Basic Concepts
MTP is used to transmit signaling messages of various UPs (such as TUP, DUP, and ISUP) in SS7 system. Its design complies with the ITU-TQ.701-710 recommendations.
The MTP provides reliable signaling message transmission. It takes measures to avoid message loss, repetition, and out-of-sequence in case of the signaling network failure. The MTP includes the functional levels of signaling data link (MTP1), signaling link (MTP2) and signaling network (MTP3).
I. MTP1
The signaling data link function is the Level 1 function (MTP1) of MTP. MTP1 defines the physical, electrical and functional features of a signaling data link and the means to access the data link. It is equivalent to the physical layer of open system interconnection (OSI)’s seven-layer model, which is to generate and receive signals over physical channels.
One signaling data link is composed of two data channels operating at the same data transmission rate and in two opposite directions. The bit rate of digital message carrier is 64 kbit/s. It is applicable to transmission links of both lower rate (such as 4.8 kbit/s) and higher rate (such as 2048 kbit/s).
II. MTP2
The signaling link function is the Level 2 function (MTP2) of MTP. It provides reliable signaling links for transmitting signaling messages to signaling data links between two directly connected SPs together with Level 1.
Signaling link function can be divided into eight parts:
l Demarcation of signal units;
l Location of signal units;
l Error detection;
l Error correction;
l Initial alignment;
l Processor faults;
l Flow control in Level 2;
l Monitoring of signaling link error rate.
III. MTP3
The network layer is the Level 3 (MTP3) of MTP. It transmits management messages between two SPs to ensure the reliable transmission of various signaling messages in case of the failure of signaling links or STPs. It is equivalent to the third layer (network layer) of the OSI model.
IV. Signaling Network Function
The signaling network function provided by the MTP3 must ensure the reliable transmission of signaling messages between SPs in case of the failure of signaling links or STPs. Therefore, this function includes functions and procedures required in notifying the remote end of fault results and the configuration function and procedure required in message routing in the signaling network.
The signaling network function is divided into signaling message processing and signaling network management, as shown in Figure 6-2.
Figure 6-2 Signaling network function
2) Signaling message processing
The signaling message processing function is to ensure the signaling messages initiated by a specific UP of a signaling point (SP) to be transmitted to the same UP of the DSP designated by this UP.
The signaling message processing function is divided into three parts.
l Message routing function determines the outgoing signaling links through which messages are transmitted to destinations from every SP.
l Message discrimination function is used to identify whether the DSP that receives a message is the local SP. If the local SP is not the DSP and is capable of transferring, the message routing function will be enabled to transfer the message.
l Message distribution function distributes received signaling messages to corresponding UPs by every SP.
3) Signaling network management function
When a signaling link or SP is faulty, signaling network management function provides the reconfiguration capability of the network. When congestion occurs, it can control signaling traffic. The reconfiguration capability is to change the routing, bypass faulty links or faulty SPs of the signaling service through proper procedures. In some cases, you must activate and align new signaling links to restore signaling traffic required between two SPs. When faulty links or SPs are restored, the opposite activities and procedures are used to rebuild the normal configuration of the signaling network.
The signaling network management consists of signaling traffic management, signaling link management and signaling route management.
When the status of a signaling link, route or SP is changed, the above three management functions can be activated under proper situations. The following describes specific contents.
l Signaling traffic management: It is to transmit signaling traffic from one link or route to multiple links or routes. It can restart MTP of an SP or slow down signaling traffic temporarily in case of congestion.
l Signaling link management: It is to restore faulty signaling links, to activate those links that are not arranged, and to deactivate arranged signaling links.
l Signaling route management: It distributes the status information of the signaling network to block or unblock signaling routes.
