1 No.7 Signaling System-SCCP
In this course, there are four parts.
In the second part, the functions and features of SCCP are introduced in detail;
The third part is mainly devoted to the addressing and routing function of SCCP;
At last, the SCCP message format is introduced.
After this course, you could:
Understand the purpose, functions, and features of SCCP.
Master the addressing and routing function of SCCP, and SCCP messages format.
1.1 General
Ok, Let’s begin with the general. In this part, the Limitation of MTP Transmission is explained, that is why the SCCP is introduced by CCITT; the location of SCCP in the OSI model is introduced.
In No.7 signaling system, MTP provides the addressing and routing function in the
signaling network, we know, MTP decides the destination according to the DPC and NI. DPC is available only in a certain network. There are four kinds of signaling network defined in specifications: international, international standby, national and national standby. In different network, encoding scheme of SPC is maybe different. For example, in China, SPC of national network is 24 bits, and in Thailand, SPC of national network is 14 bits. In this way, if only MTP is used in addressing and routing, there will be some problem. For example, We know, GSM system can realize international roaming.
If the MS in Thailand comes to China, the MS first sends the location update request to the network in China, the VLR will send the request to its HLR, but China can’t identify the DPC of HLR in Thailand, because of different network. So the location update can’t be performed, and say nothing of international roaming.
And this is the first limitation of MTP transmission.
Even if we adopt the uniform codes in the international network, the capacity of the SPC is very small, because the length of the SPC in international network is 14 bits. Then the total number of the SPC is 16384.
When MTP makes sure the message’s destination according to the DPC and NI, in the local node, the MTP then sends the signaling information to the different processing model according to the SI. The length of SI is 4 bits, that is, the most user number is 16. This is too few. It can’t satisfy the development of telecommunication in the future.
That is the another limitation of MTP in addressing and routing.
On an other hand, MTP can only provide the connectionless transmission. The routing information to the destination must be included in each message. At each relay node , MTP is required to analyze the routing information and decides where sends the message to. MTP can’t provide the connection-oriented service, that is , before the transmission of the information, the logic connection must be established.
Because of the reasons mentioned above, CCITT introduced the new layer- SCCP in 1984.
SCCP provides additional functions to MTP, and supports many new services. SCCP supplements the message carrying capabilities by the MTP. The combination of MTP-SCCP is termed “Network Service Part”(NSP). The NSP provides the full OSI layer 3 service to the users of No.7 signaling system.
1.2 Features and Functions of SCCP
Ok, now, let’s see the second part. We will study the features and functions of SCCP.
First, let’s see application features of SCCP.
The first one, enable to transfer both circuit related and non-circuit-related signaling and user information in the telecommunication networks.
In some cases it is very desirable that signaling messages can be transferred from one point to another point in the network without relation to a circuit. The MTP was originally designed for circuit-related signaling and does not meet this requirement.
For example, in GSM system, roaming between MSCs is one non-circuit related application.
SCCP makes it possible to transfer both circuit related and non-circuit related signaling and user information between exchanges and specialized centers in telecommunication networks.
Second, SCCP can provide enhanced addressing and routing function, and enable to achieve the direct global transmission between different No.7 signaling networks.
Actually, SCCP provides two kinds of new address information: GT and SSN. GT is like the dialed number, which can be numbered uniformly in different network within the world.
Last, SCCP can provide connectionless service and connection-oriented service.
After the application features, we will study the service function of SCCP. There are two kinds of service provided by the SCCP protocol. The two types of service are Connectionless and Connection-Oriented services.
There are four classes for the two kinds of services. They are: 0,1 ,2, 3
0: Basic connectionless class.
1: In-sequence delivery connectionless class.
2: Basic connection-oriented class.
3: Flow control connection-oriented class.
Class 0 and 1 is for connectionless service, class 2 and 3 is for connection-oriented service.
Ok, at first let’s see the connectionless service.
