The original Ethernet networks connected all hosts with a single cable, similar to how cable TV cables are connected in your home. All users on the network shared the bandwidth available on the cable. As Ethernet networks became more popular, connecting everyone on a single cable was no longer practical, nor even possible. Engineers developed a different type of network technology that made it easier to connect and reconnect multiple devices to the network. The first of these types of networking devices were Ethernet hubs.
Hubs contain multiple ports that are used to connect hosts to the network. Hubs are simple devices that do not have the necessary electronics to decode the messages sent between hosts on the network. Hubs cannot determine which host should get any particular message. A hub simply accepts electronic signals from one port and regenerates (or repeats) the same message out all of the other ports. All hosts attached to the hub share the bandwidth, and will receive the message. Hosts ignore the messages that are not addressed to them. Only the host specified in the destination address of the message processes the message and responds to the sender.
Only one message can be sent through an Ethernet hub at a time. It is possible for two or more hosts connected to a hub to attempt to send a message at the same time. If this happens, the electronic signals that make up the messages collide with each other at the hub. This is known as a collision. The message is unreadable by hosts and must be retransmitted. The area of the network where a host can receive a garbled message resulting from a collision is known as a collision domain.
Because excessive retransmissions can clog up the network and slow down network traffic, hubs are now considered obsolete and have been replaced by Ethernet switches.
Figure 7-5 shows how a hub delivers messages.
Figure 7-5 An Example of Hub Operation
An Ethernet switch is a device that is used at the access layer. When a host sends a message to another host connected to the same switched network, the switch accepts and decodes the frames to read the physical (MAC) address portion of the message, and then sends the message to the destination, as shown in Figure 7-6.
Figure 7-6 An Example of Switch Operation
A table on the switch, called a MAC address table, contains a list of all of the active ports and the host MAC addresses that are attached to them. When a message is sent between hosts, the switch checks to see if the destination MAC address is in the table. If it is, the switch builds a temporary connection, called a circuit, between the source and destination ports. This new circuit provides a dedicated channel over which the two hosts can communicate. Other hosts attached to the switch do not share bandwidth on this channel and do not receive messages that are not addressed to them. A new circuit is built for every new conversation between hosts. These separate circuits allow many conversations to take place at the same time, without collisions occurring. Ethernet switches also allow for the sending and receiving of frames over the same Ethernet cable simultaneously. This improves the performance of the network by eliminating collisions.