InformationWeek SMB TechEncyclopedia

The standard local area network (LAN) access method. A reference to "LAN," "LAN connection" or "network card" automatically implies Ethernet. Defined by the IEEE as the 802.3 standard, Ethernet is used to connect computers in a company or home network as well as to connect a single computer to a cable modem or DSL modem for Internet access.

     New computers come network ready with Ethernet built in. For machines without a network connection, Ethernet adapters can be added via USB or PC Card or by plugging an Ethernet card into a free PCI bus slot inside the case.

     Ethernet is also built into new TVs, Blu-ray players and other home theater equipment for downloading movies from the Internet as well as equipment configuration, control and troubleshooting via a Web browser.

Ethernet Is Wired, Wi-Fi Is Wireless
Ethernet implies a wired connection using cables; however, the wireless version of Ethernet is "Wi-Fi," and both wired and wireless operation is commonplace in companies as well as the home. See Wi-Fi and wireless router.

Megabits Per Second: 10, 100, 1,000 and 10,000
A 10/100 Ethernet port supports two speeds: 10 Mbps (10Base-T) and 100 Mbps (100Base-T). A 10/100/1000 port includes Gigabit Ethernet at 1 Gbps (1,000 megabits). Ethernet devices negotiate with each other and transmit at the highest speed possible. For high-speed backbones in metropolitan and wide area networks, 10 Gigabit Ethernet (10,000 megabits), the fastest Ethernet, is also used (see 10 Gigabit Ethernet).

Shared or Switched
Ethernet is wired in a star configuration with a hub or switch in the middle. Hubs, which predated switches, are shared media devices. All stations attached to the hub share the total bandwidth. Switches provide each sender and receiver pair with the full bandwidth and are significantly faster than hubs (see switched Ethernet). Like the client machines, Ethernet switches and hubs also support 10/100 and 10/100/1000 speeds.

Most Ethernets Use Twisted Pairs
Ethernet uses economical twisted pair cables and standard RJ-45 connectors (see cable categories). Sometimes, spare telephone wires in a building may be used, but often only at the lowest speed. To extend distances, fiber-optic cable is also used (see 100Base-T, Gigabit Ethernet and FOIRL). The first versions of Ethernet used coaxial cable (see 10Base5 and 10Base2).

Ethernet Frames
Ethernet transmits variable length frames from 72 to 1518 bytes in length, each containing a header with the addresses of the source and destination stations and a trailer that contains error correction data. Higher-level protocols, such as IP and IPX, fragment long messages into the frame size required by the Ethernet network being employed (see MTU).

Collision Detection
Ethernet uses the CSMA/CD technology to broadcast each frame onto the physical medium (wire, fiber, etc.). All stations attached to the Ethernet are "listening," and the station with the matching destination address accepts the frame and checks for errors. Ethernet is a data link protocol (MAC layer protocol) and functions at layers 1 and 2 of the OSI model.

History
Invented by Robert Metcalfe and David Boggs at Xerox PARC in 1973, Ethernet first ran at 2.94 Mbps. Metcalfe later joined Digital where he facilitated a joint venture between Digital, Intel and Xerox to collaborate further on Ethernet. Version 1 was finalized in 1980, and products shipped in the following year. In 1983, the IEEE approved the Ethernet 802.3 standard. See 100Base-T, Gigabit Ethernet, 10 Gigabit Ethernet and switched Ethernet.

ETHERNET LIMITATIONS
 Maximum Maximum
Type Segment length Devices
TWISTED PAIR (star topology)
10Base-T 328 ft. (100 m) 1
100Base-T 328 ft. (100 m) 1
1000Base-T 328 ft. (100 m) 1
COAX (bus topology)
10Base5 "thick" 1640 ft. (500 m) 100
10Base2 "thin" 607 ft. (185 m) 30
FIBER (star topology)
FOIRL .6 mi. (1 km) 1
10Base-F 1.2 mi. (2 km) 1
100Base-FX multimode 1.2 mi. (2 km) 1
100Base-FX single-mode 6 mi. (10 km) 1

Twisted Pair Ethernet
Most Ethernets use twisted pair wiring. All cables use RJ-45 connectors between the network adapters in the PC and a central hub or switch.






Ethernet Switch
This 10/100 switch from Omnitron has 16 ports and automatically senses the transmission rate of the line and adjusts accordingly.




SWFSWTCH.SWF

SWFHUB.SWF

Switch Vs. Hub
The switch provides dedicated channels between each pair of users (sender and receiver), whereas the hub shares its bandwidth with all users.







The Original Ethernet
The first Ethernet (10Base5) used a bus topology and a thick coaxial cable. Transceivers connect the network adapters to the cable via a vampire tap that "bites" into the coax.






Thin Ethernet
10Base2 Ethernet was a later variation of the original 10Base5. It used a thinner coaxial cable attached to each node using BNC T-connectors.