Ethernet FAQ

What is Ethernet?

Ethernet is the IEEE 802.3 series standard, based on the CSMA/CD access method that provides two or more stations to share a common cabling system. This access method, Carrier Sense Multiple Access with Collision Detection, is the basis for Ethernet systems which range from speeds of 1 Mb/s through 1000 Mb/s.

The design goals for Ethernet were to create a simply defined topology that made efficient use of shared resources, was easy to reconfigure and maintain, provided compatibility across many manufacturers and systems, while keeping the cost low.

What is the history of Ethernet?

The original Ethernet specification began in the early 1970's by Xerox PARC, and was eventually improved upon by Digital Equipment Corporation, Intel, and Xerox (DIX) in 1980 with the release of Ethernet Version 1. By 1982, the specification was updated and Ethernet Version 2 was released.

In 1983, Novell created their own proprietary Ethernet frame type prior to the release of the IEEE 802.3 specification (See Section [4.1]). By 1985, the IEEE 802.3 specification was completed and provided a specification for Ethernet connectivity over thick coax and thin coax. In 1990, the specification was updated to include Ethernet over twisted pair copper wiring with 10Base-T. The current IEEE 802.3 specification includes thick coax, thin coax, twisted pair cabling and fiber, with speeds of 10 Mb/s, 100 Mb/s, and 1000 Mb/s.

What is CSMA/CD?

Carrier Sense Multiple Access with Collision Detection is the basis for the Ethernet standard, and this provides specific rules for allowing stations to communicate over the same transmission medium. There are a number of steps involved in communicating with CSMA/CD.

Stations must listen for a carrier on the wire. If no carrier is detected, stations can begin transmitting. While transmitting, the station continues to listen on the wire to ensure successful communications. If two stations attempt to transmit information at the same time, the transmissions overlap and cause a collision.

If a collision occurs, the transmitting station recognizes the interference on the network and transmits a bit sequence called jam. The jam helps to ensure that the other transmitting station recognizes that a collision has occurred. After a random delay, the stations attempt to retransmit the information and the process begins again.

What are the different physical Ethernet network types?

Some of the physical Ethernet types as defined in the 802.3 specification are:

• 10BASE5 - 10BASE5 is the original design of the traditional Ethernet backbone, designed to be left in place permanently or for extended periods.

• 10BASE2 - 10BASE2 is the original design for a departmental or workgroup sized Ethernet environment. It is designed to be simple, inexpensive, and flexible as people and stations move.

• 10BROAD36 - 10BROAD36 is a seldom used Ethernet specification which uses a physical medium similar to cable television, with CATV-type cables, taps, connectors, and amplifiers.

• 1BASE5 - 1BASE5 is a specification of Ethernet that runs at 1 Mb/s over twisted pair wiring. This physical topology uses centralized hubs to connect the network devices.

• 10BASE-T - 10BASET provides Ethernet services over twisted pair copper wire.

• FOIRL - Fiber Optic Inter-Repeater Link - This specification of the 802.3 standard defines a standard means of connecting Ethernet repeaters via optical fiber.

• 10BASE-F - 10BASE-F is a set of optical fiber medium specifications which define connectivity between devices.

• 100BASE-T - 100BASE-T is a series of specifications that provides 100 megabit speeds over copper or fiber. These topologies are often referred to as Fast Ethernet.

• Gigabit Ethernet - Gigabit Ethernet provides speeds of 1000 Mb/s over copper and fiber.

What does baseband and broadband mean?

A baseband network has a single channel that is used for communication between stations. Ethernet specifications which use BASE in the name refer to baseband networks.

A broadband network is much like cable television, where different services communicate across different frequencies on the same cable. Broadband communications would allow a Ethernet network to share the same physical cable as voice or video services. 10BROAD36 is an example of broadband networking.

What is the difference between a bus topology and a star topology?

A bus topology is a networking architecture that is linear, usually by using one or more pieces of cable to form a single line, or bus. The signals sent by one station extend the length of this cable to be heard by other stations.

A star topology is an architecture that includes a central device or hub to connect all stations together. Signals sent by a station must pass through (and are usually regenerated) by these central hubs. Since the hub sits in the center and all other stations are linked through the hub, the architecture resembles a star.

What physical Ethernet topologies are no longer popular?

