Ethernet Tutorial

(Information compiled from internal and external sources., etc.)

What is Ethernet?

Ethernet (this name comes from the physical concept of ether) is a frame-based computer networking technology for local area networks (LANs). It defines wiring and signaling for the physical layer, and frame formats and protocols for the media access control (MAC)/data link layer of the OSI model. Ethernet is mostly standardized as IEEEs 802.3. It has become the most widespread LAN technology in use during the 1990s to the present, and has largely replaced all other LAN standards such as token ring, FDDI, and ARCNET. Ethernet is based on the idea of peers on the network sending messages in what was essentially a radio system, captive inside a common wire or channel, sometimes referred to as the ether. (This is an oblique reference to the luminiferous aether through which 19th century physicists believed electromagnetic radiation traveled.) Each peer has a globally unique 48-bit key known as the MAC address, factory-assigned to the network interface card (Purchase Here), to ensure that all systems in an Ethernet have distinct addresses. Due to the ubiquity of Ethernet, many manufacturers build the functionality of an Ethernet card directly into PC motherboards.

CSMA/CD shared medium Ethernet

A scheme known as carrier sense multiple access with collision detection (CSMA/CD) governs the way the computers share the channel. Originally developed in the 1960s for the ALOHAnet in Hawaii using radio, the scheme is relatively simple compared to token ring or master controlled networks. When one computer wants to send some information, it obeys the following algorithm:
  1. Start - If the wire is idle, start transmitting, else go to step 4
  2. Transmitting - If detecting a collision, continue transmitting until the minimum packet time is reached (to ensure that all other transmitters and receivers detect the collision) then go to step 4.
  3. End successful transmission - Report success to higher network layers; exit transmit mode.
  4. Wire is busy - Wait until wire becomes idle
  5. Wire just became idle - Wait a random time, then go to step 1, unless maximum number of transmission attempts has been exceeded
  6. Maximum number of transmission attempt exceeded - Report failure to higher network layers; exit transmit mode
Ethernet originally used a shared coaxial cable winding around a building or campus to every attached machine. Computers were connected to an Attachment Unit Interface (AUI) transceiver, which in turn connected to the cable (Purchase Here). While a simple passive wire was highly reliable for small Ethernets, it was not reliable for large extended networks, where damage to the wire in a single place, or a single bad connector could make the whole Ethernet segment unusable. Coax was also prone to very strange failure modes when an electrical discontinuity reflected the signal in such a manner that some nodes would work just fine while others would work slowly due to excessive retries or not at all; these could be much more painful to diagnose than a complete failure of the segment. Debugging such failures often involved several people crawling around wiggling connectors while others watched the displays of computers running PING and shouted out reports as performance changed.

Since all communications happen on the same wire, any information sent by one computer is received by all, even if that information was intended for just one destination. The network interface card filters out information not addressed to it, interrupting the CPU only when applicable packets are received unless the card is put into "promiscuous mode". This "one speaks, all listen" property is a security weakness of shared-medium Ethernet, since a node on an Ethernet network can eavesdrop on all traffic on the wire if it so chooses. Use of a single cable also means that the bandwidth is shared, so that network traffic can slow to a crawl when, for example, the network and nodes restart after a power failure.

Ethernet Repeaters and Hubs

As Ethernet grew, the Ethernet hub (Purchase Here) was developed to make the network more reliable and the cables easier to connect.

For signal degradation and timing reasons, Ethernet segments have a restricted size which depends on the medium used. For example, 10BASE5 coax cables have a maximum length of 500 metres (1,640 feet). A greater length can be obtained by using an Ethernet repeater, which takes the signal from one Ethernet cable and repeats it onto another cable. Repeaters can be used to connect up to five Ethernet segments, three of which can have attached devices. This also alleviates the problem of cable breakages: when an Ethernet coax segment breaks, all devices on that segment are unable to communicate; repeaters allowed the other segments to continue working.

Like most other high-speed buses, Ethernet segments must be terminated with a resistor at both ends. For coaxial cable, each end of the cable must have a 50-ohm resistor and heatsink attached (Part Number: BC58T), called a terminator and affixed to a male N or BNC connector. If this is not done, the result is the same as if there is a break in the cable: the AC signal on the bus will be reflected, rather than dissipated, when it reaches the end. This reflected signal is indistinguishable from a collision, and so no communication can take place. A repeater electrically isolates the segments connected to it, regenerating and retiming the signal. Most repeaters have an "auto-partition" function, which partitions (removes from service) a segment when it has too many collisions or collisions that last too long, so that the other segments are not affected by the broken one. The repeater reconnects the segment when it detects activity without collisions.

People recognized the usefulness of cabling in a star topology, and network vendors started creating repeaters having multiple ports. Multi-port repeaters are now known as hubs. Hubs can be connected to other hubs and/or a coax backbone.

The development of Ethernet on unshielded twisted-pair cables (UTP), beginning with StarLAN and continuing with 10BASE-T eventually made Ethernet over coax obsolete. These variations allowed unshielded twisted-pair Cat-3/Cat-5 cable and RJ45 telephone connectors to connect endpoints to hubs, replacing coaxial and AUI cables. Hubs made Ethernet networks more reliable by preventing problems with one cable or device from affecting other devices on the network. Twisted-pair Ethernet resolves the termination problem by making every segment point-to-point, so termination can be built into the hardware rather than requiring a special external resistor.

Type of Ethernet

  • Xerox Ethernet -- the original, 3-Mbit/s Ethernet implementation, which in turn had two versions, Version 1 and Version 2, during its development. The version 2 framing format is still in common use.
  • 10BROAD36 -- Obsolete. An early standard supporting Ethernet over longer distances. It utilized broadband modulation techniques similar to those employed in cable modem systems, and operated over coaxial cable.
  • 1BASE5 -- Also known as StarLAN, was the first implementation of Ethernet on twisted pair wiring. It operated at 1 Mbit/s and was a commercial failure.
  • 10BASE5 (also called Thickwire or Yellow Cable) -- This is the original 10 Mbit/s implementation of Ethernet. The early IEEE standard uses a single 50-ohm coaxial cable of a type designated RG-8, of maximum length 500 metres. Transceivers could be connected by a so-called "vampire tap", which was attached by drilling into the cable to connect to the core and screen, or using N connectors at the end of a cable segment. An AUI cable then connected the transceiver to the Ethernet device. Largely obsolete, though due to its widespread deployment in the early days, some systems may still be in use. It requires precise termination at each end of the cable.

    Ethernet Network Example 1

    Ethernet Network Example 2
  • 10BASE2 (also called Thinwire or Cheapernet) -- 50 ohm RG-58 coaxial cable, of maximum length 200 metres, connects machines together, each machine using a T-adaptor to connect to its NIC, which has a BNC connector. Requires termination at each end. For many years this was the dominant 10 Mbit/s Ethernet standard. Cables and Connectors are here
  • 10 Gigabit Ethernet -- The new 10 gigabit Ethernet standard encompasses seven different media types for LAN, MAN and WAN. It is currently specified by a supplementary standard, IEEE 802.3ae, and will be incorporated into a future revision of the IEEE 802.3 standard.

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