In the Connectionless service, the routing information to its destination must be included in each data packet. No logical connection is established between the end nodes.
The connectionless service is typically used to transfer small amounts of real-time critical information between remote users.
In GSM system, connectionless service is widely used in NSS. For example, when MSC requests from a database information about a MS’s location within the mobile network.
The connectionless SCCP offers two kinds of services: class 0 and class 1.
The class 0 service allows the SCCP to insert SLS values randomly, or with the aim to achieve an appropriate load sharing within the underlying MTP network;
In class 0, the receiving sequence may be different from the transmission;
The class 1 service requires the SCCP to insert the same SLS for all the SCCP-SDUs (service data unit) associated with given parameters "sequence control" and "called address" by SCCP users.
So, in class 1 service, the receiving sequence of message is same to the transmission.
There are four kinds of messages for connectionless service: UDT, UDTS, XUDT and XUDTS. Each SCCP message has one message type code whose length is one octet. XUDT can provide the segmenting function for the long message, but UDT can’t.
Then let’s see the SCCP transmission procedure in connectionless service.
In connectionless service, no logical connection is established between the end nodes.
All routing information to the destination is contained in each UDT message. The relay SCCP then analyzes the routing information in UDT after receiving and then sends it to the right destination.
When UDT message can’t be sent to the destination, the relay SCCP will then return the UDTS message to the originating node. In the UDTS message, the cause value is included, which indicates why the UDT can’t be sent to the destination SCCP. Optionally, the UDT which can’t get to the destination may be included in the UDTS.
Here, you can find, only when UDT message can’t get to the destination, UDTS is used.
The connection-oriented services are used when there are many messages to be
transferred, or when the signaling messages are so long that they have to be divided into smaller parts, so called segmenting. At the receiving side these parts then have to be reassembled again.
Connection-oriented service means the ability to transfer signaling messages via an established signaling connection.
Between the two nodes, at the same time, there are many logical connections. How to identify them?
In connection-oriented service, the logical signaling connection is achieved by giving a local reference number to the signaling messages belonging together.
The connection-oriented transfer mode can be divided into three phases:
1. Establishing the connection
2. Data transfer
3. Release of connection
In the first stage, connection request (CR) is sent to the relay SCCP, the relay SCCP will then send CR to the destination. How does the relay SCCP know the destination of the logical connection? Because the destination address is included in the CR message.
Then connection confirmation is sent to the originating SCCP.
The source local reference number (SLR) is allocated to this logical connection by the originating SCCP. The destination local reference number (DLR) is allocated by the receiving side.
In the second stage, data is transferred bi-directionally. The data message format for class 2 is DT1, and DT2 for class 3. The message “acknowledge” (AK) is only used in the class 3.
After the data transfer, the logical connection should be released.
In GSM system, class 2 service is widely used in A interface. And there is no relay SCCP because of direct connection between MSC and BSC.
1.3 Addressing and Routing Functions of SCCP
Ok, in the third part, the addressing and routing function of SCCP is introduced.
First, the address information in SCCP messages.
DPC is the destination point code, also used in MTP.
GT, Global title is an address, such as dialed digits, that needs to be translated by the SCCP before it can be used for routing in the signaling network.
GT is identified only by SCCP layer, and can’t be identified by MTP.
GT enables to transfer message directly in different signaling network. GT is numbered uniformly in the whole world.
SSN, Subsystem Number is an address identifying a part of an SCCP node which uses the SCCP either directly (like ISUP or BSSAP) or indirectly via the transaction Capabilities (like MAP).
The example of such Subsystem are: SCCP Management, ISUP, MAP, HLR, MSC….. and so on. Each Subsystem has one SSN.
SSN extends the local addressing range.
Ok, let’s see the routing mode of SCCP.
There are two basic categories of addresses for SCCP routing.
The first one is GT.
Messages contain a Called Party Address that specifies “route on Global Title”. It can be translated to different SPC and/or subsystem number depending on the current network status.
