SL Signalling Link

SL (Signaling Link) is a critical component in telecommunication networks that enables the exchange of signaling messages between various network elements, such as switches, controllers, and signaling gateways. Signaling messages are used to control and manage call setup, call routing, and other network functions.

SL signaling links are established between signaling points, which are network elements responsible for processing and forwarding signaling messages. These signaling points can be located in different network nodes, such as switches, signaling transfer points (STPs), or service control points (SCPs).

The SL signaling link operates at the network layer (Layer 3) of the OSI model and uses a protocol known as Signaling System No. 7 (SS7). SS7 is a packet-switched network protocol suite that provides signaling capabilities for various telecommunication services, including voice, data, and multimedia applications.

The SL signaling link consists of physical and logical components:

Physical Components:

• Transmission Media: The SL signaling link can be established over different physical transmission media, such as T1/E1 lines, microwave links, or even IP networks.
• Line Terminating Equipment (LTE): The LTE interfaces the transmission media with the signaling link and performs functions like framing, clock synchronization, and error detection and correction.

Logical Components:

• Message Transfer Part (MTP): MTP is a protocol layer within the SS7 protocol suite responsible for reliable and efficient transfer of signaling messages between signaling points. MTP provides functions like message routing, error handling, congestion control, and network management.
• MTP Level 2 (MTP2): MTP2 is the second layer of the MTP and handles the reliable transmission of signaling messages over the physical link. It provides error detection and correction, flow control, and signaling link alignment functions.
• MTP Level 3 (MTP3): MTP3 is the third layer of the MTP and manages the routing of signaling messages between signaling points. It ensures the reliable delivery of signaling messages and handles network congestion and traffic management.
• Signaling Connection Control Part (SCCP): SCCP is a network layer protocol that provides additional functionality to the basic MTP. It enables global title translation, allows for efficient routing of signaling messages, and provides services like connectionless and connection-oriented message transfer.
• User Part (UP): The User Part is the topmost layer of the SS7 protocol suite and represents the actual application layer protocols, such as ISUP (ISDN User Part) for circuit-switched voice services, TCAP (Transaction Capabilities Application Part) for intelligent network services, and MAP (Mobile Application Part) for mobile network services.

The SL signaling link operates in a full-duplex mode, allowing bidirectional communication between signaling points. It supports the exchange of various types of signaling messages, such as query messages, response messages, and network management messages. These messages carry information necessary for call setup, call routing, and service provisioning.

SL signaling links play a crucial role in ensuring the reliable and efficient operation of telecommunication networks. They enable the coordination and control of network resources, facilitate call routing across different network elements, and support the implementation of advanced telecommunication services.