What is the difference between a collision and a broadcast domain?

Question

I have read many answers to the quesiton but every answer includes some new term that I am not yet familiar with. Please explain as if it were to a child. (i am the child) also if the collision domain is so problematic why was it made with such high probability of collision?

Answer 1

What the previous respondent left out is that collisions are not a really big problem. Ethernet is set up so that any computer on the segment can transmit at any time, and if it doesn't work (that is, a collision occurs), the sender detects the collision and resends the data a short, random time later. On most normal networks, collisions don't affect network performance too much. If the collision domain gets too big, however, there are more and more collisions and network performance can be seriously slowed down, so networks are generally segmented with bridges or switches.

Everything that connects without passing through a router is the broadcast domain -- which means that there can be several segments (collision domains) in the broadcast domain. Broadcasts are generally used for locating a destination, after which the actual transmission only goes to the segment the destination machine is located on.

Answer 2

A collision is something that occurs physically (electrically) on the network. A broadcast is something that occurs logically on the network.

A collision domain is essentially an Ethernet segment. In the old days of Ethernet we used to use something called 10Base2 and 10Base5, which was Ethernet over coax cable, you could hook up multiple cables together through something called a repeater. All the sections of cable + repeaters were known as a segment or collision domain. The repeaters are like stereo amplifiers they just boosted the signal of whatever was on the wire, be it good packets or collisions. A collision occurs when two NICS try to send out a packet at the same time within a collision domain. You can only have one packet on the network within a collision domain at a time.

Bridges break up the collision domain. A bridge will regenerate every packet that it forwards. It must be destine for a MAC address on the other side of the bridge. However a bridge will forward a broadcast. An ARP (I know you IP address, what is your MAC address) is an example of a broadcast.

Most of todays "switches" are a combined repeater/bridge. If two workstations connected to a switch want to talk to each other the switch dynamically creates a broadcast domain between them. Early 10BaseT hubs were just repeaters.

Router break up broadcast domains. A router will respond on behalf of devices on a different domain to it's locally attached domain. For ARP, the router won't forward the broadcast but will respond with it's MAC address on behalf of the IP address, providing it knows how to find the destination IP address.