The other routing mode is DPC+SSN.
Messages contain a Called Party Address that specifies “route on DPC+SSN”. It needn’t be translated.
Then you will be given an example about SCCP routing mode.
In this network, SPA is local office. To the two STP, there are directly connected signaling links. And two STP connect to the SPD respectively through direct links.
From SPA to SPD, if DPC+SSN routing is adopted by SCCP, in the Called Party Address of SCCP message, DPC is point code of SPD, SSN depending on the different requirements. The message is sent to STPB or STPC, the Called Party Address is not necessary to be translated by STP.
From SPA to SPD, if GT routing is adopted by SCCP, in the Called Party Address of SCCP message, GT is information of SPD. Because GT can’t be identified by MTP, actually, DPC should be included in the SCCP message, in this case, DPC is point code of STP. The message is sent to STPB or STPC, the Called Party Address is necessary to be translated to the relevant point code and/or SSN by STP.
1.4 Brief Introduction to SCCP Message Format
Ok, let’s go ahead.
In the fourth part, brief introduction to the SCCP message format will be given.
SCCP message is included in SIF field of MSU. When SI is equal to 0011, it indicates in the SIF, SCCP message is contained.
There are three parts in SCCP message.
The first part is routing label. It comprises DPC, OPC and SLS. Generally, the length of point code is 14 bits, and that of SLS is 4 bits. So the length of the routing label is 4 octets. In China, the national network adopts 24 bits signaling point code. So the length of routing label is 7 octets in which 4 bits is filled.
After the routing label, the second part is message type. Each SCCP message has one message type code whose length is 1 octet. For example, for UDT message, the code is 09H. The message code decides the parameters in the following.
The last part is the parameters. It is decided by the message type.
There are three categories of the parameters: Mandatory fixed length parameter（F）; Mandatory variable length parameter（V）and Optional parameter of fixed or variable length (O).
The length is regulated in the specifications: it is fixed or variable, if it’s fixed, how long is it? If it’s variable, what is the range?
The message type decides which parameters are mandatory and which are optional.
How to describe one parameter? Generally , one parameter consists of three parts: parameter name code, the length indicator of the parameter and the contents of the parameter.
Each parameter has one parameter name code whose length is 1 octet. For example, for the Destination Local Reference, the code is 01H; for the Called Party Address, the code is 03H. In the training materials, table 4-4 shows the SCCP parameter name codes.
For the different parameter, the length is different. In the SCCP message, there is length indicator.
The last part of the parameter is the contents.
Ok, let’s see the parameters.
First , mandatory fixed length part: Those parameters that are mandatory and of fixed length for a particular message type will be contained in the "mandatory fixed part". The position, length and order of the parameters are uniquely defined by the message type in the specifications. Thus the names code of the parameters and the length indicators are not included in the message. So in the message, only the parameter contents are included in this part according the regulated sequence.
Second, mandatory variable part:Mandatory parameters of variable length will be included in the mandatory variable part. The name of each parameter and the order in which the pointers are sent is implicit in the message type. Parameter names are, therefore, not included in the message. A pointer is used to indicate the beginning of each parameter.
A pointer is also included to indicate the beginning of the optional part. If the message type indicates that no optional part is allowed, then this pointer will not be present. If the message type indicates that an optional part is possible, but there is no optional part included in this particular message, then a pointer field containing all zeros will be used.
All the pointers are sent consecutively at the beginning of the mandatory variable part. Because the length is variable, the length indicator should be included. Each parameter contains the parameter length indicator followed by the contents of the parameter.
Last, optional part: The optional part consists of a contiguous block of parameters that may or may not occur in any particular message type. The optional part may start after the pointer or after the mandatory variable part. Both fixed length and variable length parameters may be included. Optional parameters may be transmitted in any order. Each optional parameter will include the parameter name (one octet) and the length indicator (one octet) followed by the parameter contents.