6.2.2 Singnaling Message
I. Message Type
Table 6-1 Signaling network management message
| Acronym of message | Full name |
| CHM | Changeover and changeback messages |
| COO | Changeover-order signal |
| COA | Changeover-acknowledgement signal |
| CBD | Changeback-declaration signal |
| CBA | Changeback-acknowledgement signal |
| ECM | Emergency-changeover message |
| ECO | Emergency-changeover-order signal |
| ECA | Emergency-changeover-acknowledgement signal |
| FCM | Signaling-traffic-flow-control messages |
| RCT | Signaling-route-set-congestion-test signal |
| TFC | Transfer-controlled signal |
| TFP | Transfer-prohibited signal |
| TFR | Transfer-restricted signal (national option) |
| TFA | Transfer-allowed signal |
| RSM | Signaling-route-set-test message |
| RST | Signaling-route-set-test signal for prohibited destination |
| RSR | Signaling-route-set-test signal for restricted destination (national option) |
| MIM | Management inhibit messages |
| LIN | Link inhibit signal |
| LUN | Link uninhibited signal |
| LIA | Link inhibit acknowledgement signal |
| LUA | Link uninhibited acknowledgement signal |
| LID | Link inhibit denied signal |
| LFU | Link forced uninhibited signal |
| LLT | Link local inhibit test signal |
| LRT | Link remote inhibit test signal |
| TRM | Traffic-restart-allowed message |
| TRA | Traffic-restart-allowed signal |
| DLM | Signaling-data-link-connection-order message |
| DLC | Signaling-data-link-connection-order signal |
| CSS | Connection-successful signal |
| CNS | Connection-not-successful signal |
| CNP | Connection-not-possible signal |
| UFC | User part flow control messages |
| UPU | User part unavailable signal |
II. Message Structure
To meet the requirements of signaling message transmission through the MTP, three basic signaling unit formats are stipulated: message signaling unit (MSU), link status signaling unit (LSSU) and fill-in signaling unit (FISU).
l MSU is to transmit messges of various UPs, management messages, and test and maintenance messges.
l LSSU is to provide link status information so as to connect and restore signaling links.
l FISU is to maintain the normal running of signaling links and play a fill-in part when no message signaling or link status signaling unit is transmitted.
Message unit structure is shown in Figure 6-3.
Figure 6-3 Message unit format
Structurally, signaling unit is divided into two parts. One is the mantatory part of MTP part processing occupied by various signaling units, which consists of eight fixed-length fields. The other is the signaling message part of user part processing.
2) Mantatory Part of MTP Processing
This part includes flag (F), forward sequence number (FSN), forward indicator bit (FIB), backward sequence number (BSN), backward indicator bit (BIB), length indicator (LI), check bit (CK), status field (SF) and service information octet (SIO).
l Flag
Flag is also called delimiter. There is a flag at both the start and the end of every signaling unit. During the transimission of signaling units, every flag indicates the end of the last signaling unit and the start of the next one. Therefore, in the delimitation identification of signaling units, a signaling unit is identified by two flags of the start and the end in the information flow.
The pattern for a flag is 8-bit binary code 01111110.
In addition to the delimitation function, some flags can be inserted between signaling units in case of overload of signaling links to reduce load.
l FSN
FSN indicates the sequence number of the transmitted MSU and consists of 7 bits. At the transmitting end, every transmitted MSU is allocated with a FSN. FSNs are numbered in a cyclic sequence ranging from 0 to 127 At the receiving end, the FSNs in the received MSUs are used to check the sequence of the MSUs, which is a part of the confirmation function. When retransmission is necessary, it is also used to identify the signal units to be retransmitted. The FSN of FISU or LSSU uses the FSN of the MSU transmitted last time, instead of being assigned with new sequence numbers.
l FIB
It occupies one bit. FIB is used in the retransmission process of MSUs. In the normal operation, FIB has the same state with that of the received BIB. Retransmission is requested if the received BIB changes its value. Upon retransmitting MSUs, the signaling terminal will also change the value of FIB (from "1" to "0" or from "0" to "1" ), so that it is consistent with BIB again until BIB changes its value again when there is another retransmission request.
l BSN
The BSN is the sequence number of the confirmed (that is, received correctly) MSUs being sent back to the transmitting end by the receiving end.
If retransmission is requested, BSN indicates the sequence number of the unit to be retransmitted.
In the operation of a signaling network, the transmitting end and receiving end set their FSN independently.
For transmitted and unconfirmed signaling units, the limit value of FSN and BSN is 127.
l BIB
BIB is used to initiate a retransmission request for a wrong signaling unit received. If an MSU received is correct, its BIB value remains unchanged when a new signaling unit is sent. If a wrong MSU is received, the bit will be reversed (that is, from "0" to "1" or from "1" to "0") and sent, requesting the opposite end to send a correct MSU.
l LI
LI is used to indicate the number of octets following the length indicator octet and preceding the check bit (CK) so as to distinguish three types of signaling units.
LI is a number in binary code in the range 0–63 (decimal numeral). The field of LI is 6 bits.
