Introduction to Classification

A switching fabric is the internal hardware network within a switching node that physically connects input ports to output ports. The design and architecture of this fabric are critical, determining the node's performance, cost, and capabilities. To understand these trade-offs, we classify switching fabrics based on several key criteria. These classifications are not mutually exclusive; a single switch will have characteristics from each category.

Classification by Technology

This classification reflects the technological evolution of switching.

• Electromechanical: The earliest form, using physical moving parts like rotary selectors and crossbar switches. They were slow, noisy, and required significant maintenance.
• Electronic: Based on semiconductor components like transistors and logic gates. This offered massive improvements in speed, size, and reliability, forming the basis of modern digital switching.
• Optical: Uses light to switch signals without converting them to electricity. Technologies like MEMS (micro-mirrors) physically direct light beams, offering the potential for enormous bandwidths independent of data rate.

Classification by I/O Ports and Directionality

This category describes the physical layout and the flow of information through the fabric.

Port Configuration

• Two-sided fabric: Has distinct, separate sets of input and output ports. A connection is always made from an input to an output.
• One-sided fabric: Has a single set of ports where each port can function as either an input or an output in a given connection.

Number of I/O Ports ($N_1$ inputs, $N_2$ outputs)

• Concentrator: More inputs than outputs $(N_1 > N_2)$. Used to aggregate traffic from many slower links to fewer faster links.
• Expander: More outputs than inputs $(N_2 > N_1)$. Used to distribute traffic from a single source to multiple destinations.
• Distributor: Equal number of inputs and outputs $(N_1 = N_2)$. Standard configuration for routing traffic between peers.

Information Flow Direction

• Unidirectional: Signal flow is strictly from inputs to outputs only.
• Bidirectional: Signal flow can occur in both directions. This is often achieved using two unidirectional paths (a "four-wire" configuration) to create a full-duplex link.

Classification by Combinatorial Properties (Blocking)

This is one of the most critical classifications, defining whether a new connection can always be established through the fabric. A blocking state occurs when a free input port cannot be connected to a free output port because all available internal paths are already occupied by existing connections.

• Blocking Fabrics: Fabrics in which blocking states can occur under certain traffic patterns. They are simpler and cheaper but may lead to dropped calls or rejected connection requests.
• Non-Blocking Fabrics: Fabrics designed to avoid blocking states. They can be further categorized:
  • Non-Blocking in the Strict Sense (SNB): This is the gold standard. A path can always be found between any free input and free output without having to re-route any existing connections.
  • Non-Blocking in the Wide Sense (WNB): A connection is always possible, provided that a specific routing algorithm is used to choose paths for all connections. The algorithm intelligently pre-allocates paths to prevent future conflicts.
  • Rearrangeable Non-Blocking (RNB): If a new connection is blocked, it's always possible to make it succeed by changing the paths of one or more existing connections. This is less desirable for real-time traffic due to the momentary disruption.

Classification by Path Separation

This describes how different communication paths are kept separate as they traverse the switching fabric, and is closely related to multiplexing techniques.

• Space-Division: Each connection is given its own physically separate path through the switch fabric (e.g., a specific set of crosspoints). This is the most common type for electronic and optical switches.
• Time-Division: Multiple connections share the same physical paths but at different points in time, using assigned time slots. This is the basis of TDM switching.
• Wavelength-Division: Used in optical systems. Connections are separated by being assigned different wavelengths (colors) of light, all travelling through the same optical components.
• Code-Division: Connections share the same space and time, but are separated using unique orthogonal codes (similar to CDMA).