Message Transfer Part Functions and Protocol

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.

MTP1 Signaling Data Link Layer
Bidirectional transparent transmission channel (64kbit/s)

MTP2 Signaling Link Layer
MTP2 provide reliable signaling links for transmitting signal messages between two directly connected signaling points.
- 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.
- 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.
- It is used to check whether the bit streams of a signal unit are transmitted incorrectly.
- It is used to re-obtain correct signal units if some errors are found. There are two correction methods: basic method and preventive cyclic re-transmission method.
- 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
- It is used to process the congestion problem detected by the second layer to lighten the congestion condition.
- It is used to mark or remove the faulty status of a processor.

MTP3 Functional Layer of Signaling Network
A normal signaling link sends an SLTM message every minute. The signaling point that receives the SLTM message sends an SLTA message. The value of test code in the SLTA message must be same to that in the SLTM message. Also, the number of SLTA messages must be equal to that of SLTM messages.

After MTP2 is aligned successfully, MTP3 sends an SLTM message and starts a six-second timer. If an SLTA message is received before the timer is time out, the link service is activated. If an SLTA message is not received, the SLTM message is transmitted again after the timer is timeout. If the SLTA message is not received within six seconds, the link service cannot be activated.

If a signaling link is unavailable, the changeover is performed. During the changeover process, signaling services are transferred from an unavailable signaling link to one or more replaced link and avoiding message loss, duplication or mis-sequencing. Figure 1-9 shows the changeover procedure.

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The Architecture of Signaling Number 7 (SS7)

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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Basic Concept of Signaling SS7

Message Transfer Part (MTP) is mainly responsible for transmitting "User Part" signaling messages across the SS No. 7 network. If a system or signaling network is faulty, MTP can take measures to prevent messages from losing, and repeatedly or disorderly transmitting. MTP consists of signaling data links, signaling link functions, and signaling network functions.

Here are the following exchange in SS7

Signaling point (SP):
A signaling point is a node of SS7. It can be identified by signaling point code (SPC). Source signaling point refers to a signaling point that generates signaling messages. Destination signaling point refers to a signaling point that receives signaling messages. In general, international SPC is 14-bit and national SPC is 24-bit.

Signaling link: 
A signaling link is a physical link that connects signaling points and transmits signaling messages.

Signaling network:
Logically, a signaling network is separated from a communication network. It is used to transmit signaling messages. A signaling network mainly consists of signaling points and signaling links.

Signaling link set:
A signaling link set is a set of parallel signaling links that directly connects two signaling points.

Signaling route:
A signaling route is a route that transmits signaling messages from source signaling point to destination signaling point.

Signaling transfer point (STP):
A signaling transfer point is neither a source signaling point, nor a destination signaling point. It is a signaling point that transfers signaling messages. If a signaling transfer point can serve as a signaling point, it is called an integrated STP. If a signaling transfer point cannot serve as a signaling point, it is called an individual STP.

There are the following two working modes:

Associated: 
Three signaling points (VMSC, GMSC, and VMSC) cannot serve as an STP. The signaling messages between VMSC and GMSC are transmitted through a directly connected signaling link. GMSC can serve as a voice transfer path; however, GMSC cannot serve as a signaling transfer point.If SS7 signaling messages are transmitted through two or more signaling links with one or more STPs, it is called quasi-direct connection. Figure 1-2 shows the quasi-direct connection.

Quasi-associated: 

There is a direct voice path between VMSC and VMSC; however, there is no direct signaling link between VMSC and VMSC. Signaling messages between VMSC and VMSC are transferred by GMSC. GMSC serves as an integrated STP.            

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