The LIs of the three types of signaling units are as follows:
Length indicator LI=0 Fill-in signaling unit
Length indicator LI=1 or 2 Link status signaling unit
Length indicator LI> 2: Message signaling unit
In a signaling network, if the signaling information field of an MSU is more than 62 octets, the length indicator is set to 63. However, if the LI is 63, the maximum length indicated must not exceed 272 octets.
Note that the numbers of bits and octets between two flags of a signaling unit have to be calculated frequently in the receiving and processing of signaling units. The CCITT regulates that the number of bits between two flags of a signaling unit must be an integral number of octets. The number of octets can be "0" (if only the flag is transmitted) or 5 (for FISU). The number can also be less than or equal to m+7 octets (m is equal to 272). If the number is beyond the range, it is regarded that there is a signaling unit error.
l CK
The field is used for error detection of signaling units. It consists of 16 bits.
The seven fields above mentioned are available in all the three kinds of signaling units. (Eight fields including end F have been discussed). They are mandatory for every signaling unit.
l SF
Status field is unique to each LSSU, which indicates the statuses of signaling links.
The length of SF can be an octect (8-bit) or two octects (16-bit).
When it is an octect, the lower 3 bits indicates link statuses. The meanings are shown in Table 6-2.
Table 6-2 Meanings of SF status indication
| CBA | Identifier | Indication | Meaning |
| 000 | SIO | Status indication “O” | Out of location |
| 001 | SIN | Status indication “N” | Normal location |
| 010 | SIE | Status indication “E” | Emergency location |
| 011 | SIOS | Status indication “OS” | Out of service |
| 100 | SIOP | Status indication “OP” | Processor outage |
| 101 | ISB | Status indication “EB” | Busy |
l SIO
The field of SIO is unique to MSUs. It consists of SI and SSF, as shown in Figure 6-4.
The field has 8 bits. SI and SSF occupy 4 bits respectively.
Figure 6-4 Format and codes of SIO
3) SI
SI indicates to which user part the transmitted message belongs. In the MTP of a signaling network, the message processing function distributes a message to the user part indicated by SI.
SI is coded as Figure 6-4. The capacity of SI allows it to represent messages of16 kinds of UPs. This diagram only lists those that are frequently used.
4) SSF
ISSF consists of 4 bits, of which the higher two bits act as the network indicator, and the lower two bits, coded 00, are reserved presently.
The network indicator serves to distinguish the network attribute of the transmitted message, that is, to distinguish between an international signaling network message and a national signaling network message. Figure 6-4 illustrates the codes and network allocation of SSF.
According to CCITT stipulations, the use of the reserved codes in SSF is determined by the domestic signaling network conditions in different countries.
5) Signaling Information Part Processed by User Part
Signaling information part processed by user parts is the SIF in the format of MSU. SIF is unique to MSUs. It consists of message addressing tag, user signaling information heading and user signaling information.
l Tag
Tag includes the information required in sending messages to the destination. Standard routing tag has 32 bits, which is at the beginning of SIF. The tag includes destination point code (DPC), originating point code (OPC) and SLS.
Signaling point code is a digital address, which identifies every signaling point in the No.7 singnaling network uniquely. When the DPC in a message represents the receiving sigaling point, the message is distributed to the correponding user part (such as ISUP or SCCP) in the SIO as indicated by the service indicator.
The SLS ensures the sequencing of messages. Any two messages sent with the same SLS always arrive at the destination in the same sequence as that in which they are sent. Equal traffic load can be shared among all available links. If a user part regulary sends messages and distributes the SLS value cyclically, all the service levels to the destination should be equal. There are four types of tag structures, as shown in
Figure 6-5.
Type A MTP management message
Type B TUP message
Type C ISUP message
Type D SCCP message
Since TCAP messages must be sent through SCCP, TCAP messages belong to the type of SCCP messages, that is, type D.
Figure 6-5 Four types of tag structures
l Label
It is the field immediately following the tag field. It is composed of H1 and H0, occupies 4 bits respectively, and indicates the group and type of messages. For example, H0= 0001 and H1=0001 in a telephone user part (TUP) message mean that the transmitted message is an initial address message (IAM); H0=0001 and H1=0100 mean that the transmitted message is an address complete message (ACM). For signaling network management messages, if both H0=0001 and H1=0001, it means that the transmitted message is changeover order signaling (COO); if H0=0001 and H1=0100, it mean that it is a transmission forbidden message. Since both H1 and H0 occupy 4 bits, a certain user message can accommodate 256 messages.
l Signaling message
Signaling message part is also called service message part. This part can be further divided into several sub-fields. These sub-fields may be mandatory or optional, and meanwhile they may be of fixed length or variable length so as to meet the needs of various functions and expansion. Thus the SMU is adaptable to different user messages with different features, so that these various user messages can be transmitted through common channels.
