ISUP

1 Overview

I. Basic Concepts
ISUP is one kind of UPs of the No.7 public channel signaling system. It provides the signaling function required for supporting voice and non-vocie basic bearer services and supplementary services in the integrated services digital network (ISDN).
ISUP is applied to the hybrid digital/analog network, telephony network, and dedicated circuit-switched data network. ISUP meets the requirements of CCITT for international semi-automatic and automatic telephony services and circuit-switched data services.
II. Protocol stack architecture
The ISUP uses services provided by the MTP to transfer information between ISUPs. Figure 6-7 shows the protocol stack of the ISUP. ISUP information is carried to the MTP or from MTP to ISUP by primitives in the form of parameters.
Figure 6-7 ISUP protocol stack
Primitives used between MTP and ISUP include the transfer primitive, recovery primitive, suspension primitive, and status primitive.
The MTP transfer primitive receives and sends ISUP singaling messages by encapsulating the messages.
When MTP sends the MTP suspension primitive, it means MTP cannot send messages to a specific destination as parameters.
When MTP sends the MTP recovery primitive, it means MTP can be recovered to parameters and can send messages to a specific destination in an unrestricted manner.
When MTP sends the MTP status primitive, it means that the signaling route to a specific destination is congested or that there is no ISUP on the destination. This may be because ISUP is not installed or ISUP cannot be accessed. Other unkown factors may cause this problem too.
III. Application in NGN
ISUP has three application modes in NGN solutions, as shown in Figure 6-8, Figure 6-9 and Figure 6-10.
1) Application of ISUP in NGN (MTP-MTP)
Figure 6-8 Application of ISUP in NGN (MTP-MTP)
A MTP link goes directly from the SoftX3000 to connect a STP, thus realizing the interoperation of SS7 with a PSTN switch through an SS7 network. In the voice channel, the SoftX3000 controls the TMG8010 to implement the interconnection with the PSTN.
2) Application of ISUP in NGN (M2UA-MTP)
Figure 6-9 Application of ISUP in NGN (M2UA-MTP)
The SoftX3000 provides an M2UA link to connect with the TMG8010, and exchanges SS7 with a PSTN switch through the TMG8010, which has the function of an embedded signaling gateway. In the voice channel, the SoftX3000 controls the TMG8010 to implement the interconnection with the PSTN.
3) Application of ISUP in NGN (M3UA-MTP)
Figure 6-10 Application of ISUP in NGN (M3UA-MTP)
The SoftX3000 provides an M3UA link to connect with the SG7000, and exchanges SS7 with a PSTN switch through the SG7000. In the voice channel, the SoftX3000 controls the TMG8010 to implement the interconnection with the PSTN.
6.3.2 Singnaling Message
ISUP messages are transferred on the signaling link through the MSU. The messages are encapsulated in the SIF of the MSU. An ISUP message consists of six parts: routing tag, circuit identification code (CIC), message type code, mandatory fixed part, mandatory variable part, and optional part, as shown in Figure 6-11.
For details of the routing tag and CIC, refer to 6.2.2 Singnaling Message. The following introduces other parts of the ISUP message.
Figure 6-11 Structure of the ISUP message
I. Message Type Code
A message type code is an SIO field, which is required for all messages. The message type code defines the function and format of each kind of ISUP message. See Table 6-4.
Table 6-4 ISUP message code
Code
Abbreviation
Meaning
00000001
IAM
Initial address message: A message sent in the forward direction to initiate occupancy of an outgoing circuit and to transmit number and other information relating to the routing and handling of a call.
00000010
SAM
Subsequent address message: A message that may be sent in the forward direction following an initial address message, to convey additional called number information.
00000011
INR
Information request: A message sent by a switch to request information in association with a call.
00000100
INF
Information: A message sent to convey information in association with a call, which may have been requested in an information request message.
00000101
COT
Continuity message: A message indicating whether or not there is continuity on the preceding circuit as well as of the selected circuit to the following switch, including verification of the communication path across the switch with the specified degree of reliability.
00000110
ACM
Address complete message: A message indicating that all the address signals required for routing the call to the called party have been received.
00000111
CON
Connect message: A message indicating that all the address signals required for routing the call to the called party have been received and that the call has been answered.
00001000
FOT
Forward transfer message: A message sent in the forward direction on semi-automatic calls when the outgoing international switch operator wants the help of an operator at the incoming international switch. The message will normally serve to bring an assistance operator into the circuit if the call is automatically set up at the switch. When the call is completed through an operator at the incoming international switch, the message should preferably cause this operator to be recalled.
00001001
ANM
Answer message: it indicates that the call has been answered. In semi-automatic working, this message has a supervisory function. In automatic working, this message is used in conjunction with charging information.
