Curs PC si internet cap 5.5 Collisions and Collision Domains in Shared Layer Environments

5.5 Collisions and Collision Domains in Shared Layer Environments 5.5.4 Collisions as natural functions of shared media environments and collision domains Instructor Note The purpose of this target indicator is to deepen the students understanding of a shared media environment. The Hawaiian Islands serve as an example of a shared, broadcast media for Electromagnetic Wave Signals. In a similar way, the nodes on an Ethernet share can share a copper media for voltage pulse signals. There is historical importance to this analogy -- Hawaii is where the early networking protocol Aloha was developed. Aloha evolved into Ethernet! Normally, people think that collisions are bad because they decrease network performance. However, a certain amount of collisions are a natural function of a shared media environment (i.e. collision domain) because large numbers of computers are all trying to communicate with each other, at the same time, by using the same wire. The history of how Ethernet handles collisions and collision domains dates back to research at the University of Hawaii, and its attempts to develop a wireless communication system for the Islands of Hawaii. Researchers developed a protocol called Aloha, which was eventually instrumental in the development of Ethernet. The main graphic shows the Island of Hawaii and an Ethernet segment. Both are shared media environments; both are collision domains. 5.5 Collisions and Collision Domains in Shared Layer Environments 5.5.5 Shared access as a collision domain Instructor Note The most basic collision domain occurs when multiple computers have access to the same medium. Design goals are to minimize the number of hosts in a single collision domain and to minimize the physical extant of collision domains. As a networking professional, one important skill is the ability to recognize collision domains. If you connect several computers to a single medium that has no other networking devices attached, you have a basic shared access situation, and you have a collision domain. Depending on the particular technology used, this situation limits the number of computers that can use that part of the medium, also called a segment. 5.5 Collisions and Collision Domains in Shared Layer Environments 5.5.7 Hubs and collision domains Instructor Note The purpose of this target indicator is to illustrate that networking devices which solve one problem can help cause another. In this case, a hub is shown extending a collision domain. You have already learned that another name for a hub is a multiport repeater. Any signal that comes in one port of the hub is regenerated, retimed, and sent out every other port. Therefore, hubs, which are useful for connecting large number of computers, extend collision domains. The final result is diminished network performance if all the computers on that network are demanding large bandwidths, simultaneously. 5.5 Collisions and Collision Domains in Shared Layer Environments 5.5.8 Hubs and repeaters as causes of collision domains Instructor Note The purpose of this target indicator is to illustrate that networking devices which solve one problem can help cause another. In this case, a hub and a repeater are shown extending a collision domain. Since both repeaters and hubs are Layer 1 devices, and therefore perform no filtering of network traffic, extending a run of cable with a repeater, and ending that run with a hub, results in a larger collision domain. 5.5 Collisions and Collision Domains in Shared Layer Environments 5.5.9 The four repeater rule Instructor Note The purpose of this target indicator is to highlight the importance of the 4 repeater rule, also known as the 4 hub rule or the 5-4-3-2-1 rule, for Ethernet. 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. Each repeater adds latency or slows down the bits as their signal strength is boosted. 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 increase. 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 referred to as consumption delay. As consumption delay and latency increase, network performance decreases. This Ethernet rule of thumb is also known as the 5-4-3-2-1 rule. Five sections of the network, four repeaters or hubs, three sections of the network are "mixing" segments (with hosts), two sections are link segments (for link purposes), and one large collision domain. 5.6 Basic Topologies Used in Networking 5.6.10 Cellular network topology Instructor Note The purpose of this target indicator is to introduce the cellular topology. A good activity is to have the students draw 10 nodes in a cellular topology. Mathematical PerspectiveThe cellular topology consists of circular or hexagonal areas, each of which has an individual node at its center. Physical PerspectiveThe cellular topology is a geographic area that is divided into regions (cells) for the purposes of wireless technology a technology that becomes increasingly more important each day. There are no physical links in a cellular topology, only electromagnetic waves. Sometimes the receiving nodes move (e.g. car cell phone), and sometimes the sending nodes move (e.g. satellite communication links).The obvious advantage of a cellular (wireless) topology is that there are no tangible media other than the earth's atmosphere or the vacuum of off-planet space (and satellites). The disadvantages are that signals are present everywhere in a cell and, thus, are susceptible to disruptions (man-made and environmental) and to security violations (i.e. electronic monitoring and theft of service).Logical PerspectiveCellular technologies communicate with each other directly (though distance limitations and interference sometimes make it extremely difficult), or communicate only with their adjacent cells, which is extremely inefficient. As a rule, cellular-based topologies are integrated with other topologies, whether they use the atmosphere or satellites.