For specific format and codes of SIF, refer to the descriptions of codes and format of user messages.
l MTP
The fields F, BSN, BIB, FSN, FIB, LI and CK in a signaling unit are mainly used for the sending and receiving sequence, error detection and correction of signaling message units. These fields are analyzed and processed in the second function level of a signaling network, that is, signaling link level. FISU is mainly composed of those fields with transmission control function. It has the function to "fill-in" in the signaling link, and therefore the signaling unit of this type is generated and processed by the second function level.
LSSU is used to transmit the status indication information of a signaling link. It is also generated and processed on the second function level. The second function level may generate and transmit corresponding status signaling units according to instructions from the third level or its judgement, or receive and process the signaling link status indication sent by the opposite end. If necessary, it reports such statuses as congestion and processor error to the third level.
The MSUs transmitted in a signaling network fall into three types according to their roles in the network: MSU used for signaling network management (MSU-SNM), MSU used for signaling network test and maintenance (MSN-SNT), and MSU generated by the user part (MSU-UP). The first two types are of type A structure, which are transmitted between MTPs. They are generated and processed by Level 3. The third type consists of messages of type B, C and D structures. These messages are transmitted through the MTP to a specific UP. Level 3 analyzes its message tag, and determines the message destination; while the generation and processing of its service message part (SIF) are performed in Level 4, that is, the user part.
The most important message in the MTP layer is the signaling network management message. The following introduces the general format of the signaling network management message.
In the signaling network, the signaling network management message is identified by the SI bit “S1=10000” of the SIO in the signaling unit.
As one kind of message signaling unit, the signaling information of the signaling network management message is transferred by the SIF. The Figure 6-6 shows the structure of the signaling management message.
Figure 6-6 General format of the signaling network management message
l Tag
The tag comprises three parts: DPC, OPC, and SLC.
The DPC and OPC are the same as those described above.
The SLC refers to the signaling link code that connects the DSP and the originating signaling point (OSP). If transferred messages are not related to the signaling link or another special code is not specified, the SLC is “0000”. Currently a four-bit code is used. The standby four-bit code is “0000”.
l Heading code
The heading code comprises two four-code bits: H0 and H1.
H0 identifies a management message group, and H1_determines the messages in a message group. Since H0 and H1 both have four codes, the total message capacity is up to 256 types. That is, there can be 16 message groups, and each group has 16 types of messages. At present only some of the message types are used. Refer to Table 6-3.
Table 6-3 Distribution of the heading code of the management message of the signaling network
| Message Group | H1 H0 | 0000 | 0001 | 0010 | 0011 | 0100 | 0101 | 0110 | 0111 | 1000 | 1001 | 1010 | 1011 | 1100 | 1101 | 1110 | 1111 |
| | 0000 | | | | | | | | | | | | | | | | |
| CHM | 0001 | | COO | COA | | | CBD | CBA | | | | | | | | | |
| ECM | 0010 | | ECO | ECA | | | | | | | | | | | | | |
| FCM | 0011 | | RCT | TFC | | | | | | | | | | | | | |
| TFM | 0100 | | TFP | * | TFR | | TFA | * | | | | | | | | | |
| RSM | 0101 | | RST | RSR | | | | | | | | | | | | | |
| MIM | 0110 | | LIN | LUN | LIA | LUA | LID | LFU | LLT | LRT | | | | | | | |
| TRM | 0111 | | TRA | | | | | | | | | | | | | | |
| DLM | 1000 | | DLC | CSS | CNS | CNP | | | | | | | | | | | |
| | 1001 | | | | | | | | | | | | | | | | |
| UFC | 1010 | | UPU | | | | | | | | | | | | | | |
| | 1011 | | | | | | | | | | | | | | | | |
| | 1100 | | | | | | | | | | | | | | | | |
| | 1101 | | | | | | | | | | | | | | | | |
| | 1110 | | | | | | | | | | | | | | | | |
| | 1111 | | | | | | | | | | | | | | | | |
Example
The following is the format of the TFA message:
The heading code H1 contains a signaling code, as shown beblow:
| D | C | B | A | |
| 0 | 1 | 0 | 1 | TFA |
III. Signaling Procedure
1) Message routing
The message routing function is based on the information contained in the routing tag, that is, the information on DPC and SLC field.
