Signalling Connection Control Part Overview

The Signalling Connection Control Part (SCCP) is a crucial component in telecommunications networks.

It provides a way for different networks to communicate with each other, allowing for the establishment of connections between nodes.

The SCCP is responsible for managing the connections between networks, ensuring that data is delivered efficiently and effectively.

In essence, the SCCP acts as a translator, facilitating communication between different networks and protocols.

It does this by breaking down complex data into manageable packets, which can then be transmitted over the network.

Signaling Connection Control Part (SCCP) is a network layer protocol that provides features of extended routing. It's used in Signaling System 7 (SS7) telecommunications networks to relay messages.

SCCP provides several key functions, including flow control, segmentation, connection orientation, and error detection and correction facilities. These features help ensure that data is transmitted accurately and efficiently.

One of the unique aspects of SCCP is that it relies on the services of MTP (Message Transfer Part) for basic routing and error detection. This allows SCCP to focus on more advanced routing and error correction.

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SCCP is often used as an interface between the Cisco call manager and Cisco VOIP phones. It's also used in Tcap-based applications, where it's placed over the MTP3 layer.

Here are some key facts about SCCP:

SCCP uses a unique identifier called Point Code to direct messages to a specific node within an SS7 network.

The Point Code is used in conjunction with a Subsystem Number to enable targeted communication with a specific application or service within a node.

SCCP employs Global Title Translation (GTT) to map logical addresses (Global Titles) to physical addresses (Point Codes), guiding MTP on how to route messages through the network.

The structured message format defined by SCCP includes a version number, reserved field, message class, message type, message length, and message data.

A key element of the message structure is the Message Type, which can be UDT (Unitdata) for connectionless services.

The Message Type is used to determine the type of message being sent, with UDT being one of the possible types.

SCCP uses a structured message format to ensure communication across the network is efficient and effective.

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The SCCP manages an additional global title address translation function, converting an address into a DPC and sub-system number (SSN) for voice, data, ISDN, or mobile networks.

This process is essential for identifying SCCP users at a signaling point, which are examples of SCCP users like SCCP management, OMAP, and ISDN-UP.

SCCP uses the following addresses to enable an S7 to route MAP messages: Calling address and Called address.

SCCP addressing is very flexible and makes use of three separate elements: Destination Point Code (DPC), Global Title (GT), and SubSystem Numbers (SSN).

The Address Indicator shows which information elements are present in the address information for the called and the calling party.

The GT is of variable length and can contain specified combinations of Address Information (AI), Nature of Address (NA), Numbering Plan (NP), and Translation Type (TT).

Here's a breakdown of the GT elements:

The GT does not contain information that allows routing in the signaling network, making the translation function required for proper routing.

The Signalling Connection Control Part (SCCP) plays a crucial role in the SS7 protocol stack, sitting above the Message Transfer Part (MTP) and below the Transaction Capabilities Application Part (TCAP). This positioning allows SCCP to leverage MTP's reliable transport capabilities while providing enhanced routing and addressing services to TCAP and other higher-layer protocols.

SCCP is situated between MTP3, the Signalling Network Layer, and TCAP, the Transaction Capabilities Application Part. This is a key point to understand, as it highlights the importance of SCCP in facilitating communication between different layers of the protocol stack.

The SCCP architecture is divided into two main components: ASE (Application Service Elements) and OMAP (Operations, Maintenance, Administration, and Provisioning). ASE includes application-specific protocols and services, while OMAP handles network management functions.

Here's a breakdown of the SCCP architecture:

In terms of protocol classes, SCCP supports four distinct classes, each designed to accommodate different communication needs. These classes are: Class 0 (Basic Connectionless Class), Class 1 (Sequenced Connectionless Class), Class 2 (Basic Connection-Oriented Class), and Class 3 (Flow Control Connection-Oriented Class).

GSM uses two numbering plans: NP=1 (E.164, ISDN) and NP=7 (E.214, MGT, Mobile Global Title).

The E.164 numbering plan is normally used, while MGT is used to address some messages from the MSC/VLR to the HLR.

E.164 is the numbering plan used in the PSTN/ISDN, and addresses using E.164 resemble "regular" phone numbers.

The E.214 number (MGT) is derived from the subscriber's IMSI by replacing the Mobile Country Code and Mobile Network Code with the ISDN Country Code and National Destination Code.

In GSM, the numbering plan used in the PSTN/ISDN is E.164, which is also used in ISDN.

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In GSM, the protocol between the Mobile services Switching Center/Visitor Location Register (MSC/VLR) and the Base Station Controller (BSC) is called Base Station System Application Part (BSSAP). BSSAP requires both the Connection Oriented (CO) and the ConnectionLess (CL) service.

The protocol between the MSC/VLR, HLR, and GMSC communicates via the Mobile Application Part (MAP), using only the ConnectionLess (CL) mode. This is supported by the Transaction Capabilities Application Part (TCAP).

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BSSAP allows for both CO and CL services, but it's worth noting that MAP only uses the CL mode. This is in contrast to the protocol between the MSC/VLR and BSC, which requires both CO and CL services.

Here's a brief overview of the protocols involved in GSM:

In GSM, two numbering plans are used: NP=1 (E.164, ISDN) and NP=7 (E.214, MGT, Mobile Global Title).

E.164 is the numbering plan used in the PSTN/ISDN. Addresses using E.164 resemble “regular” phone numbers.

The E.214 number (MGT) is derived from the subscriber’s IMSI by replacing the Mobile Country Code and Mobile Network Code by the ISDN Country Code and National Destination Code.

Two numbering plans are used in GSM: NP=1 (E.164, ISDN) and NP=7 (E.214, MGT, Mobile Global Title).

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The SCCP structure is quite fascinating.

SCCP uses a Point Code to direct messages to a specific node within an SS7 network. This is a unique identifier assigned to each node or network element.

The SCCP structure also employs a Subsystem Number to identify a specific application or service within a node, enabling targeted communication.

Global Title Translation (GTT) is used by SCCP to map logical addresses (Global Titles) to physical addresses (Point Codes), guiding the Message Transfer Part (MTP) on how to route messages through the network.

A structured message format is essential for communication across the network, defined by SCCP as per RFC3868. This structure includes key elements such as Version, Reserved, Message Class, Message Type, Message Length, and Message Data.

The SCCP functional structure is divided into two main categories: Connection-Oriented Services and Connectionless Services.

For Connectionless Services, the Message Type is UDT (Unitdata), which is a fundamental component of SCCP messages.

The parameters of some SCCP messages, such as DT1 (Data Form 1), are also defined within the SCCP structure.

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The Signalling Connection Control Part (SCCP) is a crucial component in the S7 system, providing additional functionality to fulfill the need for extended services in certain applications. It serves as a common platform for various functional elements, including the Message Transfer Part (MTP) and other user parts.

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SCCP's main goal is to provide a way for communication with databases, Home Location Register (HLR), and Visitor Location Register (VLR), without any speech connection. This is especially useful during Location Updating.

The combination of MTP and SCCP is called the Network Service Part (NSP). This partnership enables SCCP to support two network services: Connection Oriented (CO) and ConnectionLess (CL).

Here's a breakdown of the two network services supported by SCCP: