means for:
- The reliable transport and delivery "User Part" signaling information across the SS7 network.
- The ability to react to system and network failures that will affect, and take the necessary action to ensure that is achieved.
MTP has the following three functional levels:
- Signaling data link
- Signaling link function
- Signaling network function
Signaling Data Link Layer (MTP1):
- Bidirectional transparent transmission channel (64kbit/s)
- Switching function
- Signaling data link layer switches an SS7 terminal to a trunk device through semi-permanent
Signaling Link Layer (MTP2):
MTP 1 and MTP2 provide reliable signaling links for transmitting signal messages between two directly connected signaling points. MTP2 has the following functions:
Signal Unit Delimitation
It indicates the starting point and the ending point of a signal unit, namely, recognizing signal units from bit streams in a signaling data link.
Signal Unit Alignment
It is used to check whether signaling links in service is loss of alignment. Loss of alignment will cause a change in the mode of operation of the signal unit error rate monitor.
Signal Unit Error Detection
It is used to check whether the bit streams of a signal unit are transmitted incorrectly.
Error Correction
It is used to re-obtain correct signal units if some errors are found. There are two correction methods: basic method and preventive cyclic retransmission method.
Initial Alignment
The initial alignment procedure is appropriate to both first time initialization (e.g. after "switch-on") and alignment in association with restoration after a link failure.
Signaling Link Error Monitoring
It is used to monitor the errors of a signaling link to ensure high-quality service. It include signal unit error detection and the alignment error rate detection
Flow Control
It is used to process the congestion problem detected by the second layer to lighten the congestion condition.
Processor Outage
It is used to mark or remove the faulty status of a processor.
Three types of signal unit are described as follows:
Fill-In Signal Unit (FISU)
If no signal unit is transmitted on a SS7 signaling link, FISU (The length identity (LI) is 0) is sent to the peer end for notifying that the MTP2 of local office is normal. There is no information field in FISU.
Link Status Signal Unit (LSSU)
The LSSU provides the information of link status to achieve the connection or recovery of signal link. Information field obtained in LSSU is called Status Field (SF). The status indicator represents the status of the MTP2 of the signaling point. The length of SF is 1 or 2.
Message Signal Unit (MSU)
It has a longer information field, which has two sections. The first section is a service information octet (SIO) that consists of eight bits. The second section is a signaling information field (SIF) that consists of multiple eight bits, that is, MTP signaling messages and MTP user part messages, such as TUP and ISUP (The length identity (LI) is greater than 2). All the signals transmitted on a SS7 link belong to one of the three signal units
Signaling link alignment process
Initial alignment is a control program used for initializing or recovering a signaling link. There are two initial alignment procedures: normal initial alignment and emergent initial alignment. The decision to apply either the "normal" or "emergency" procedures is made unilaterally at MTP3. The proving period of normal initial alignment is different from that of emergent initial alignment. For a 64 kbit/s link, the nominal value of normal initial alignment stipulated in the protocol is 8.2 s, whereas the nominal value of emergent initial alignment is 0.5 s. During the proving period, the alignment error rate monitoring (AERM) function is used to check whether the error rate of a link is within the acceptable scope.
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