Understanding collision domains requires understanding what collisions are and how they are caused. To help explain collisions, Layer 1 media and topologies are reviewed here.
Some networks are directly connected and all hosts share Layer 1. 
Examples are listed in the following:
- Shared media environment – Occurs when multiple hosts have access to the same medium. For example, if several PCs are attached to the same physical wire, optical fiber, or share the same airspace, they all share the same media environment.
- Extended shared media environment – Is a special type of shared media environment in which networking devices can extend the environment so that it can accommodate multiple access or longer cable distances.
- Point-to-point network environment – Is widely used in dialup network connections and is the most familiar to the home user. It is a shared networking environment in which one device is connected to only one other device, such as connecting a computer to an Internet service provider by modem and a phone line.
It is important to be able to identify a shared media environment, because collisions only occur in a shared environment. A highway system is an example of a shared environment in which collisions can occur because multiple vehicles are using the same roads. As more vehicles enter the system, collisions become more likely. A shared data network is much like a highway. Rules exist to determine who has access to the network medium, but sometimes the rules simply cannot handle the traffic load and collisions occur.
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 without transmissions 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 1 devices do not break up collision domains, Layer 2 and Layer 3 devices do break up collision domains. Breaking up, or increasing the number of collision domains with Layer 2 and 3 devices is also known as segmentation.
Layer 1 devices, such as repeaters and hubs, serve the primary function of extending the Ethernet cable segments. 
By extending the network more hosts can be added. However, every host that is added increases the amount of potential traffic on the network. Since Layer 1 devices pass on everything that is sent on the media, the more traffic that is transmitted within a collision domain, the greater the chances of collisions. The final result is diminished network performance, which will be even more pronounced if all the computers on that network are demanding large amounts of bandwidth. Simply put, Layer 1 devices extend collision domains, but the length of a LAN can also be overextended and cause other collision issues.
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. 
To assure that a repeated 10BASE-T network will function properly, the round-trip delay calculation must be within certain limits otherwise all the workstations will not be able to hear all the collisions on the network. Repeater latency, propagation delay, and NIC latency all contribute to the four repeater rule. 
Exceeding the four repeater rule can lead to violating the maximum delay limit. When this delay limit is exceeded, the number of late collisions dramatically increases. 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 (no hosts)
- One large collision domain
The 5-4-3-2-1 rule also provides guidelines to keep round-trip delay time in a shared network within acceptable limits.
Segmentation
| 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. Layer 2 devices segment or divide collision domains. These smaller collision domains will have fewer hosts and less traffic than the original domain. Layer 3 devices, like Layer 2 devices, do not forward collisions. Because of this, the use of Layer 3 devices in a network has the effect of breaking up collision domains into smaller domains. Layer 3 devices perform more functions than just breaking up a collision domain. Layer 3 devices and their functions will be covered in more depth in the section on broadcast domains. |
Layer 2 broadcasts
To communicate with all collision domains, protocols use broadcast and multicast frames at Layer 2 of the OSI model. When a node needs to communicate with all hosts on the network, it sends a broadcast frame with a destination MAC address 0xFFFFFFFFFFFF. This is an address to which the network interface card (NIC) of every host must respond.
Layer 2 devices must flood all broadcast and multicast traffic. The accumulation of broadcast and multicast traffic from each device in the network is referred to as broadcast radiation. In some cases, the circulation of broadcast radiation can saturate the network so that there is no bandwidth left for application data. In this case, new network connections cannot be established, and existing connections may be dropped, a situation known as a broadcast storm. The probability of broadcast storms increases as the switched network grows.
Because the NIC must interrupt the CPU to process each broadcast or multicast group it belongs to, broadcast radiation affects the performance of hosts in the network. Figure shows the results of tests that Cisco conducted on the effect of broadcast radiation on the CPU performance of a Sun SPARCstation 2 with a standard built-in Ethernet card. As indicated by the results shown, an IP workstation can be effectively shut down by broadcasts flooding the network. Although extreme, broadcast peaks of thousands of broadcasts per second have been observed during broadcast storms. Testing in a controlled environment with a range of broadcasts and multicasts on the network shows measurable system degradation with as few as 100 broadcasts or multicasts per second.
Most often, the host does not benefit from processing the broadcast, as it is not the destination being sought. The host does not care about the service that is being advertised, or it already knows about the service. High levels of broadcast radiation can noticeably degrade host performance. The three sources of broadcasts and multicasts in IP networks are workstations, routers, and multicast applications.
Workstations broadcast an Address Resolution Protocol (ARP) request every time they need to locate a MAC address that is not in the ARP table. Although the numbers in Figure might appear low, they represent an average, well-designed IP network. When broadcast and multicast traffic peak due to storm behavior, peak CPU loss can be orders of magnitude greater than average. Broadcast storms can be caused by a device requesting information from a network that has grown too large. So many responses are sent to the original request that the device cannot process them, or the first request triggers similar requests from other devices that effectively block normal traffic flow on the network.
