What is switched ethernet?

Switched ethernet is a packet-switching technology that dramatically reduces the network congestion caused by using the CSMA/CD medium access control over shared media. Sometimes known as a switched hub, a switch offers guaranteed bandwidth per port and operates at the medium access control sublayer of the network access layer of the TCP/IP protocol suite.

The introduction of switching can overcome congestion, due to the non-deterministic nature of CSMA/CD, by dynamically routing frames across networks. Don't confuse switches with routers - strictly speaking, switches are layer two devices and routers operate at layer three. Just to confuse matters, layer three switches are being developed to support the growing needs of large corporate intranets. The first generation of layer 3 switches were known as High Function switches. These devices employed a mixture of hardware switching and software routing. This represented a compromise because the performance benefits of switching were sometimes lost with the reliance on slow software. The new generation of layer three switches route using hardware only.

A switch will examine the incoming frame's MAC header to determine the physical address of the destination device. Whereas a hub simply repeats an incoming frame out of all its ports, the switch creates a virtual path to forward frames to the port where it knows the destination device exists.

CSMA/CD is no longer required, each device on a switched network has guaranteed access to the media through the switch. Switches can also provide some flow control to network traffic by temporarily storing frames in a buffer. Flow control is necessary when switches are used between clients operating over 10BaseT and servers using 100BaseT.

Switches can also generate backpressure to cause transmitting devices to wait long enough for the switch to deal with backlogged traffic. To create backpressure, the switch broadcasts false collision detection signals to cause network interface cards to back off. It is ironic that switches must sometimes mimic the CSMA/CD access methodology that they negate.

In order to operate effectively, switches maintain their own MAC look-up tables. These table illustrate which network device is at which port on the switch. The look-up tables are constantly updated. When a switch is newly installed, it will construct its table via an Address Resolution Protocol (ARP) broadcast. ARP is a TCP/IP layer 3 protocol.

The animated gif image shows the essential difference between a switch and a hub. Note how the switch routes the frame to the right hand segment, whereas the hub merely repeats the frame out of both ports.

What sort of switches are there?

There are three sorts of layer 2 switches:

• Cut-through switches. These switches only examine the address in the MAC header. The switch consults its MAC address look-up table to decide which port to forward the frame to. It then creates a virtual link (point-to-point connection) and forwards the frame. Cut-through switches do not examine the frame's check sequence (cyclic redundancy check) to ascertain if it is corrupt, therefore they can forward corrupted data. The destination will then request a retransmission. This, and the resultant retransmission will increase network traffic.
  
  
• Store and forward switches. A store and forward switch reads the whole incoming frame into a buffer and calculates its own frame check sequence. The switch then compares its CRC figure to the frame's CRC. If the two CRCs are the same, the switch will consult its address look-up table, create the virtual link and then forward the frame. Store and forward is slower than cut-through, but it won't forward corrupted frames.
  
  
• Error-free cut-through switches. These switches read the MAC address and the CRC for each frame. If the CRC is correct, the switch configures its port for cut-through (fast) switching. If the CRC is incorrect, the switch configures its port for store and forward (error-checked) switching. As the amount of corrupted frames diminishes to a pre-set threshold, the switch will reconfigure the store and forward port back to cut-through.

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