Switching Fabric

The term "switching fabric" refers to the component at the heart of a data communications switch that allows any input port to send data to any output port. Many different kinds of switching fabric have been used over the years, depending on the manufacturer, the size and type of the data communications switch, and the technology available at the time. Sometimes a switching fabric will directly connect to all ports, but usually there are a group of ports on a single card called a line card and the switching fabric connects the line cards together. There are many different types of switching fabrics available on the market today. An example of one of the most basic is the "crossbar" switching fabric, which consists of a matrix of rows and columns, where each row is connected to an input port and each column is connected to an output port. The resulting diagram looks like a fabric with threads crossing at right angles. A switch or "crosspoint" is located at each intersection between a row and a column. By closing the right crosspoints, each input port can be connected to each output port. Crossbar fabrics are very general, but expensive to create in large sizes because the number of crosspoints is equal to the number of input ports times the number of output ports. For instance, if you had a small eight port switch you would have eight potential input and output ports making a total of 64 crosspoints, but if you had a large switch with 100 ports you would need 10,000 crosspoints to allow every port to connect with each other. Other types of switching fabric use buffering, queuing, packet shaping, switching logic, and specialized application specific integrated circuits (ASIC) to enhance switching fabric performance. A well-designed switching fabric will reach switching speeds equal to the line rate of the port. For instance, a port with a theoretical speed of 100 Mbsp should be able to pass packets across the switching fabric to the destination port or ports at 100 Mbps, which is also known as line-rate or wire-speed switching. A poorly designed switching fabric has delays or other latency that will drop the data rate as packets travel through the switching fabric. The variety and performance of switching fabrics depend on many different variables as well as the manufacturer. However, one thing is for certain future trends in switching fabrics are hard to anticipate, but the switching fabric will always remain at the heart of the data communication switch.

Below is a simplified illustration of switching fabric which shows how any input port (i.e. line card) can transmit data to any output port, essentially linking all of the line cards together. Notice a "crosspoint" is depicted with a solid black dot indicating the intersection between a row and a column of the fabric.


Source: http://choonho.files.wordpress.com/2007/09/capture4.jpg