Ethernet Switches

Ethernet Switches (sometimes called switching hubs) are devices that have changed the way Ethernet's bus arbitration works. 10 Mbps Ethernet is based on CSMA/CD, a shared bus where all nodes listen and fight for use of the bus. It is a first come first serve protocol.

Ethernet switches changed that. They are based on circuit switching technology. When a node on a port wants to talk to another node, a dedicated circuit is made between the two ports. No one else is allowed to use the connection. It is not a shared line anymore. The complete bandwidth is available for the conversation.

In addition, starting with 100 Mbps Ethernet, all Ethernet switches are full duplex which means that they can transmit data and receive data at the same time. This effectively doubles the bandwidth between the two ports. Modern Ethernet switches run at 100 Mbps and 1 Gbps full duplex.

If A wanted to communicate with B, a dedicated 100 Mbps connection would be established between the two. If C wanted to communicate with D, another dedicated 100 Mbps connection would be established.

The Ethernet switch examines the MAC address of the Ethernet frame to determine which port to send the frame. They operate the same as Ethernet Bridges and the proper name is a multi-port bridge. Review the Ethernet bridge section to get a good understanding of how an Ethernet switch works. Ethernet switches which work only at the Data Link layer are called layer 2 switches.

A new generation of Ethernet switches in addition to examining the frame information, can examine the packet information at the Network layer. They can make decisions based on the Network layer information which used to be the domain of routers. Ethernet switches which can make decisions at the Network layer are called layer 3 switches.

Virtual LANs - VLANs

Another feature of Ethernet switches is the ability to divide the LAN into virtual networks called VLANs. Rather than have separate switches for each department of a company, one switch can be virtually divided so that it appears as if it is multiple switches. You would do this to segment the broadcast domains.

A good portion of network traffic is "behind the scenes" traffic that is overhead communication between network devices and not data traffic. They communicate by sending broadcast updates to each other. Routers send routing protocol updates, devices connecting up to the network for the first time request DHCP services, there is a lot of ARP requests: "whose MAC address does this IP address belong to?", etc.. Breaking up the broadcast domains, frees bandwidth for data traffic.

Another reason for VLAN'ing a network is security. For example, only the accounting dept can see the accounting server on the network. VLANs will be discussed more deeply in the Network section.

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Copyright July 2013 Eugene Blanchard