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. Web Links Carrier Sense Multiple Access with Collision Detection (CSMA/CD) http://www.erg.abdn.ac.uk/users/gorry/course/ lan-pages/ csma-cd.html
    Content 8.2 Collision Domains and Broadcast Domains 8.2.2 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: The 5-4-3-2-1 rule also provides guidelines to keep round-trip delay time in a shared network within acceptable limits. Web Links Collision Domain http://www.transition-french.com/learning/ whitepapers/ downloads/ colldom_wp.pdf
    Content 8.2 Collision Domains and Broadcast Domains 8.2.3 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. Connecting several computers to a single shared-access medium that has no other networking devices attached creates a collision domain. This situation limits the number of computers that can use the medium, also called a segment. Layer 1 devices extend but do not control collision domains. Layer 2 devices segment or divide collision domains. Controlling frame propagation using the MAC address assigned to every Ethernet device performs this function. Layer 2 devices, bridges, and switches, keep track of the MAC addresses and which segment they are on. By doing this these devices can control the flow of traffic at the Layer 2 level. This function makes networks more efficient by allowing data to be transmitted on different segments of the LAN at the same time without the frames colliding. By using bridges and switches, the collision domain is effectively broken up into smaller parts, each becoming 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. As long as the traffic between bridged segments is not too heavy a bridged network works well. Otherwise, the Layer 2 device can actually slow down communication and become a bottleneck itself. 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. Interactive Media Activity Fill in the blank: Segmentation After completing this activity, the student will learn about segmentation of a network. Web Links Basics of Network SEgmentation: Switching and Bridging http://www.oreillynet.com/pub/a/network/ 2001/03/ 16/net_2nd_lang.html
    Content 8.2 Collision Domains and Broadcast Domains 8.2.4 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