Network Basics: Collision Overview

A collision occurs on your network when something happens to the data sent from the physical network medium that prevents it from reaching its destination. Mainly, it encounters another signal from another host on the network that yields a resulting useless signal on the network when the signals combine. The collision occurs when the sending device does not receive a clear response back within the allotted time.

This causes an issue for both network devices because they both need to wait for an ever-increasing period until they are able to transmit the data clearly. If the network is busy enough, the network devices can spend an inordinate amount of time retransmitting data.

A collision can only occur at the physical layer in the OSI model. When multiple devices share a common media at the physical layer, which happens when you have multiple devices connected with a hub, there is a possibility that you will have a collision. The network area where a collision may occur is called a collision domain.

So, what benefit do you get from the switch? The switch acts like a multiport bridge that, yes, bridges two collision domains. What happens with the introduction of the bridge? The bridge breaks the network into two or more pieces, with each piece being a separate collision domain. Fewer network devices in a collision domain reduce the chance of a collision, just like fewer cars on a street reduce the chance of an accident.

The following illustration shows a network before and after a bridge is added. The addition of the bridge reduces the collisions on the network by reducing the number of devices sharing either portion of the shared media.

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No special configuration is required to implement a switch or bridge onto your network, and since it is so easy to implement this product, here is the flow of traffic in this scenario.

  1. A network host sends an Ethernet frame destined for another host.

  2. Regardless of where the destination host is, the bridge looks at the source address of the frame, and if the bridge does not recognize the MAC address of the source host, the bridge records the MAC address in the bridge’s Address Database, which includes the port on which the address is found.

  3. The bridge looks at the destination MAC address in the Ethernet frame and does one of the following.

    • If the bridge does not know where the destination host is, it will flood the frame out to all ports on the bridge.

    • If the bridge does know where the destination host is, it will send the frame out on the port on which the destination host is located, unless it is the port on which the frame originally came from, as the destination host will already have received the frame.

  4. The data should arrive at the host to which it was targeted, and the destination host will send data back to the original source host. At this point, if the bridge did not know where this host was, Step 2 will place the original destination host's MAC address in the Address Database.

The great thing about switches and bridges is that they require almost no configuration to function. If you have a heavily congested network, adding a switch to the middle of the network reduces the collision rate on the network and you achieve an increase in network performance and throughput.

This process has no impact on your network layer protocols because it takes place at Layer 2, and your network-level devices (Layer 3) will not see any difference in the traffic that they see. If you replace your network hubs with switches, each network device will be in a collision domain by itself, and your rate of collisions should reduce to zero.

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