The Heart of the Switch

The switching fabric is the internal hardware that creates paths between a switch's inputs and outputs. Think of it as a highly automated version of an old-fashioned telephone exchange, where an operator manually connected patch cords to establish a call. The goal is to build a fabric that can efficiently connect any of its $N$ inputs to any of its $N$ outputs.

Most fabrics are built from a simple building block: a 2x2 switching element. This element has two inputs and two outputs and can be in one of two states: "bar" (straight-through) or "cross". By arranging these simple elements in different ways, we can create large and complex switching fabrics.

The Crossbar Fabric

The crossbar is one of the most fundamental switching architectures. It is structured as a grid of switching points, with $N^2$ elements for an $N \times N$ fabric.

Operation and Characteristics

• Operation: To connect input $i$ to output $j$, the switching element at the intersection of the $i$-th row and $j$-th column is activated.
• Advantage: The main advantage is that it is strictly non-blocking. As long as the desired input and output ports are free, a connection can always be made.
• Disadvantage: Its primary drawback is the varying optical path length for different connections. A connection from input 0 to output 0 passes through only one element, while a connection from input 3 to output 3 may pass through $2N-1$ elements. This variation causes different levels of signal attenuation and delay, complicating signal reception. The $N^2$ complexity also makes it costly for large switches.

Multi-Stage Fabrics: Clos and Beneš

To reduce the complexity of the crossbar switch while maintaining good performance, multi-stage interconnection networks (MINs) were developed. The Clos and Beneš networks are foundational examples of this approach.

Clos Network

A Clos network is a three-stage fabric. It consists of an input stage with $r$ switches of size $n \times m$, a middle stage with $m$ switches of size $r \times r$, and an output stage with $r$ switches of size $m \times n$. Its key property is that it can be strictly non-blocking if the number of middle-stage switches, $m$, is sufficiently large, specifically when the condition $m \ge 2n-1$ is met.

Beneš Network

A Beneš network is a special, recursive form of a Clos network. It is not strictly non-blocking but is rearrangeably non-blocking. This means that any desired connection can be made, but it might be necessary to change the paths of existing connections to avoid blocking. While more complex to control, it uses fewer switching elements than a strictly non-blocking Clos network.

Other Fabric Architectures

Beyond the classic crossbar and Clos-type networks, other architectures have been developed with different trade-offs between performance, cost, and complexity.