Broadcast Domain and Collision Domain

Broadcast Domain:

Broadcast domain is a restricted area in which information can be transmitted for all devices in the domain to receive. More specifically, Ethernet LANs are broadcast domains. Any devices attached to the LAN can transmit frames to any other device because the medium is a shared transmission system. Frames are normally addressed to a specific destination device on the network. While all devices detect the frame transmission on the network, only the device to which the frame is addressed actually receives it. A special broadcast address consisting of all 1s is used to send frames to all devices on the network.

• A repeater is a device that joins two LANs to extend the distance of the LAN. All network traffic is sent across the repeater unaltered.
• A bridge is a device that joins two LANs into a single broadcast domain, but isolates them so that problems on one LAN do not propagate to the other LAN. In addition, bridges maintain separate collision domains, so that computers on each segment only contend with other computers on the same segment for access.

Collision Domain:

Ethernet networks use a collision-sensing protocol called CSMA/CD (carrier sense multiple access/collision detection). The protocol allows multiple devices connected to a shared network cable to use that cable by taking turns accessing it. The basic strategy goes like this:

1. A computer listens on the cable to see if another computer is transmitting, which is indicated by a voltage change on the cable. If busy, the computer waits and listens.
2. When the cable is not busy, a computer attempts to transmit.
3. Another computer may attempt to transmit at the same time, which causes a collision.
4. Both computers that attempted to transmit must back off, wait, and then attempt to transmit again.

Computers on the network detect collisions by looking for abnormally changing voltages. Signals from multiple systems overlap and distort one another. Overlapping signals will push the voltage above the allowable limit. This is detected by attached computers, which reject the corrupted frames (called runts).