Signalling Connection Control Part
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{{SS7stack The Signalling Connection Control Part (SCCP) is a
network layer In the seven-layer OSI model of computer networking, the network layer is layer 3. The network layer is responsible for packet forwarding including routing through intermediate routers. Functions The network layer provides the means of transfe ...
ITU-T Recommendation Q.1400
protocol that provides extended
routing Routing is the process of selecting a path for traffic in a network or between or across multiple networks. Broadly, routing is performed in many types of networks, including circuit-switched networks, such as the public switched telephone netw ...
, flow control, segmentation, connection-orientation, and
error correction In information theory and coding theory with applications in computer science and telecommunication, error detection and correction (EDAC) or error control are techniques that enable reliable delivery of digital data over unreliable communica ...
facilities in
Signaling System 7 Signalling System No. 7 (SS7) is a set of telephony signaling protocols developed in 1975, which is used to set up and tear down telephone calls in most parts of the world-wide public switched telephone network (PSTN). The protocol also perform ...
telecommunications networks. SCCP relies on the services of MTP for basic routing and error detection.


Published specification

The base SCCP specification is defined by the
ITU-T The ITU Telecommunication Standardization Sector (ITU-T) is one of the three sectors (divisions or units) of the International Telecommunication Union (ITU). It is responsible for coordinating standards for telecommunications and Information Commu ...
, in recommendation
Q.711
t
Q.714
with additional information to implementors provided b
Q.715
an
Q.716
ITU-T Q-Series Recommendations
There are, however, regional variations defined by local standards bodies. In the United States,
ANSI The American National Standards Institute (ANSI ) is a private non-profit organization that oversees the development of voluntary consensus standards for products, services, processes, systems, and personnel in the United States. The organi ...
publishes its modifications t
Q.713
as ANSI T1.112. The TTC publishes as JT-Q.711 to JT-Q.714, and Europe
ETSI The European Telecommunications Standards Institute (ETSI) is an independent, not-for-profit, standardization organization in the field of information and communications. ETSI supports the development and testing of global technical standard ...
publishes
ETSI EN 300-009-1
'' both of which document their modifications to the
ITU-T The ITU Telecommunication Standardization Sector (ITU-T) is one of the three sectors (divisions or units) of the International Telecommunication Union (ITU). It is responsible for coordinating standards for telecommunications and Information Commu ...
specifications.


Routing facilities beyond MTP

Although MTP 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 Codes from Global Titles 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: * Address Indicator ** Routing indicator *** ''Route on Global Title'' *** ''Route on Point Code/Subsystem Number'' ** 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'' ** Subsystem indicator *** ''Subsystem Number present'' *** ''Subsystem Number not present'' ** Point Code indicator *** ''Point Code present'' *** ''Point Code not present'' * Global Title ** Address Indicator Coding ** ''Address Indicator coded as national'' (the Address Indicator is treated as international if not specified)


Protocol classes

SCCP provides 4 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. 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.ITU-T Recommendation Q.714
- Signalling connection control part procedures


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 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 service Connectionless-mode Network Service (CLNS) or simply Connectionless Network Service is an OSI network layer datagram service that does not require a circuit to be established before data is transmitted, and routes messages to their destinatio ...
. 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
SIGTRAN SIGTRAN is the name, derived from ''signaling transport'', of the former Internet Task Force (I) working group that produced specifications for a family of protocols that provide reliable datagram service and user layer adaptations for Signaling ...
suite of protocols, there are two primary methods of transporting SCCP applications across
Internet Protocol The Internet Protocol (IP) is the network layer communications protocol in the Internet protocol suite for relaying datagrams across network boundaries. Its routing function enables internetworking, and essentially establishes the Internet. IP h ...
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 The ITU Telecommunication Standardization Sector (ITU-T) is one of the three sectors (divisions or units) of the International Telecommunication Union (ITU). It is responsible for coordinating standards for telecommunications and Information Commu ...
also provides for the transport of SCCP users over
Internet Protocol The Internet Protocol (IP) is the network layer communications protocol in the Internet protocol suite for relaying datagrams across network boundaries. Its routing function enables internetworking, and essentially establishes the Internet. IP h ...
using the ''Generic Signalling Transport'' service specified i
Q.2150.0
the ''signalling transport converter'' for
SCTP The Stream Control Transmission Protocol (SCTP) is a computer networking communications protocol in the transport layer of the Internet protocol suite. Originally intended for Signaling System 7 (SS7) message transport in telecommunication, the p ...
specified i
Q.2150.3
and a specialized ''Transport-Independent Signalling Connection Control Part'' (TI-SCCP) specified i
T-REC-Q.2220
TI-SCCP can also be used with the ''Generic Signalling Transport'' adapted for MTP3 and MTP3b as described i
Q.2150.1
or adapted for SSCOP or SSCOPMCE as described i
Q.2150.2


References

ITU-T recommendations Signaling System 7 Transport layer protocols