Collision Domains

What is a Collision Domain

A collision domain is restricted to a single physical link or circuit. If two devices on the same link try to send frames to a third device simultaneously then the frames (which are carried as electrical signals) will collide. If frames collide then when they reach the intended recipient on the network, the data will be corrupted. Consequently, there is a protocol defined to help prevent these collisions: Carrier Sense Multiple Access with Collision Detection (CSMA/CD).

CSMA/CD

When a host using CSMA/CD wants to transmit an Ethernet frame, it follows the following procedure:

  1. Monitor the link to see if any other device is transmitting, if they are then wait until they’ve finished.

  2. Start transmitting data. Keep listening to see if anyone else starts.

  3. If another device starts transmitting, stop and send a ‘jam’ signal.

  4. Both devices stop transmitting and wait for a random length of time (determined by a backoff algorithm)

  5. Try again.

CSMA/CD helps to prevent collisions but within a single collision domain or on a single physical circuit, the bandwidth is shared between all of the hosts which can significantly reduce data speeds.

Hubs and Bridges

On old computer networks, network devices called hubs were used to connect multiple hosts. Hubs are physical layer devices with multiple ports that just take a signal in on one port and repeat it out of all of the other ports. This meant that devices connected by an Ethernet hub were effectively on a single physical circuit and in the same collision domain. Any two devices transmitting at the same time would have to use CSMA/CD to prevent collisions and the bandwidth of the physical link was shared between all devices con-nected to the hub.

To help solve the problems of connecting devices using hubs, a new device called a ‘bridge’ was intro-duced. Bridges were used to connect multiple hubs together and give them each a separate collision do-main. A bridge understands CDMA/CD and only forwards frames when it is safe to do so.

Switches

Ethernet switches can be thought of as combining the functionality of hubs and bridges – as well as many other additional features. Each port on a switch is a separate collision domain and therefore each link can be used to full capacity. Because hubs are rarely used today, most ports will just have a single device con-nected such as a host or another switch. Therefore, the link can be used to it’s full capacity by the two connected devices.

Full-Duplex vs Half-Duplex

With two devices on a link, there can still be a situation which both devices are transmitting data at the same time. Originally, Ethernet was a half-duplex standard which meant that only one devices could send data at a time. With half-duplex Ethernet, we need to use CSMA/CD to make sure that both devices don’t transmit at the same time and cause collisions. This means that both devices share the bandwidth on the link. To make everything better, support for full-duplex communication was introduced into the Ethernet standard. On a full duplex link, separate wires within the cable allow both devices to transmit and receive data simultaneously on separate physical media. This means that collisions cannot occur, and we no longer need to use CSMA/CD. Furthermore, the bandwidth on the link can be fully utilised by both devices at the same time. For example, on a 100Mbit/s link, both devices could send 100Mbit/s simultaneously.

Broadcast Domains

A broadcast domain is a group of Ethernet devices which will receive an Ethernet broadcast frame sent by another device in the group. Network switches will flood Ethernet broadcasts out of all ports associated with a given LAN, therefore devices connected to a switch are typically in the same broadcast domain. The exception to this is when a switch has different Virtual LANs (VLANs) configured – in this instance, each VLAN is a separate broadcast domain.

Routers in a network act as a boundary between different broadcast domains. This is because routers act at Layer 3, the IP layer, and therefore do not forward broadcast Ethernet frames which reach them.

In networks broadcast domains can contain many hosts which means they can all communicate at layer two. However, with too many devices there can be a lot of broadcast frames flying around the network which must each be processed by every other device in the broadcast domain.

Collision Domains vs Broadcast Domains

  • A collision domain is a layer one property grouping devices on the same physical circuit. A broadcast do-main is a layer two property grouping devices which can send broadcast frames to each other.

  • Collision domain boundaries are typically switches whereas broadcast domain boundaries are typically routers.

  • There will typically be multiple collision domains within each broadcast domain.

  • In modern Ethernet networks we always want to stop collisions having lots of small collision domains. The number of hosts we want in a broadcast domains is more complex design decision.

Other Related Skills

VLANs are used in modern computer networks to support logically separate LANs on a single device.