Question

Suppose two packets arrive to two different input ports of a router at exactly the same time. Also suppose there are no other packets anywhere in the router. a. Suppose the two packets are to be forwarded to two different output ports. Is it possible to forward the two packets through the switch fabric at the same time when the fabric uses a shared bus? b. Suppose the two packets are to be forwarded to two different output ports. Is it possible to forward the two packets through the switch fabric at the same time when the fabric uses a crossbar? c. Suppose the two packets are to be forwarded to the same output port. Is it possible to forward the two packets through the switch fabric at the same time when the fabric uses a crossbar?

Short answer

a. No; b. Yes; c. No.

Step-by-step solution

Understanding the Scenario

Consider two packets arriving simultaneously at two input ports of a router, each intended for different processing scenarios as defined in parts a, b, and c.

Analyzing Part A - Shared Bus Fabric

For a shared bus architecture, all data travels through a single communication path. Since both packets are destined for different output ports but must use the shared bus, only one packet can be transferred at a time to avoid collision on the bus.

Analyzing Part B - Crossbar Fabric

In a crossbar fabric, each input port can be connected to any output port simultaneously, as it allows multiple communications paths. Therefore, packets heading to different output ports can be forwarded concurrently without interference.

Analyzing Part C - Crossbar to Same Output

With crossbar switching, although multiple paths are possible, two packets cannot be forwarded to the same output port at the same time. Output ports can handle one packet at a time, which means queuing will occur if multiple packets arrive for the same port.

Key concepts

Shared Bus

In router architecture, a shared bus operates like a single highway where all data packets must travel along the same path. Think of it as everyone in a city trying to use one road to get different places.

This configuration can be simple but has a significant limitation: only one packet can traverse the bus at any given time to avoid collision. If two packets try to use the shared bus simultaneously, they would interfere with each other, much like a traffic jam of data.

Benefits of shared bus architecture include ease of implementation and cost-effectiveness. However, because only one transmission can occur at once, there can be a bottleneck, particularly when multiple packets arrive at the same time destined for different outputs.

Crossbar Switching

Crossbar switching is more like having multiple roads, each leading to a different destination. It is a type of switch fabric structure that provides a dedicated path between each input port and any output port they need to reach.

This configuration allows for multiple packets to be transmitted simultaneously, as there is no single shared path. Each packet independently finds its way to its designated output without being held up by others.

A major advantage of using crossbar switching is the ability to handle multiple, concurrent data transfers, which significantly increases throughput and reduces delay when outputs differ. However, this type of switching can be more complex and expensive to build and maintain.

Packet Forwarding

Packet forwarding is a critical function in router architecture, responsible for the movement of packets from one side of a network to another. When a packet arrives at an input port, the router determines the appropriate output port it should be sent to. This decision is based on routing information within the router.

Routers forward packets based on their destination addresses and the routing paths defined in its routing tables. Efficient packet forwarding is essential for network performance, minimizing delay and ensuring packets reach their destination correctly.

Packet forwarding involves various algorithms and can be influenced by the type of switch fabric used, be it a shared bus or a crossbar, which can affect how quickly and efficiently the forwarding occurs.

Switch Fabric

The term 'switch fabric' refers to the internal framework that routers use for routing data packets from input ports to output ports. It is an essential part of how routers manage data traffic and can greatly influence their efficiency and performance.

There are several types of switch fabric design, including the shared bus and crossbar switching methods. Each has its own advantages and trade-offs, such as simplicity versus performance.

Switch fabric choices are crucial, as they can dictate how data moves within the router and impacts the overall speed and efficiency of data forwarding. Understanding the implications of each switch fabric type helps in designing networks that can efficiently handle various traffic loads.