Each signaling point has routing information which determines the signaling link. Messages are sent in the siganaling link according to the DPC and SLS field.
Typically, the DPC is associated with more than one signaling links which are used to bear messages. A specific signaling link is selected through the SLS field, thus realizing load sharing.
Two basic exmples of load sharing:
l Load sharing among links in the same link group.
l Load sharing among links not in the same link group.
Any SLC can be allocated to messages unrelated to the signaling link so that the messages can be load-shared. The other way is to allocate the default SLC, such as “0000” to these messages. The messages route according to the normal routing function. In this function, the SLC is used as the SLS to realize load sharing.
2) Switchover
The switchover program ensures that the signaling services borne on unavailable links are switched over to alternative signaling links as soon as possible. It also avoids message loss, repetition and sequencing errors.
To implement this function, buffer updating and recovery are included in the switchover process. The process is started before the signaling link starts switching over the service. Buffer updating includes identifying all the messages in the retransmission buffer of the available signaling link. These message are not received by the remote end. Recovery includes forwarding relevant messages to the transfer buffer of the alternative link.
When one signaling link is unavailable, switchover is implemented in the signaling point. Then the following is conducted:
l Terminating the sending and receiving of MSUs on relevant signaling links.
l Sending LSSUs or FLSUs.
l Determining the alternative signaling link.
l Running the content updating process of the re-buffer of the unavailable signaling link.
l Transferring signaling services to the alternative signaling link.
3) Changback
The changback program ensures that the signaling services are transferred from alternative signaling links to available signaling links as soon as possible. It also avoids message loss, repetition and sequencing errors. Changback includes the basic program that uses opposite activities for switchover.
When a signaling link becomes available due to reconnection, recovery or unlocking, changback is implemented at the signaling point. Then the following is conducted:
l Determining the alternative signaling link that forwarded normals servies in the past.
l Stopping the transmitting of related services on the alternative signaling link, and storing the services in the changback buffer.
l Sending a changback notification through a related alternative signaling link to the remote signaling point of the signaling link that becomes available. This message indicates that the message service on the alternative signaling link can be sent through the available signaling link.
l When receiving the changback confirmation sent by the remote signaling point of the available signaling link, the relevant signaling point will restart the forwarded service on the available signaling link.
4) Activating the signaling link.
When it is decided to activate an inactive signaling link, initial alignment starts:
l If initial alignment succeeds, the signaling link becomes activated and the signaling link test starts.
l If the signaling link test succeeds, the link prepares to transmit signaling services.
l If initinal alignment fails, a new initial alignment process starts on the same signaling link after time-out for the timer.
l If the signaling link test fails, start link recovery until the signaling link is activated or conduct manual operations.
5) Recovering the signaling link.
When a signaling link fault is detected, signaling link initial alignment will occur.
l If initial alignment succeeds, the signaling link test starts.
l If the signaling link test succeeds, the link is recovered and can be used for signaling transmission.
l If initial alignment fails, a new initial alignment process may be started on the same signaling link.
l If the signaling link test fails, repeat the recovery process until the link is recovered, or intervene manually.
6) Deactivating the signaling link.
If not bearing signaling services, an active signaling link can turn inactive through the deactivating process. If a signaling link is deactivated, the signaling terminals of the signaling link exit services.
7) Signaling route management process
The purpose of signaling route management is to ensure that information is reliably exchanged between signaling points; that is, to ensure the signaling route is available.
The unavailability, restriction, and availabitity of the signaling route are implemented by the transfer-prohibited, transfer-restricted, and transfer-allowed procedures.
8) Transfer-prohibited procedure
To facilitate description, three letters are used to represent three kinds of signaling points: Y for OSP, X for DSP, and Z for signaling transfer point (STP).
l When Y selects the signaling route from Z to X, Z is unavailable for Y. In this case, the TFP transfer-prohibited message is sent to Z.
l When Z confirms that X is difficult to reach, the transfer-prohibited message is sent to all reachable adjacent signaling points (by broadcasting).
l When Z receives a message sent to X and Z cannot forward the message, the transfer-prohibited message is sent to an adjacent signaling point and relevant messages are received at this point.
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