00001100
REL
Release message: A message sent in either direction to indicate that the circuit is being released due to the cause supplied and is ready to be put into the idle state on receipt of the release complete message. When the call is redirected, the redirection message will also carry the redirection number.
00001110
RES
Resume message: A message sent in either direction indicating that the calling or called party, after having been suspended, is reconnected.
00010000
RLC
Release complete message: A message sent in either direction in response to the receipt of a release message, or if appropriate to a reset circuit message, when the circuit concerned has been brought into the idle condition.
00010001
CCR
Continuity check request message: A message sent by a switch for a circuit on which a continuity check is to be performed, to the switch at the other end of the circuit, requesting continuity checking equipment to be attached.
00010010
RSC
Reset circuit message: A message sent to release a circuit, due to memory broken or other causes.
0010011
BLO
Blocking: A message sent only for maintenance purposes to the switch at the other end of a circuit, to cause an engaged condition of that circuit for subsequent calls outgoing from that switch. When a circuit is used in the dual-circuit mode of operation, a switch receiving the blocking message must be capable of accepting incoming calls on the concerned circuit unless it has also sent a blocking message. Under certain conditions, a blocking message is also a proper response to a reset circuit message.
00010101
BLA
Blocking acknowledgement: A message sent in response to a blocking message, indicating that the circuit has been blocked.
00010111
GRS
Circuit group reset: A message sent to release an identified group of circuits.
00011000
CGB
Circuit group blocking message: A message sent to the switch at the other end, indicating the specified circuit group has been blocked.
00011001
CGU
Circuit group unblocking: A message sent to the switch at the other end of an identified group of circuits to cause cancellation in that group of circuits of an engaged condition activated earlier by a blocking or circuit group blocking message.
00011010
CGBA
Circuit group blocking acknowledgement: A message sent in response to a circuit group blocking message to indicate that the requested group of circuits has been blocked.
00011011
CGUA
Circuit group unblocking acknowledgement: A message sent in response to a circuit group unblocking message to indicate that the requested group of circuits has been unblocked.
00011111
FAR
Facility request: A message sent from a switch to another switch to request activation of a facility.
00100000
FAA
Facility accepted: A message sent in response to a facility request message, indicating that the requested facility has been activated.
00100001
FRJ
Facility rejected: A message sent in response to a facility request message to indicate that the facility request has been rejected.
00100100
LPA
Loop-back acknowledgement message: A message sent in the backward direction in response to a continuity check request message indicating that a loop (or transceiver in the case of a 2-wire circuit) has been connected.
00101000
PAM
Pass-along message
00101001
GRA
Circuit group reset acknowledgement: A message sent in response to a circuit group reset message and indicating that the requested group of circuits has been reset. The message also indicates the maintenance blocking state of each circuit.
00101010
CQM
Circuit group query message: A message sent on a routine or demand basis to request the far-end switch to give the states of all circuits in a particular range.
00101011
CQR
Circuit group query response: A message sent in response to a circuit group query message to indicate the states of all circuits in a particular range.
00101100
CPG
Call progress: A message sent in either direction during the set-up or active phase of the call, indicating that an event, which is of significance, and should be relayed to the originating or terminating access, has occurred.
00101111
CFN
Confusion message: A message sent in response to any message (other than a confusion message) if the switch does not recognize the message or detects a part of the message as being unrecognized.
00110000
OLM
Overload message: A message sent in the backward direction, on non-priority calls in response to an IAM, to invoke temporary trunk blocking of the circuit concerned when the switch generating the message is subject to load control.
00110001
CRG
Charging information: Information sent in either direction for accounting and/or call charging purposes.
00110010
NRM
Network resource management message: A message sent in order to modify network resources associated with a certain call. The message is sent along an established path in any phase of the call.
00110011
FAC
Facility: A message sent in either direction at any phase of the call to request an action at another switch. The message is also used to carry the results, error or rejection of a previously requested action.
00110110
IDR
Identification request
00110111
IDS
Identification response
00111000
SGM
Segmentation message: A message that may be sent in either direction to convey an additional message segement of an extremely long message.
00011101
Reserved.
00011100
00011110
00100111

