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Introduction During last three decades Ethernet has become the most widely deployed internetworking topology in the world. Ethernet, which is the common name for IEEE 802. 3 CSMA/CD-carrier sense multiple access, collision detection technology, is the dominant cabling and low-level data delivery standard used in local area networks (LANs). Ethernet technology, developed by Robert Metcalfe and outlined in his 1974 Ph. D. thesis at Harvard University; later it was implemented and further developed as an open standard by DEC, Intel, and Xerox (or DIX) (Loomis 45).
The Institute of Electrical and Electronics Engineers (IEEE) described these specifications as formal standard. The term Ethernet was coined by Metcalfe. The part "ether" describes the main feature of the technology: as the one able to connect different computer systems. Below are some basic Ethernet features: There are several Ethernet technologies: 10 Mbps (million bits per second), to 100 Mbps (Fast Ethernet), and further to 1000 Mbps (Gigabit Ethernet) and the latest development 10000 Mbps (10 Gigabit Ethernet). Currently, 10 Base T and 100 Base T (Fast Ethernet) Ethernet's are the most common, and both can be built with twisted-pair cabling. Data is transmitted over the network in discrete packets (frames), which are between 64 and 1518 bytes in length (46 to 1500 bytes of data, plus a mandatory 18 bytes of header and cyclical redundancy code [CRC] information).
Each device on an Ethernet operates independently and equally, precluding the need for a central controlling device. Ethernet supports a wide array of data types, including TCP/IP, AppleTalk, IPX, and more. To prevent the loss of data, when two or more devices attempt to send packets at the same time, Ethernet detects collisions. All devices immediately stop transmitting and wait a randomly determined period of time before they attempt to transmit again. Ethernet is the most popular physical layer LAN technology in use today. CIBC World Markets estimates that this year there are 326 million installed Ethernet connections (Caplan).
Ethernet is popular because it strikes a good balance between speed, cost and ease of installation. These benefits, combined with wide acceptance in the computer marketplace and the ability to support virtually all popular network protocols, make Ethernet an ideal networking technology for most computer users today. An international standard, IEEE 802. 3, technical specification of all aspects of Ethernet design and construction. This standard, first established in 1983, ensures that components supplied by different vendors will work harmoniously together when used on a network.
As a result, users enjoy a technology which has been the subject of immense investment over the years from many major semiconductor and network product manufacturers. Switched Ethernet Traditionally, many LAN access methods, like the MAC (Media Access Protocol) protocols for Ethernet, Token Ring and FDDI (Fiber Distributed Data Interface), were characterized by shared environment. Lets take for example a traditional Ethernet LAN, it has a common bus-type design. Workstations are physically attached to this bus through a hub, repeater or concentrator, constituting a broadcast domain. Every station is capable of receiving all transmissions from all stations, but only in a half-duplex mode.
In other words computers on this type of LAN can not talk simultaneously. Still, nodes on an Ethernet network send information strictly observing the following rule: they listen before speaking. Therefore, in Ethernet environment, only one node on the segment is allowed to broadcast at any time due to the CSMA/CD protocol (Carrier Sense Multiple Access/Collision Detection). Though this handles packet collisions, it increases transmission time in two ways. First, if two nodes start transmitting simultaneously, their packets collide, so they both must stop broadcast and wait for better time. Second, once a packet is transmitted from a node, an Ethernet LAN will send any other information until that packet reaches its endpoint.
This often cause LAN congestion's. The inefficient pattern described above, along with increased traffic due to ever growing number of network users and amount of data transported between client / server applications, responsible for bandwidth shortages that plague many existing networks. Switching technology is aimed to increase efficiency of the traffic pattern, making current systems more useful. Switching is an intelligent traffic regulator: it forwards information directly from the port of origin to its destination port.
