The Signalling System No. 7 protocol stack, often known as the SS7 protocol stack, is a critical communication protocol widely used in telecommunications. SS7 allows for effective interchange of signalling messages between network parts, which is critical in establishing and controlling phone conversations and other telecommunication services.
The SS7 protocol stack supports communication across heterogeneous networks by operating on the network layer of the OSI model. Understanding the differences between the OSI and SS7 protocols provides significant insights into their respective capabilities in network communication.
Table of Content:
What is the SS7 Protocol Stack?
What is SS7, and how does it work?
What is the difference between OSI and SS7?
Scope and Purpose
Layers and Functionality
Network Coverage
Implementation
What is the network layer of SS7?
MTP Level 1
MTP Level 2
MTP Level 3
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What is the SS7 Protocol Stack?
The SS7 protocol stack is a set of telecommunication protocols used to set up and manage phone calls across the public switched telephone network (PSTN) and other signalling networks. Services like call setup, routing, and network administration are only possible because of the infrastructure it provides for exchanging signalling signals between network nodes.
The SS7 protocol stack consists of several layers, each performing a different signalling procedure task. The Physical layer is the lowest layer of a network’s architecture, and it is responsible for transmitting signalling messages across the network via media such as TDM (Time Division Multiplexing) and IP (Internet Protocol).
The Data Link layer is above it and is in charge of ensuring data gets sent and received without any problems.
The Network layer coordinates the delivery of data packets to their intended destinations. Messages can be sent to specific nodes in the network with the help of the Signalling Connection Control Part’s (SCCP) global title addressing.
To guarantee proper communication between nodes in a network, the Transport layer ensures the timely delivery of signalling messages via protocols such as the Reliable Transfer Service (RTS) and the Transaction Capabilities Application Part (TCAP).
The Application layer of the SS7 protocol stack provides protocols for various services. Signalling in mobile networks is handled by the Mobile Application Part (MAP), whereas in the public switched telephone network (PSTN), call setup and teardown are handled by the Integrated Services Digital Network User Part (ISUP).
What is SS7, and how does it work?
Signalling System 7 (or SS7) is a group of protocols used in telecommunications networks to handle call signalling and control. It is essential to connect and communicate with other telecommunications service providers worldwide. The SS7 protocol has been around since the 1970s, although it sees regular use even now.
The primary purpose of SS7 is to initiate, manage, and close user-to-user communication sessions. It functions at the OSI model’s network layer, offering features like call setup, call routing, call teardown and a host of optional extras like call forwarding and caller ID.
While SS7 is most commonly associated with older, circuit-switched networks, it can also be found in more recent, IP-based infrastructures.
Signalling points, which form the backbone of the SS7 network, are organised hierarchically. These nodes in the network that process signalling messages can be switches, databases, or anything else. SS7 signalling links are either actual wires or virtual connections between nodes that allow signalling points to communicate with one another.
SS7 signalling is used to link callers when they make the first move. To begin, the switch that initiated the call sends a signalling message to a central hub for routing calls called a Signal Transfer Point (STP). The signalling transfer point (STP) evaluates the call’s routing options and sends the signalling message along the most efficient path.
When the message reaches the target signalling point, it alerts the destination switch to begin calling. After a call is made, SS7 keeps tabs on it to ensure it goes where it’s supposed to, handles call transfers, and manages any optional extras the user has signed up for.
SS7 operates using a set of standardised protocols and message formats, which enable interoperability between different vendors and networks. The protocols specify the format, transmission, and processing of signalling messages, facilitating efficient and effective interaction between signalling nodes.
What is the difference between OSI and SS7?
The OSI (Open Systems Interconnection) model and SS7 (Signaling System 7) are both frameworks used in telecommunications to ensure efficient and reliable communication between networked devices. While they serve similar purposes, their scope and specific functionalities differ. Let’s explore the key differences between OSI and SS7.
Scope and Purpose:
The OSI model is a conceptual framework that specifies the roles and interactions of different protocols in a network. This document serves as a useful reference by breaking down the process of creating network communication protocols into seven distinct layers.
The Signalling System 7 protocol suite is the gold standard for making and managing phone calls via a network. Telecom signalling and control features such as call setup, teardown, and routing are highlighted.
Layers and Functionality:
Each of the seven layers in the OSI model performs a different set of duties. Physical, Data Link, Network, Transport, Session, Presentation, and Application are the several layers that make up a network. There are distinct borders between each layer, and each contributes to the overall communication process.
SS7 differs from the OSI model in that it is not a layered paradigm but a collection of protocols designed to perform certain signalling tasks. Included are protocols such as Signalling Connection Control Part (SCCP), ISUP (ISDN User Part), and MAP (Mobile Application Part) that serve specific tasks.
When managing and regulating the delivery of telecommunications services, SS7 is all about conveying signalling information between nodes in the network.
Network Coverage:
The OSI model is a universal framework that may be used on any network, from a small LAN to a global WAN. It is not restricted to any one technology or field of use.
In the context of circuit-switched networks like the public switched telephone network (PSTN) and mobile networks like GSM and CDMA, Signalling System 7 is widely utilised. Communication between network switches and SCPs is primarily its purview.
Implementation:
The OSI model provides a framework for thinking about how to build network protocols. It is not a protocol specification document but rather a set of guidelines for developing systems that can communicate with one another.
SS7 is a standardised suite of protocols with clear guidelines and requirements. It provides a standardised signalling protocol that allows various network devices from various manufacturers to work together.
What is the network layer of SS7?
The network layer of SS7 (Signaling System No. 7) is responsible for the reliable transport of signalling messages between network elements in a telecommunication network. It communicates between various components, such as switches, service control points, and databases.
The network layer in SS7 is also known as the Message Transfer Part (MTP). It ensures the efficient and secure delivery of signalling messages across the SS7 network. The network layer is divided into three levels: MTP Level 1, MTP Level 2, and MTP Level 3.
MTP Level 1: Also known as the physical layer, MTP Level 1 specifies the electrical and physical properties of SS7 signalling lines. It defines physical network connections’ transmission medium, line encoding, modulation, and signalling speeds.
MTP Level 2 oversees error checking and repair during signalling message delivery. MTP Level 2 includes message sequencing, flow management, error detection, and retransmission procedures. It ensures the secure transmission of messages through signalling lines.
MTP Level 3: MTP Level 3 handles routing and managing signalling messages on the SS7 network. It finds the best path for message delivery based on the destination point code (DPC) and other routing parameters. MTP Level 3 also controls congestion control, network management, and status monitoring.
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