Each kind of message is composed of a message type code and several parameters. Each parameter has a name that is coded by a single octet. The length of a parameter can be fixed or variable, and each parameter has an LI, the length of which is one octet.
II. Mandatory Fixed Part
For a specified message type, those mandatory parameters with fixed lengths are contained in the mandatory fixed part. The locations, lengths and order of them are defined by the message type. In this case, the message does not include the name and LI of its parameter.
III. Mandatory Variable Part
Those mandatory parameters with variable lengths are contained in the mandatory variable part. A pointer is used to indicate the start of each parameter, and it is coded based on a single octet. The parameter name and pointer transmitting order are implicit in the message type, and the numbers of parameters and pointers are defined by the message type.
A pointer also can be used to indicate the start of an optional part. If a message type specifies that the optional part is not allowed, this pointer does not exist. If a message type specifies that there may be optional part, but this specific message does not include optional part, the pointer field is all 0.
All pointers are transmitted consecutively at the start of mandatory variable part. Each parameter includes parameter LI and contents.
IV. Optional Part
Optional part is composed of parameters, which may appear or not appear in any specific message type. Parameters may be of fixed or variable lengths, and optional parameters can be transmitted in any order. Each optional parameter should contain a parameter name (one octet), a LI (one octet) and the parameter content.
"Optional parameter end" octet: If there are optional parameters, the "optional parameter end" octet will be transmitted after all optional parameters are sent out, and the octet is all 0.
6.3.3 Basic Signaling Flow
Figure 6-12 shows the call setup and release flow originated by an IP trunk media gateway. When an IP trunk media gateway originates a call, the media gateway is connected with subscribers through the circuit trunk of a circuit-switched network, and call signaling enters the SoftX3000 through an SS7 gateway.
This flow example is based on the following conventions:
l The caller is controlled by MG1 and SG1.
l The callee is controlled by MG2 and SG2.
l ISUP is taken as an example of SS7.
l MG1 and MG2 are controlled by the same SoftX3000.
Figure 6-12 ISUP call setup and release flow originated by a trunk media gateway
1) After the caller dials the number of the callee, an IAM is sent to the SoftX3000 through SG1.
2) A context is created in MG1. TDM termination and RTP termination are added in the context. [Mode] is set to “SendReceive”. The jitter buffer and voice compression algorithm are also set. MG1 returns its RTP port number and voice compression algorithm through the Reply command.
3) A context is created in MG2. TDM termination and RTP termination are added in the context. [Mode] is set to “SendReceive”. The jitter buffer and voice compression algorithm are also set. MG2 returns its RTP port number and voice compression algorithm through the Reply command.
4) The SoftX3000 sends an IAM to the circuit-switched network through SG2. The circuit-switched network returns an ACM. The phone set of the callee rings.
5) The SoftX3000 sends the Modify command to MG1 and reports the remote RTP port number.
6) The SoftX3000 sends an ACM to SG1.
7) The callee hooks off. SG2 sends an ANM to the SoftX3000.
8) The SoftX3000 sends an ANM to SG1.
9) The callee hooks off. SG2 sends an REL to the SoftX3000. The SoftX3000 sends an REL to SG1.
10) The SoftX3000 sends the Subtract command to MG1 and MG2.

MTP

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 H_1.

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.