Collision Domain and Broadcast Domain in Computer Network

Prerequisite – Network Devices, Transmission Modes

The most common network devices used are routers and switches. But we still hear people talking about hubs, repeaters, and bridges. Do you ever wonder why these former devices are preferred over the latter ones? One reason could be: ‘because they are more efficient and powerful’. But what actually is the reason behind their efficiency? This is when terms like “Collision Domains” and “Broadcast Domains” come into picture.

Before going further, let us recall that a hub is a multiple-port repeater. Similarly, a switch is a multiple-port bridge so that you can understand why repeaters and bridges are not typically used in production networks(because of less number of ports).

Now, narrowing down to Hubs, Switches and Routers, let us discuss them in reference with the blow domains.

  1. Collision Domain –
    A Collision Domain is a scenario in which when a device sends out a message to the network, all other devices which are included in its collision domain have to pay attention to it, no matter if it was destined for them or not. This causes a problem because, in a situation where two devices send out their messages simultaneously, a collision will occur leading them to wait and re-transmit their respective messages, one at a time. Remember, it happens only in case of a half-duplex mode.
  2. Broadcast Domain –
    A Broadcast Domain is a scenario in which when a device sends out a broadcast message, all the devices present in its broadcast domain have to pay attention to it. This creates a lot of congestion in the network, commonly called LAN congestion, which affects the bandwidth of the users present in that network.

    From this, we can realize that more the number of collision domains and more the number of broadcast domains, the more efficient is the network providing better bandwidth to all its users.

So, which of our network devices break collision domains and which of them break broadcast domains?

  • HUB –
    We start with a hub because we should get rid of it as soon as possible. The reason being, it neither breaks a collision domain nor a broadcast domain,i.e a hub is neither a collision domain separator nor a broadcast domain separator. All the devices connected to a hub is in a single collision and single broadcast domain. Remember, hubs do not segment a network, they just connect network segments.
  • SWITCH –
    Coming to switches, we have an advantage over the hub. Every port on a switch is in a different collision domain, i.e a switch is a collision domain separator. So messages that come from devices connected to different ports never experience a collision. This helps us during designing networks but there is still a problem with switches. They never break broadcast domains, means it is not a broadcast domain separator. All the ports on the switch are in still in a single broadcast domain. If a device sends a broadcast message, it will still cause congestion.
  • ROUTER –
    Last, but not least, we have our savior. A router not only breaks collision domains but also break broadcast domains, means it is both collision as well as broadcast domain separator. A router creates a connection between two networks. A broadcast message from one network will never reach the other one as the router will never let it pass.

Also, as repeaters and bridges differ from hubs and switches only in terms of the number of ports, a repeater does not break collision and broadcast domains, while a bridge breaks only collision domains.

References –
CCNA, Todd Lammle

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