Anyway, in the parameters, for the mandatory fixed part, only contents included in the regulated sequence in the specifications; for the variable length, there are the pointer which indicates the beginning of the parameter, and the length indicator (one octet) followed by the parameter contents; for the optional part: A pointer is also included to indicate the beginning of the optional part after the pointers of the mandatory variable part. Besides, the parameter name code, the length indicator and the contents.
Ok, then take the UDT as example and introduce the message format and several
common-used parameters. UDT is one kind of connectionless service message, which is widely used in NSS of GSM system.
As the SCCP message, the UDT message also includes three parts.
The first part: routing label, which includes DPC, OPC, SLS.
The second part: message type code: 09H, which decides the following parameters.
The third part: parameters.
For the UDT message, only one mandatory fixed length parameter is included: Protocol Class. I will then introduce this parameter.
The "protocol class" parameter field is a one-octet parameter and is structured as follows:
Bits 1-4 indicating protocol class is coded as shown in the slide: code 0000, 0001, 0010, 0011 indicate the class 0, 1, 2 and 3 respectively.
When bits 1-4 are coded to indicate a connection-oriented-protocol class (class 2, class 3), bits 5-8 are spare. When bits 1-4 are coded to indicate a connectionless protocol class (class 0, class 1), bits 5-8 are used to specify message handling as follows: Code 1000 means “return the UDT on error”. Code 0000 means “no return on error”. In other words, whether UDT is included in the UDTS when the UDT can’t be sent to the destination. Ok, this is the parameter: protocol class. Then let’s continue the UDT’s parameters.
For the UDT message, there are three mandatory variable length parameters: Called party address, Calling party Address and Data. No optional parameter in UDT message.
So, after the mandatory fixed length parameter, there are three ordinal pointers to the Called party address, Calling party Address and Data. After the three pointers, orderly, Called Party Address (CDA), Calling Party Address (CGA), and Data. Before contents of each parameter, there is the length indicator for each parameter.
Then the Called party address and Calling Party address are introduced. The format of the two parameters is same, namely address. The "called/calling party address" is a variable length parameter.
The address consists of the address indicator and address information.
Now, the address indicator.
We know, there are three types of address information in SCCP address. The various elements, when provided, occur in the order: point code, subsystem number, and global title.
The "address indicator" indicates the type of address information contained in the address field. The address consists of one or any combination of the following elements: signalling point code; global title (for instance, dialled digits); subsystem number.
Bit 1 is point code indicator, "1" indicates that the address contains a signalling point code and "0" not.
Bit 2 is Sub-system indicator, "1" indicates that the address contains a sub-system number and "0" not.
The Subsystem Number (SSN) identifies an SCCP user function and, when provided, consists of one octet coded in this slide.
Then we continue the Address indicator.
GT indicator indicates by four bits (bit 3~6). The format of the Global Title (GT) is of variable length. GT provides four possible formats.
GT provides four possible formats.
It means that global title is not included when GT indicator is equal to “0000”.
The other four codes are valid: 0001, 0010, 0011 and 0100 which show the GT type 1, 2, 3 and 4 respectively.
For different GT type, there are different contents in GT code.
In GSM system, type 4 GT is used. This global title format (0100) is used for international network applications, specially, for the NSS of GSM system. But in the A-interface, the SCCP message doesn't contain the global title.
Then I mainly introduce the format of type 4 GT.
The first octet is translation type, it is set to “00” in GSM system. 1-254 used for the SC(Short Message center) interface.
The second octet is divided into two sub-fields. The high 4 bits express numbering plan, the common-used codes are 0001 and 0111, indicate E.164 and E.214 format. The low 4 bits express the encoding scheme. “0001” means odd BCD code in following GT information; “0010” means even BCD code in the following GT.
The third octet is Nature of Address Indicator (NAI). The highest bit is spare. “03H” means national number, “04H” means international number.
In the following octets, GT information is included.
Ok, let’s return to the address indicator.