Switching not only enhances network performance, it also boosts up flexibility of a network. Switching allows simultaneous links between various ports, by establishing direct lines of communication between origin and destination ports. It proficiently handles network traffic by reducing media sharing - traffic is contained to the segment for which it is destined, be it a server, station or workgroup. Switched Ethernet brings new quality to traditional bridged and routed networks. A 10 Mbps or 100 Mbps shared channel can be changed to 10 Mbps or 100 Mbps of dedicated bandwidth. Bridges and routers typically have multiply devices connected to their ports, sharing the available bandwidth.
Switches allow connection between a shared segment (a workgroup) or a dedicated one (a power user or server) to each port. Most importantly, this is accomplished without upgrading any software or hardware already running on the workstations. In addition to that, installing a switch is less complex than a bridge / router configuration; this ease of use makes switching an attractive solution. Ethernet switches break LANs single shared environment a into bunch of parallel dedicated lines that enable smooth efficient network traffic. In addition to that switches provide scalable architecture. A switch port serve as a segment with many stations attached to it or with a single station connected to it.
In the former set-up, the segment is a collision domain and the rules of contention based on CSMA/CD are applied. However, to be effective Switching has to be properly implemented. There has to be a clear picture about information traffic. For example, if a number of workstations frequently access a server on the same segment, it would make little point to connect the server to a switch only by one port because only one workstation can be served at a time.
On the other hand, connecting the server to a switch using three ports will allow three parallel bit flows to three different workstations in the same segment, tripling the existing bandwidth. Poorly install switch can even worsen traffic situation. For instance, segments of different departments connected to the same switch will create an overwhelming flow causing congestion's because each segment adds its traffic to the entire network. Traffic generated by a particular segment should remain on that segment. In this case traffic is segment specific and there is little point to broadcast in outside. At the same time a switch will greatly speed up information flow if connected to a single resource, say email server, which is accessed by all departments.
Each group will have a dedicated link to that server. The usability of the aggregated bandwidth is further augmented by the fact that a switch establishes an instant change of dedicated connections: a direct connection from sales department can immediately turn into the one from accounting. Switching technology will definitely be present networks, allowing more efficient traffic control. Moreover, switching flexible architecture facilitates migrations to more efficient networks; it cuts down the cost of an upgrade by narrowing down the lines with the most traffic. However, installing switches in an existing network can be only a temporary solution, besides it requires through analysis of existing traffic, which can be altered with time, in turn putting switching advantages to a minimum. Fast Ethernet The urging demand for higher transmission speeds prompted by fast network growth has been answered by the creation Fast Ethernet specification (IEEE 802. 3 u) known as 100 BASET.
Fast Ethernet has put the plank for data transmission rate from 10 Megabits per second to 100 Megabits, but most importantly it was possible to achieve with only minimal changes to the existing cable structure. Moreover, the emerged technology has feature called Auto negotiation that provide compatibility with 10 BASET legacy equipment and several variations of Fast Ethernet. Fast Ethernet utilizes twisted-pair and fiber optic cabling. An important part of Fast Ethernet specifications is Auto-Negotiation of the media speed.
This makes it possible to employ dual-speed Ethernet interfaces that can be installed and run at either 10 -Mbps or 100 -Mbps automatically. New technology fits great in existing network structure effectively; in combination with switches, it eliminates bottlenecks in most congested segments with minimal costs. Fast Ethernet repeaters are often used between switches allowing a connect at 100 Mbs speed. Fast Ethernet uses the same RJ 45 jack and the wiring at the connector is indistinguishable. However, on 100 Mbs segments Category 5 rated twisted pair cable has to be installed. An essential part of Fast Ethernet is repeater, a network device aimed to translate or replicate a signal.
The two types of Fast Ethernet repeaters offered on the market today are: Class I Repeater: the device operates translates line signals on the incoming port to a digital signal. This ensures compatibility between different types of Fast Ethernet like 100 BASE-TX and 100 BASE-FX. The downside is that only one repeater can be put in a single Fast Ethernet LAN segment. Class II Repeater: The Class II repeater instantly repeats the signal on an incoming port to all the ports on the repeater. Typically Class II repeaters, have all ports of the same Fast Ethernet type...
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