What is a Switching Node?
A switching node is a device in a communication network that directs traffic based on control information and configuration. It receives data, selects an output port or channel for the connection, and forwards the data to the next node. These nodes are basic elements of switched networks, from the global telephone system to the internet.
The name of a switching node depends on the network type, while its basic role remains selecting an egress and forwarding data.
- Telephone Exchanges: In the traditional telephone network (PSTN), these switch voice calls.
- Routers: In IP networks like the internet, they forward packets using addressing and forwarding tables.
- ATM Switches: In ATM networks, they switch fixed-size cells.
- Optical Cross-Connects (OXC): In optical networks, they switch established optical paths.
General Architecture of a Switching Node
Regardless of the technology, a modern switching node consists of several functional blocks. They cooperate in receiving, switching, controlling, and managing traffic.
Core Components Explained
- Input/Output Modules (and Links): These are the physical entry and exit points for data. Each input link is connected to an , and each output link is driven by an . Their specific functions depend on the transmission method used on the link (e.g., handling SDH frames, Ethernet packets, etc.).
- Switching Fabric: This is the heart of the switch. It transfers data from input modules to selected output modules. Depending on the design, it can be a single-stage matrix, a multistage network, or a shared-memory path. Its primary task is to connect inputs and outputs so that user information can cross the node.
- Control Unit: This unit processes control and signaling information, makes forwarding decisions, initiates setup and teardown of connections within the switching fabric, and coordinates responses to errors during operation.
- Management Unit:This block handles the overall administration of the node. Its responsibilities include managing the node's configuration, running diagnostic tests, ensuring system security, and collecting billing data or performance statistics for network operators.
Functions of Input and Output Modules in Detail
The input and output modules adapt signals from physical links for switching and prepare them for further transmission.
Input Module Functions
When a signal arrives, the input module performs several operations. An ATM switch provides an example:
- Physical Signal Reception: Receiving the optical signal and converting it into an electrical one.
- Framing and Synchronization: For framed transmission like SDH, the module must find the frame boundaries (bit and octet synchronization) and process the frame header.
- Cell Extraction: Extracting the ATM cells from the payload of the higher-level transmission frame.
- Header Analysis: Identifying individual cells and checking header integrity with HEC before the forwarding lookup.
- Switching Lookup: Looking up the incoming VPI/VCI in a switching table to determine the output port and the VPI/VCI values used on the next link.
Output Module Functions
After a cell has traversed the switching fabric, the output module prepares it for transmission:
- Buffering: Storing incoming cells temporarily while they wait for their turn to be transmitted on the physical link.
- Header Recalculation: Recomputing HEC when the forwarding stage changes VPI/VCI or another header octet before the cell is sent on the next link.
- Framing: Placing the cell back into the payload of a physical layer frame (e.g., SDH).
- Physical Transmission: Converting the internal signal to the outgoing line representation, such as optical form on an optical link.