Signaling Connection and Control Part
Encyclopedia
The Signalling Connection Control Part (SCCP) is a network layer
protocol that provides extended routing
, flow control
, segmentation, connection-orientation
, and error correction facilities in Signaling System 7 telecommunications networks. SCCP relies on the services of MTP
for basic routing and error detection.
, in recommendations Q.711 to Q.714, with additional information to implementors provided by Q.715 and Q.716. There are, however, regional variations defined by local standards bodies. In the United States, ANSI
publishes its modifications to Q.713 as ANSI T1.112. The TTC
publishes as JT-Q.711 to JT-Q.714, and Europe ETSI publishes ETSI EN 300-009-1: both of which document their modifications to the ITU-T
specifications.
provides routing capabilities based upon the Point Code
, SCCP allows routing using a Point Code
and Subsystem number
or a Global Title
.
A Point Code is used to address a particular node on the network, whereas a Subsystem number
addresses a specific application available on that node. SCCP employs a process called Global Title Translation to determine Point Code
s from Global Title
s so as to instruct MTP
on where to route messages.
SCCP messages contain parameters which describe the type of addressing used, and how the message should be routed:
The connectionless protocol classes provide the capabilities needed to transfer one Network Service Data Unit (NSDU) in the "data" field of an XUDT, LUDT or UDT message. When one connectionless message is not sufficient to convey the user data contained in one NSDU, a segmenting/reassembly function for protocol classes 0 and 1 is provided. In this case, the SCCP at the originating node or in a relay node provides segmentation of the information into multiple segments prior to transfer in the "data" field of XUDT (or as a network option LUDT) messages. At the destination node, the NSDU is reassembled.
The connection-oriented protocol classes (protocol classes 2 and 3) provide the means to set up signalling connections in order to exchange a number of related NSDUs. The connection-oriented protocol classes also provide a segmenting and reassembling capability. If an NSDU is longer than 255 octets, it is split into multiple segments at the originating node, prior to transfer in the "data" field of DT messages. Each segment is less than or equal to 255 octets. At the destination node, the NSDU is reassembled.
. As a connectionless protocol, no network connection is established between the sender and the receiver.
suite of protocols, there are two primary methods of transporting SCCP applications across Internet Protocol
networks: SCCP can be transported indirectly using the MTP level 3 User Adaptation protocol (M3UA)
, a protocol which provides support for users of MTP-3
—including SCCP. Alternatively, SCCP applications can operate directly over the SCCP User Adaptation protocol (SUA) which is a form of modified SCCP designed specifically for use in IP networking.
ITU-T
also provides for the transport of SCCP users over Internet Protocol
using the Generic Signalling Transport service specified in Q.2150.0, the signalling transport converter for SCTP
specified in Q.2150.3 and a specialized Transport-Independent Signalling Connection Control Part (TI-SCCP) specified in T-REC-Q.2220.
TI-SCCP can also be used with the Generic Signalling Transport adapted for MTP3
and MTP3b as described in Q.2150.1, or adapted for SSCOP or SSCOPMCE as described in Q.2150.2.
Network layer
The network layer is layer 3 of the seven-layer OSI model of computer networking.The network layer is responsible for packet forwarding including routing through intermediate routers, whereas the data link layer is responsible for media access control, flow control and error checking.The network...
protocol that provides extended routing
Routing
Routing is the process of selecting paths in a network along which to send network traffic. Routing is performed for many kinds of networks, including the telephone network , electronic data networks , and transportation networks...
, flow control
Flow control
In data communications, flow control is the process of managing the pacing of data transmission between two nodes to prevent a fast sender from outrunning a slow receiver. It provides a mechanism for the receiver to control the transmission speed, so that the receiving node is not overwhelmed with...
, segmentation, connection-orientation
Connection-oriented
Connection-oriented communication is a data communication mode in telecommunications whereby the devices at the end points use a protocol to establish an end-to-end logical or physical connection before any data may be sent. In case of digital transmission, in-order delivery of a bit stream or...
, and error correction facilities in Signaling System 7 telecommunications networks. SCCP relies on the services of MTP
Message Transfer Part
The Message Transfer Part is part of the Signaling System 7 used for communication in Public Switched Telephone Networks. MTP is responsible for reliable, unduplicated and in-sequence transport of SS7 messages between communication partners....
for basic routing and error detection.
