Switching Fabric Architectures

Switching fabric definition

The switching fabric is the internal hardware that creates paths between a switch's inputs and outputs. A fabric can be single-stage, connecting ports in one step, or multistage, combining smaller modules into a route. Its controller selects connections that satisfy transmission requests. For a fabric with $N$ inputs and $N$ outputs, the design must specify which pairs can operate simultaneously.

Many fabrics use a 2x2 switching element. It has two inputs, two outputs, and two states: bar (straight-through) and cross. Independent elements can operate in parallel. Cascading these elements creates multistage fabrics with more inputs and outputs.

The Crossbar Fabric

A crossbar is a grid of switching points with $N^2$ elements for an $N \times N$ fabric. Each point can be controlled independently, so a connection does not share an intermediate stage with another connection. Under one-to-one traffic, all idle input-output pairs can operate in parallel.

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: With an idle input and an idle output, the fabric is strictly non-blocking and can establish the requested connection regardless of other active connections.
• Disadvantage: A single-stage crossbar has $N^2$ switching points, so cost and wiring grow quadratically with $N$. Multistage fabrics built from 2x2 elements reduce the point count relative to a full grid, but add stages and more complex control. Path length and delay then depend on the particular multistage architecture, so the design must balance cost, signal equalization, and control complexity.

Multistage fabrics: Clos and Beneš

Multistage interconnection networks (MINs) reduce the element count relative to a large crossbar by combining smaller modules into stages. Clos and Beneš networks are standard examples.

Clos Network

A Clos network has three stages: 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 $m \times n$. It has $nr$ inputs and the same number of outputs. The number of middle-stage switches, $m$, determines the number of available paths between stages. It is strictly non-blocking when the middle-stage count satisfies $m \ge 2n-1$, while $m \ge n$ is sufficient for rearrangeable non-blocking operation.

Beneš Network

A Beneš network is a recursive form of a Clos network. It is rearrangeably non-blocking. Establishing a new connection may require changing existing paths to remove a blocking condition. For $N$ ports, with $N$ a power of two, the network has $2\log_2N-1$ stages with $N/2$ 2x2 elements in each stage. For a comparable number of ports, it uses fewer switching elements than a strictly non-blocking Clos network, with more complex control.

Other fabric architectures

Other architectures combine performance, cost, and control complexity in different ways.