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Saturday, September 6, 2008

Understanding the Basics of Ethernet

Ethernet was developed at Xerox's Palo Alto Research Center (PARC) by Robert Metcalfe and David Boggs with Chuck Thacker and Butler Lampson in the early 1970s. Xerox filed a patent application for Ethernet in 1975. Today, Ethernet is based on IEEE standard 802.3 (Institute of Electrical and Electronic Engineers). Metcalfe left Xerox in 1979 and founded 3Com to promote local area networks and personal computers. He persuaded Digital Equipment Corporation (DEC) and Intel to work together with Xerox to promote the DIX (Digital/Intel/Xerox) Ethernet standard. Ethernet is named for the invisible, massless substance that 19th century scientists believed filled the universe. Ethernet was originally based on the same rules as those for polite conversation. Each computer wanting to transmit data waits until there's a lull in network traffic before attempting to transmit its data. That technology was called CSMA/CD for Carrier Sense Multiple Access Collision Detection and used coaxial cables as a transmission medium. Today, Ethernet uses full duplex transmission over unshielded twisted pair copper cables or fiber optic cables with a system of hubs and/or switches.

Ethernet operates at layer two of the OSI reference model. Layer two, also known as the Data Link Layer, is subdivided into the Logical Link Control (LLC) layer and the Media Access Control (MAC) layer. Ethernet nodes use a globally-unique 48-bit address called the MAC address to communicate within a network. Datagrams at layer two are called frames. The frame structure used by modern Ethernet is the same as that used by earlier coaxial-cabled Ethernet networks, thus providing a level of backwards compatibility.The original Ethernet operated at a speed of three megabits per second. Today, typical transmission rates for Ethernet are 10 Mbps, 100 Mbps, and 1000 Mbps (Gigabit Ethernet). 10,000 Mbps (10 Gigabit Ethernet) is now starting to emerge. Faster data rates are always under development.


Ethernet Cable Standards

  • 10-Base-2, also known as thinnet, uses coaxial cable, is limited to 10 Mbps, and a maximum segment length of 185 meters. 10-Base 2 is falling into disuse due to the lower cost and greater simplicity associated with UTP (unshielded twisted pair) cabling.
  • 10-Base-5, also known as thicknet, uses coaxial cable, is limited to 10 Mbps, and a maximum segment length of 500 meters. 10-Base-5 is rarely seen anymore.
  • 10-Base-T uses unshielded twisted pair (UTP) cable over a maximum of 100 meters (328 feet) at a data rate of 10 Mbps. 10-Base-T uses only two of the four wire pairs in the cable.
  • 10-Base-FL uses fiber optic lines up to 2000 meters with a maximum data rate of 10 Mbps.
  • 100-Base-TX uses UTP cable over a maximum segment length of 100 meters with a maximum data rate of 100 Mbps. 100-Base-TX also uses only two of the four wire pairs in the cable.
  • 100-Base-FX uses fiber optic cable over a maximum segment length of 2000 meters with a maximum data rate of 100 Mbps.
  • 1000-Base-FX uses fiber optic cable over a maximum segment length of 2000 meters with a maximum data rate of 1000 Mbps (one gigabit per second).
  • 1000-Base-TX uses UTP cable cable over a maximum segment length of 100 meters with a maximum data rate of 1000 Mbps (one gigabit per second). Unlike 100-Base-TX, 1000-Base-TX uses all four wire pairs in the cable.

Copper Cable Categories

Although there are a total of nine categories of unshielded twisted pair (UTP) copper cable, there are really only three that you're likely to encounter in your local area network. The others are either obsolete or designed for use in backbone networks. The three categories are:

  • Category 5e: Provides performance of up to 100 MHz, and is frequently used for both 100 Mbit/s and Gigabit Ethernet networks.
  • Category 6: Provides performance of up to 250 MHz, more than double category 5 and 5e.
  • Category 6a: Provides performance of up to 500 MHz, double that of category 6 and is even suitable for 10 Gigabit Ethernet networks.

What should you use in your network?

Build your networks with the fastest cable you can afford. Your bandwidth demands will increase over time and retro-fitting your cable plant is disruptive, time-consuming, and expensive.

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