Published specification
The base SCCP specification is defined by the ITU-TITU-T
The ITU Telecommunication Standardization Sector is one of the three sectors of the International Telecommunication Union ; it coordinates standards for telecommunications....
, in recommendations Q.711 to Q.714, with additional information to implementors provided by Q.715 and Q.716. There are, however, regional variations defined by local standards bodies. In the United States, ANSI
Ansi
Ansi is a village in Kaarma Parish, Saare County, on the island of Saaremaa, Estonia....
publishes its modifications to Q.713 as ANSI T1.112. The TTC
Telecommunication Technology Committee
The Telecommunication Technology Committee was established in 1985. This standardization organization is authorized by Japan's Ministry of Internal Affairs and Communications to conduct research and to develop and promote standards for telecommunications in Japan.The TTC is a founding partner...
publishes as JT-Q.711 to JT-Q.714, and Europe ETSI publishes ETSI EN 300-009-1: both of which document their modifications to the ITU-T
ITU-T
The ITU Telecommunication Standardization Sector is one of the three sectors of the International Telecommunication Union ; it coordinates standards for telecommunications....
specifications.
Routing facilities beyond MTP
Although MTPMessage Transfer Part
The Message Transfer Part is part of the Signaling System 7 used for communication in Public Switched Telephone Networks. MTP is responsible for reliable, unduplicated and in-sequence transport of SS7 messages between communication partners....
provides routing capabilities based upon the Point Code
Point Code
An SS7 point code is similar to an IP address in an IP network. It is a unique address for a node , used in MTP layer 3 to identify the destination of a message signal unit ....
, SCCP allows routing using a Point Code
Point Code
An SS7 point code is similar to an IP address in an IP network. It is a unique address for a node , used in MTP layer 3 to identify the destination of a message signal unit ....
and Subsystem number
Subsystem number
Signalling Connection Control Part subsystem numbers are used to identify applications within network entities which use SCCP signalling.- GSM and UMTS SSNs :...
or a Global Title
Global Title
A Global Title is an address used in the SCCP protocol for routing signaling messages on telecommunications networks. In theory, a global title is a unique address which refers to only one destination, though in practice destinations can change over time....
.
A Point Code is used to address a particular node on the network, whereas a Subsystem number
Subsystem number
Signalling Connection Control Part subsystem numbers are used to identify applications within network entities which use SCCP signalling.- GSM and UMTS SSNs :...
addresses a specific application available on that node. SCCP employs a process called Global Title Translation to determine Point Code
Point Code
An SS7 point code is similar to an IP address in an IP network. It is a unique address for a node , used in MTP layer 3 to identify the destination of a message signal unit ....
s from Global Title
Global Title
A Global Title is an address used in the SCCP protocol for routing signaling messages on telecommunications networks. In theory, a global title is a unique address which refers to only one destination, though in practice destinations can change over time....
s so as to instruct MTP
Message Transfer Part
The Message Transfer Part is part of the Signaling System 7 used for communication in Public Switched Telephone Networks. MTP is responsible for reliable, unduplicated and in-sequence transport of SS7 messages between communication partners....
on where to route messages.
SCCP messages contain parameters which describe the type of addressing used, and how the message should be routed:
- Address Indicator
- Subsystem indicator: The address includes a Subsystem Number
- Point Code indicator: The address includes a Point Code
- Global title indicator
- No Global Title
- Global Title includes Translation Type (TT), Numbering Plan Indicator (NPI) and Type of Number (TON)
- Global Title includes Translation Type only
- Routing indicator
- Route using Global Title only
- Route using Point Code/Subsystem number
- Address Indicator Coding
- Address Indicator coded as national (the Address Indicator is treated as international if not specified)
Protocol classes
SCCP provides 5 classes of protocol to its applications:- Class 0: Basic connectionless.
- Class 1: Sequenced connectionless.
- Class 2: Basic connection-oriented.
- Class 3: Flow control connection oriented.
- Class 4: Error recovery and flow control connection oriented.
The connectionless protocol classes provide the capabilities needed to transfer one Network Service Data Unit (NSDU) in the "data" field of an XUDT, LUDT or UDT message. When one connectionless message is not sufficient to convey the user data contained in one NSDU, a segmenting/reassembly function for protocol classes 0 and 1 is provided. In this case, the SCCP at the originating node or in a relay node provides segmentation of the information into multiple segments prior to transfer in the "data" field of XUDT (or as a network option LUDT) messages. At the destination node, the NSDU is reassembled.
