Unit 6

Lesson 1 - Repeaters and Hubs

   Ethernet Collision Domains

A collision domain is a portion of a network where all nodes receive every frame transmitted by all other nodes, and compete for access to the shared medium. For example, in a small, one-hub 10-megabits-per-second (Mbps) Ethernet network, every node in the network receives every frame transmitted by any other node. Thus, all nodes attached to the hub share the same 10-Mbps bandwidth. The Ethernet Collision Domain Diagram illustrates this principle.

Ethernet Collision Domain

Ethernet Collision Domain

In this diagram, Node A wants to send information to Node G. A frame is sent from Node A to the hub. The hub, which is essentially a multiport repeater, repeats the frame out every port. Each node attached to the hub receives the frame. However, only the node that has a NIC address (in this case, Node G) that matches the frame address will process the frame and pass its contents to the next highest layer.

Hub-to-Hub Connectivity

After an Ethernet hub fills to capacity, additional computers cannot be connected to the hub. As a network grows and more nodes are needed, hubs can be added to provide more physical ports to connect additional devices. The Ethernet Hub-to-Hub Diagram illustrates this principle.

Ethernet Hub-to-Hub

Ethernet Hub-to-Hub

In this diagram, Node H is removed from the first hub and a connection is made to another hub. Many hubs provide the ability to use one of the hub ports for either device connectivity or hub connectivity. A switch is normally mounted under this port for switching between computer connectivity and hub connectivity. (Another method used to connect hubs built without a switch is to use an Ethernet crossover cable.) The switch is put in one position for attaching a computer, and the opposite position when attaching to another hub.

As noted on the diagram, the port that was used to attach to Node H is now used to attach to the hub. Node H now resides on the second Ethernet hub. Once again this configuration represents a single collision domain. If a frame is generated from Node A to the hub, the hub will repeat the frame out each port. This includes the uplink port that attaches to the second hub. The second hub will then take this frame and repeat it out each of its ports as well.

As networks grow, more and more hubs may be added to increase the number of nodes attached to a network. At some point, servers must be added to provide options not available in a strictly peer-to-peer network. However, when hubs are interconnected, all of their nodes are still in the same collision domain. Regardless of where the information is going to or coming from, each node receives the frame transmission.

As traffic increases in the network, there will be a point where performance is unacceptable. In other words, the 10-Mbps bandwidth shared by all devices in this broadcast network will no longer be adequate. To correct this problem, other devices can be used to divide the network into separate collision domains. We will learn about these devices in upcoming lessons.

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