Bit 7 is routing indicator. “0” means GT routing, and “1” used for DPC+SSN.
After the address indicator, address information is followed.
That’s all for the address format in SCCP.
For UDT message, the last parameter is DATA. The content of the data is TCAP message in NSS, and BSSAP message in A interface.
Ok, so much for the UDT message format.
Ok, the explanation about this SCCP message.
In the traced message, the structure is as following: the whole message belongs to MTP. SCCP messages is contained in the MSU. Here the octets before SIO are ignored by us, let’s begin with SIO. SCCP portion is after 83 and to the last. The meaning of each octet is as following：83----SIO, national network, SCCP message. You can see H and G is network indicator, “10” means national network, bit D, C, B and A is service indicator, “0011” means SCCP message.
SIO: Bit H G F E D C B A
1 0 0 0 0 0 1 1
After the SIO, according to the format of MSU, SIF is followed, that is, SCCP message.
Then for SCCP message, the first part is routing label. This message is traced in China Mobile network. In China, 24 bits point code is used in national network, so, 7 octets for this field. “11 FF 03” is DPC field, and DPC is 03 FF 11, “09 FF 05” is OPC field, OPC is 05 FF 09, “0D” is SLS Value.
Actually, SLS is 4 bits code, “0” is filled for the length of MSU.
The second part of SCCP message is message type. 09H indicates the message type is UDT。
Then the parameters are followed.
The first one is mandatory fixed length parameter: Protocol Class. 81H is the content of protocol class, it means return message on error, and class 1.
After the mandatory fixed length parameter, the mandatory variable length parameters followed. According to the format of SCCP message, there are three pointers to the called party address, calling party address and data. So “03” is pointer to the called party address;“03” means that the called party address begin with the third octet after "03".
“0E” is pointer to the calling party address;“0E”means that the calling party address begin with the fourteenth octet after "0E". “18” is the pointer to the data;“18”means that the data begin with the twenty-fourth octet after "18".
Then “0B” is called address length. The length is 11 octets. The followed 11 octets is the contents of the called party address, which consists of address indicator and address information. “12” is address indicator，in binary, it is “00010010”:
Bit 8 is spare.
Bit 7 is routing indicator. “0” means routing on GT.
Bit 6/5/4/3 is Global Title indicator.“0100” means type 4 GT is included.
Bit 2 is SSN indicator. “1” indicates the address contains a subsystem number.
Bit 1 is Signaling Point indicator. “0” means no SPC included in the address
After the address indicator, the address information is followed. From the address indicator, we can see, SSN and GT included in the address. “06” is SSN which indicates the Subsystem is HLR. Then type 4 GT is followed: 00 is translation type, then “12”: bit 8/7/6/5 is numbering plan，E.164 is used, bit 4/3/2/1 is encoding scheme, even BCD code in GT information. “04” is Nature of address indicator. It means international number. Then GT information is followed. “68 31 39 31 00 00” is GT information, so “86139313000” is the called party address: HLR No. That is the called party address.
Then the following “0A 12 07 00 12 04 68 31 09 40 67” is calling party address, the analysis way is same as called party address. The calling party address is a VLR, the VLR No. is 8613900476.
Then the last parameter for the UDT message. “28” is the length of data, that is the length of TCAP message. For this part, we don’t analyze it.
Ok, after this message example, you should understand the SCCP message format.
Ok, that’s all for the SCCP. At last, let’s make a review.
In this course, we studied the functions of the SCCP, SCCP provides the new addressing and routing function, and supplements the message carrying capabilities of the MTP. In the first part, the purpose of the SCCP was introduced. Then you were given introduction to the features and functions of SCCP. In the third part, the addressing and routing function was introduced in detail. At last, SCCP message format was introduced.
And now, hope you have:
Understood the purpose, functions, and features of SCCP.
Mastered the addressing and routing function of SCCP, and SCCP messages format.
Thank you
