Signalling Connection Control Part (SCCP)

Signalling Connection Control Part

SS7 protocol suite
Layer
Protocols
Application
INAP, MAP, IS-41...
TCAP, CAP, ISUP, ...
Transport
SCCP
Network
MTP Level 3
Data link
MTP Level 2
...
Physical
MTP Level 1
...
The Signalling Connection Control Part (SCCP) is a transport layer protocol which 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.
Contents[hide]
1 Published specification
2 Routing facilities beyond MTP-3
3 Classes of service
3.1 Class 0: Basic connectionless
3.2 Class 1: Sequenced connectionless
3.3 Class 2: Basic connection-oriented
3.4 Class 3: Flow control connection oriented
4 Transport over IP Networks
5 References
6 External links
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[edit] Published specification
The base SCCP specification is defined by the ITU-T, 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 or JT-Q.711 to JT-Q.714, whilst in Europe ETSI publishes ETSI EN 300 009, which documents its modifications to the ITU-T specification.

[edit] Routing facilities beyond MTP-3
Although MTP-3 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, whilst a Subsystem number addresses a specific application available on that node. SCCP employs a process called Global Title Translation (which is similar to DNS resolution in IP networks) in order to determine Point Codes from Global Titles so as to instruct MTP-3 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 Indiciator (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)

[edit] Classes of service
SCCP provides 5 classes of service 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.[1]

[edit] Class 0: Basic connectionless
The SCCP Class 0 service is the most basic of SCCP transports. 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 service. As a connectionless protocol, no transport-level dialog is established between the sender and the receiver.

[edit] 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 service with in-sequence delivery.

[edit] 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.

[edit] 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.

[edit] Transport over IP Networks
In the SIGTRAN suite of protocols, there are two primary methods of transporting SCCP applications across Internet Protocol networks: SCCP can be transported directly 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 over the SCCP User Adaptation protocol (SUA) which is a form of modified SCCP designed specifically for use in IP networking.