There are a number of physical networking components specified in the IEEE 802.3 specification, but many of those early physical networking components are not used in most modern Ethernet networks. However, there may be instances where an existing legacy network still exists which uses these older components. Since these older pieces of equipment are still part of the 802.3 specification, there are no technical reasons why an Ethernet network would not operate properly with these components. The two most popular older Ethernet technologies are 10BASE5 and 10BASE2.

• 10BASE5

10BASE5 is the original Ethernet backbone, and is occasionally referred to as thicknet or thick Ethernet because of the thick 50 ohm coax that was used as the physical medium. 10BASE5 is a bus topology that uses transceiver cables to attach stations to the central 10BASE5 cable.

• Maximum segment length: 500 meters
• Maximum number of segments connected with repeaters: 5 (2500 meters)
• Maximum attachments per segment: 100
• Minimum separation between attachments: 2.5 meters

• 10BASE2

10BASE2 is designed as a smaller and less expensive alternative to 10BASE5, and is sometimes referred to as Thinnet or Thin Ethernet because of the much smaller cables. 10BASE2 is also a bus topology, but each of the workstations use a 'T' BNC connector to connect workstations to the central bus.

• Maximum segment length: 200 meters
• Maximum number of segments connected with repeaters: 5 (1000 meters)
• Maximum attachments per segment: 30
• Minimum separation between attachments: .5 meters

What are the most common physical Ethernet networks used today?

Most modern Ethernet networks use twisted pair copper cabling or fiber to attach devices to the network. The 10BASE-T, 100BASE-T, and Gigabit Ethernet topologies are well suited for the modern cabling and fiber infrastructures.

What pin assignments are used in twisted-pair Ethernet cabling?

Twisted-pair Ethernet (10BASE-T, 100BASE-T, or 1000BASE-T) uses an RJ-45 connector, which is an eight-pin modular connector.

• Contact 1 Transmit +
• Contact 2 Transmit -
• Contact 3 Receive +
• Contact 4 Not Used
• Contact 5 Not Used
• Contact 6 Receive -
• Contact 7 Not Used
• Contact 8 Not Used

When looking at an RJ-45 wall jack (female), contact 1 is on the left and contact 8 is to the right. When looking at the RJ-45 connector on the end of a cable (male) with the tab on the bottom and the contacts on the top, contact 8 is on the left and contact 1 is to the right.

Can two Ethernet stations be directly attached with 10BASE-T?

Two Ethernet stations can be directly attached to each other, but the cabling will be wired differently than a normal 10BASE-T Ethernet network connection. The 802.3 specification refers to this direct connection between two stations as a crossover function.

The crossover function is accomplished by simply wiring the receive pins to the transmit pins:

• Contact 1 - Contact 3
• Contact 2 - Contact 6
• Contact 3 - Contact 1
• Contact 6 - Contact 2

What is propagation delay?

The propagation speed of a medium refers to the speed that the data travels through that medium. Propagation delays differ between mediums, which affect the maximum possible length of the Ethernet topology running on that medium.

In the following table, c refers to the speed of light in a vacuum, or 300,000 kilometers per second.

Medium Propagation Speed
------ -----------------
Thick Coax    .77c (231,000 km/sec)
Thin Coax     .65c (195,000 km/sec)
Twisted Pair  .59c (177,000 km/sec)
Fiber  .66c (198,000 km/sec)
AUI Cable     .65c (195,000 km/sec)

From these values, the size of a bit on 10BaseT can be calculated. 10BaseT is twisted pair, which has a propagation delay of 177,000 km/sec. 177,000 km/sec divided by 10 million bits per second is 17.7 meters, or the size of a single bit on a 10BaseT network.

The maximum propagation delay through the network can be calculated by dividing the maximum length by the speed. For 10Base2 thin coax network, this is 185 meters divided by 195,000 km/sec, or 950 nanoseconds. If the actual propagation delay from one end of the network to the other is greater than 950 nanoseconds, late collisions may occur. See section [5.4] for more information on late collisions.

What is an interframe gap?

The inteframe gap is the amount of time that is specified between frames transmitted from a workstation. The designers of the Ethernet specification arbitrarily chose 96 bit times to occur between frames from a transmitting station.

This delay is designed to provide the workstations on the Ethernet network with some 'breathing time' between frames to perform normal Ethernet housekeeping functions on the network interface card.