The connection-oriented protocol classes (protocol classes 2 and 3) provide the means to set up signalling connections in order to exchange a number of related NSDUs. The connection-oriented protocol classes also provide a segmenting and reassembling capability. If an NSDU is longer than 255 octets, it is split into multiple segments at the originating node, prior to transfer in the "data" field of DT messages. Each segment is less than or equal to 255 octets. At the destination node, the NSDU is reassembled.
Class 0: Basic connectionless
The SCCP Class 0 protocol class is the most basic of the SCCP protocol classes. Network Service Data Units passed by higher layers to the SCCP in the originating node are delivered by the SCCP to higher layers in the destination node. They are transferred independently of each other. Therefore, they may be delivered to the SCCP user out-of-sequence. Thus, this protocol class corresponds to a pure connectionless network serviceConnectionless protocol
In telecommunications, connectionless describes communication between two network end points in which a message can be sent from one end point to another without prior arrangement. The device at one end of the communication transmits data addressed to the other, without first ensuring that the...
. As a connectionless protocol, no network connection is established between the sender and the receiver.
Class 1: Sequenced connectionless
SCCP Class 1 builds on the capabilities of Class 0, with the addition of a sequence control parameter in the NSDU which allows the SCCP User to instruct the SCCP that a given stream of messages should be delivered in sequence. Therefore, Protocol Class 1 corresponds to an enhanced connectionless protocol with assurances of in-sequence delivery.Class 2: Basic connection-oriented
SCCP Class 2 provides the facilities of Class 1, but also allows for an entity to establish a two-way dialog with another entity using SCCP.Class 3: Flow control connection oriented
Class 3 service builds upon Class 2, but also allows for expedited (urgent) messages to be sent and received, and for errors in sequencing (segment re-assembly) to be detected and for SCCP to restart a connection should this occur.Transport over IP Networks
In the SIGTRANSIGTRAN
SIGTRAN is the name, derived from signaling transport, of the former Internet Engineering Task Force working group that produced specifications for a family of protocols that provide reliable datagram service and user layer adaptations for Signaling System 7 and ISDN communications protocols....
suite of protocols, there are two primary methods of transporting SCCP applications across Internet Protocol
Internet Protocol
The Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...
networks: SCCP can be transported indirectly using the MTP level 3 User Adaptation protocol (M3UA)
M3UA
M3UA stands for MTP Level 3 User Adaptation Layer as defined by the IETF SIGTRAN working group in RFC 4666 . M3UA enables the SS7 protocol's User Parts to run over IP instead of telephony equipment like ISDN and PSTN...
, a protocol which provides support for users of MTP-3
Message Transfer Part
The Message Transfer Part is part of the Signaling System 7 used for communication in Public Switched Telephone Networks. MTP is responsible for reliable, unduplicated and in-sequence transport of SS7 messages between communication partners....
—including SCCP. Alternatively, SCCP applications can operate directly over the SCCP User Adaptation protocol (SUA) which is a form of modified SCCP designed specifically for use in IP networking.
ITU-T
ITU-T
The ITU Telecommunication Standardization Sector is one of the three sectors of the International Telecommunication Union ; it coordinates standards for telecommunications....
also provides for the transport of SCCP users over Internet Protocol
Internet Protocol
The Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...
using the Generic Signalling Transport service specified in Q.2150.0, the signalling transport converter for SCTP
Stream Control Transmission Protocol
In computer networking, the Stream Control Transmission Protocol is a Transport Layer protocol, serving in a similar role to the popular protocols Transmission Control Protocol and User Datagram Protocol...
specified in Q.2150.3 and a specialized Transport-Independent Signalling Connection Control Part (TI-SCCP) specified in T-REC-Q.2220.
TI-SCCP can also be used with the Generic Signalling Transport adapted for MTP3
Message Transfer Part
The Message Transfer Part is part of the Signaling System 7 used for communication in Public Switched Telephone Networks. MTP is responsible for reliable, unduplicated and in-sequence transport of SS7 messages between communication partners....
and MTP3b as described in Q.2150.1, or adapted for SSCOP or SSCOPMCE as described in Q.2150.2.