Collision and Broadcast Domains

Collision Domain

The “collision domain” describes a network where packet collisions can occur when two devices on a shared network medium send packets simultaneously. The colliding packets discarded and must be sent again, which reduces network speed and efficiency.

Usually, collisions occur in a hub environment, because each port on a hub is in the same collision domain. So, all devices connected to the hub are in the same collision domain and only one device can transmit at a time, and all other devices must listen to the network in order to avoid collisions. Total network bandwidth is shared among all devices.

In contrast to hubs, each port on a bridge, switch, or a router different collision domain which reduces and eliminates the possibility of collisions and enables the devices to use the full-duplex communication. The full-duplex communication effectively doubles the speed of data capacity. To understand the collision domains, examine the following figure:

Collision and Broadcast Domains 2

You can see that there is eight collision domain marked in the above topology. Because the hub is single collision domain or all ports of the hub are in single collision domain but each port of the router, bridge and switch are separate collision domain.

Broadcast Domain

All the devices in the broadcast domain can reach via broadcast at the data link layer. A Broadcast Domain can receive any broadcast packet originating from any device within the network segment. All ports of hub and switch belong to same broadcast domain but all ports of the router

All ports of the hub and switch are in the same broadcast domain. Hub and Switches send broadcasts out all interfaces except the interface on which it received. Routers do not transmit broadcasts because when a router receives a broadcast, it does not forward it out to other interfaces.

Each interface of the router belongs to the different broadcast domain and each broadcast is only propagated within its specific domain. Routers separate the boundaries of the broadcast domains. Now examine the same figure for the broadcast domain.

Collision Domain

In the figure above you can see four broadcast domains marked. Because all ports on a hub, bridge and a switch are in the same broadcast domain and all interfaces of the router are in a different broadcast domain.

Layer 2 devices send broadcasts known as ARP to a known IPv4 address on the local network to discover the associated MAC address. The host can get IP address configuration using the Dynamic Host Configuration Protocol (DHCP) from the DHCP server. A large broadcast domain can connect many hosts. A problem with a large broadcast domain is to generate excessive broadcasts and negatively affect the network.

A large number of Broadcasts also decrease the bandwidth of the network for normal traffic because the broadcast traffic is forwarded to all the devices in the domain. It also decreases the processing power of computers and network devices. Because the computers and network devices need to process all the broadcast packets received a part of the CPU power spent on processing the broadcast packets.

So we need to decrease broadcast. For decreasing broadcast, we need to enter a router into the network. The router is an expansive device, So it is not possible to add a router into many networks. Another solution for decreasing is subnetting.

Subnetting is the process which decreases network traffic and improves network performance and makes management very easy. We can isolate network segments very easily using subnetting. We can also apply security policies such as which subnets allowed or not allowed to communicate together. There are different ways of using subnets. Network administrators can also set services into subnets such as:

  • Different floors in a building
  • Different Organizations
  • A different section of the organizations
  • Types of Different Devices such as servers, printers, and hosts
  • Any other division that makes sense for the network.