Signaling Connection and Control Part
From Wikipedia, the free encyclopedia
| Layer | Protocols | |
|---|---|---|
| Application | INAP, MAP, IS-41... |
|
| Transport | SCCP | |
| Network | MTP Level 3 | |
| Data link | MTP Level 2 | ... |
| Physical | MTP Level 1 | ... |
Contents |
[edit] Introduction
Signaling Connection and 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.
[edit] Published specification
The base SCCP specification is defined by the ITU-T, in recommendation Q.713. There are however regional variations defined by local standards bodies. In the United States, ANSI publishes its modifications to Q.713, 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
[edit] Class 0: Basic connectionless
The SCCP Class 0 service is the most basic of SCCP transports. No provision is made for breaking blocks of data which are larger than the maximum size of an SCCP message into smaller pieces. Class 0 is typically used for non-critical messaging. As a connectionless protocol, no transport-level dialog is established between the sender and the receiver.
[edit] Class 1: Sequenced connectionless
SCCP Class 1 allows for sending blocks of data which are larger than the maximum size of a single SCCP message. It permits this by splitting the data into pieces small enough to fit into an SCCP message and sending each piece separately. Upon receipt of a sequence of class 1 messages, the receiver will re-assemble the original data block in the correct order. As with SCCP Class 0, the Class 1 service is also connectionless.
[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] Class 4: Error recovery and flow control connection oriented
Whilst SCCP Class 3 allows for error detection, Class 4 supports re-transmitting messages when errors 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 be operate over the SCCP User Adapation protocol (SUA) which is a form of modified SCCP designed specifically for use in IP networking.