As an example, the command telnet mumble.com translates into an IP address through a Domain Name System (DNS) search. To locate the corresponding MAC address an ARP request is broadcast. Generally, IP workstations cache 10 to 100 addresses in their ARP tables for about two hours. The ARP rate for a typical workstation might be about 50 addresses every two hours or 0.007 ARPs per second. Thus, 2000 IP end stations produce about 14 ARPs per second.
The routing protocols that are configured on a network can increase broadcast traffic significantly. Some administrators configure all workstations to run Routing Information Protocol (RIP) as a redundancy and reachability policy. Every 30 seconds, RIPv1 uses broadcasts to retransmit the entire RIP routing table to other RIP routers. If 2000 workstations were configured to run RIP and, on average, 50 packets were required to transmit the routing table, the workstations would generate 3333 broadcasts per second. Most network administrators only configure a small number of routers, usually five to ten, to run RIP. For a routing table that has a size of 50 packets, 10 RIP routers would generate about 16 broadcasts per second.
IP multicast applications can adversely affect the performance of large, scaled, switched networks. Although multicasting is an efficient way to send a stream of multimedia data to many users on a shared-media hub, it affects every user on a flat switched network. A particular packet video application can generate a seven megabyte (MB) stream of multicast data that, in a switched network, would be sent to every segment, resulting in severe congestion.
Broadcast domains
| A broadcast domain is a grouping of collision domains that are connected by Layer 2 devices. Breaking up a LAN into multiple collision domains increases the opportunity for each host in the network to gain access to the media. This effectively reduces the chance of collisions and increases available bandwidth for every host. But broadcasts are forwarded by Layer 2 devices and if excessive, can reduce the efficiency of the entire LAN. Broadcasts have to be controlled at Layer 3, as Layer 2 and Layer 1 devices have no way of controlling them. The total size of a broadcast domain can be identified by looking at all of the collision domains that the same broadcast frame is processed by. In other words, all the nodes that are a part of that network segment bounded by a layer three device. Broadcast domains are controlled at Layer 3 because routers do not forward broadcasts. Routers actually work at Layers 1, 2, and 3. They, like all Layer 1 devices, have a physical connection to, and transmit data onto, the media. They have a Layer 2 encapsulation on all interfaces and perform just like any other Layer 2 device. It is Layer 3 that allows the router to segment broadcast domains. In order for a packet to be forwarded through a router it must have already been processed by a Layer 2 device and the frame information stripped off. Layer 3 forwarding is based on the destination IP address and not the MAC address. For a packet to be forwarded it must contain an IP address that is outside of the range of addresses assigned to the LAN and the router must have a destination to send the specific packet to in its routing table. |
| Data flow in the context of collision and broadcast domains focuses on how data frames propagate through a network. It refers to the movement of data through Layer 1, 2 and 3 devices and how data must be encapsulated to effectively make that journey. Remember that data is encapsulated at the network layer with an IP source and destination address, and at the data-link layer with a MAC source and destination address. A good rule to follow is that a Layer 1 device always forwards the frame, while a Layer 2 device wants to forward the frame. In other words, a Layer 2 device will forward the frame unless something prevents it from doing so. A Layer 3 device will not forward the frame unless it has to. Using this rule will help identify how data flows through a network. Layer 1 devices do no filtering, so everything that is received is passed on to the next segment. The frame is simply regenerated and retimed and thus returned to its original transmission quality. Any segments connected by Layer 1 devices are part of the same domain, both collision and broadcast. Layer 2 devices filter data frames based on the destination MAC address. A frame is forwarded if it is going to an unknown destination outside the collision domain. The frame will also be forwarded if it is a broadcast, multicast, or a unicast going outside of the local collision domain. The only time that a frame is not forwarded is when the Layer 2 device finds that the sending host and the receiving host are in the same collision domain. A Layer 2 device, such as a bridge, creates multiple collision domains but maintains only one broadcast domain. Layer 3 devices filter data packets based on IP destination address. The only way that a packet will be forwarded is if its destination IP address is outside of the broadcast domain and the router has an identified location to send the packet. A Layer 3 device creates multiple collision and broadcast domains. Data flow through a routed IP based network, involves data moving across traffic management devices at Layers 1, 2, and 3 of the OSI model. Layer 1 is used for transmission across the physical media, Layer 2 for collision domain management, and Layer 3 for broadcast domain management. |
As with many terms and acronyms, segment has multiple meanings. The dictionary definition of the term is as follows:
In the context of data communication, the following definitions are used:
To properly define the term segment, the context of the usage must be presented with the word. If segment is used in the context of TCP, it would be defined as a separate piece of the data. If segment is being used in the context of physical networking media in a routed network, it would be seen as one of the parts or sections of the total network |
| Cisco Systems, Inc. |
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