Function of Hubs

Function of Hubs

A hub is one type of networking device that is installed at the Access Layer of an Ethernet network. 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.

Remember that the NIC on a host accepts messages only addressed to the correct MAC address. Hosts ignore 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.

 All of the ports on the Ethernet hub connect to the same channel to send and receive messages. Because all hosts must share the bandwidth available on that channel, a hub is referred to as a shared-bandwidth device.

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.

 A collision causes the messages to become garbled and unreadable by the hosts. A hub does not decode the messages; therefore it does not detect that the message is garbled and repeats it out all the ports. The area of the network where a host can receive a garbled message resulting from a collision is known as a collision domain.

 Inside a collision domain, when a host receives a garbled message, it detects that a collision has occurred. Each sending host waits a short amount of time and then attempts to send, or retransmit, the message again. As the number of hosts connected to the hub increases, so does the chance of collisions. More collisions cause more retransmissions. Excessive retransmissions can clog up the network and slow down network traffic. For this reason, it is necessary to limit the size of a collision domain.