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Collision domains / Segmentation

Collision domains
8.2.2 This page will define collision domains.


Collision domains are the connected physical network segments where collisions can occur. Collisions cause the network to be inefficient. Every time a collision happens on a network, all transmission stops for a period of time. The length of this period of time varies and is determined by a backoff algorithm for each network device.

The types of devices that interconnect the media segments define collision domains. These devices have been classified as OSI Layer 1, 2 or 3 devices. Layer 2 and Layer 3 devices break up collision domains. This process is also known as segmentation.

Layer 1 devices such as repeaters and hubs are mainly used to extend the Ethernet cable segments. This allows more hosts to be added. However, every host that is added increases the amount of potential traffic on the network. Layer 1 devices forward all data that is sent on the media. As more traffic is transmitted within a collision domain, collisions become more likely. This results in diminished network performance, which will be even more pronounced if all the computers use large amounts of bandwidth. Layer 1 devices can cause the length of a LAN to be overextended and result in collisions.

The four repeater rule in Ethernet states that no more than four repeaters or repeating hubs can be between any two computers on the network. For a repeated 10BASE-T network to function properly, the round-trip delay calculation must be within certain limits. This ensures that all the workstations will be able to hear all the collisions on the network. Repeater latency, propagation delay, and NIC latency all contribute to the four repeater rule. If the four repeater rule is violated, the maximum delay limit may be exceeded. A late collision is when a collision happens after the first 64 bytes of the frame are transmitted. The chipsets in NICs are not required to retransmit automatically when a late collision occurs. These late collision frames add delay that is referred to as consumption delay. As consumption delay and latency increase, network performance decreases.

The 5-4-3-2-1 rule requires that the following guidelines should not be exceeded:

• Five segments of network media
• Four repeaters or hubs
• Three host segments of the network
• Two link sections with no hosts
• One large collision domain

The 5-4-3-2-1 rule also provides guidelines to keep round-trip delay time within acceptable limits.

The next page will discuss segmentation.

Segmentation
8.2.3 This page will explain how Layer 2 and 3 devices are used to segment a network.


The history of how Ethernet handles collisions and collision domains dates back to research at the University of Hawaii in 1970. In its attempts to develop a wireless communication system for the islands of Hawaii, university researchers developed a protocol called Aloha. The Ethernet protocol is actually based on the Aloha protocol.

One important skill for a networking professional is the ability to recognize collision domains. A collision domain is created when several computers are connected to a single shared-access medium that is not attached to other network devices. This situation limits the number of computers that can use the segment. Layer 1 devices extend but do not control collision domains.

Layer 2 devices segment or divide collision domains. They use the MAC address assigned to every Ethernet device to control frame propagation. Layer 2 devices are bridges and switches. They keep track of the MAC addresses and their segments. This allows these devices to control the flow of traffic at the Layer 2 level. This function makes networks more efficient. It allows data to be transmitted on different segments of the LAN at the same time without collisions. Bridges and switches divide collision domains into smaller parts. Each part becomes its own collision domain.

These smaller collision domains will have fewer hosts and less traffic than the original domain. The fewer hosts that exist in a collision domain, the more likely the media will be available. If the traffic between bridged segments is not too heavy a bridged network works well. Otherwise, the Layer 2 device can slow down communication and become a bottleneck.

Layer 2 and 3 devices do not forward collisions. Layer 3 devices divide collision domains into smaller domains.
Layer 3 devices also perform other functions. These functions will be covered in the section on broadcast domains.

The next page will discuss broadcasts

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