From Mycomputer Notes

Revision as of 01:20, 11 September 2006

What is Ethernet? It is the LAN technology most widely used and designed to enable sharing ressources on a local workgroup level. Mainly designed for applications in which a local communicatin medium must carry sporadic, ocasionally heavy traffic at high-pack data rates.

Ethernet designed goals:

• Simplicity
• Low Cost
• Compatibility
• Fairness
• Low Delay
• Highspeed

Ethernet Fundamentals

LANs are high-speed, low error data network that cover a relatively small geographic area (up to a few thousand meters) LAN connects workstations, peripherals, terminals and other devices in a single building or other geographically limited area. The success of Ethernet is a result of its simplicity and ease of maintenance, its capability to incorporate new technologies, its reliability and its low cost of installlation and upgrade.

First Ethernet Standard was published by a consortium of Digitasl Equipment Company, Intel & Xerox (DIX) in 1980. Ethernet Standards Intentions was to be open, that everyone could benefit. First Ethernet Standard products was transmitted at 10Mbps over Thick Coaxial up to a distance of 2km

The IEEE (Institute of Electrical and Electronics Engineers is a professional organization that defines network standards. In 1985 the IEEE standars committee for local and metropolitan networks published its standard for LANs. The IEEE LAN standards are the predominant and best known LAN standards today. Its standards starts with 802. The standard basedon Ethernet is 802.3

The IEEE 802.3 standard made some modifiations to the Ethernet standard, some differences exits between the DIX and IEEE 802.3 specification. The difference is so minor that any Ethernet network interface card (NIC) can transmit and receive Ethernet and 802.3 packets and frames. Bottom line, Ethernet and IEEE 802.3 are the same standard. Now a days the official standard is Ethernet (IEEE 802.3)

Ethernet (IEEE802.3)

Ethernet Standard Includes

• 10Mbps => Original
• 100Mbps => Fast Ethernet
• 1000Mbps => Gigabit Ethernet
• 10GB Mbps => 10 Gigabit Ethernet

• 10BASE2 (IEEE 802.3a)
• 10BASE5 (IEEE 802.3)
• 100BASE-T (IEEE 802.3i)
• 1000BASE-TX (IEEE 802.3x)

The above descriptions consist of:

• A number indicating the number of megabists per second transmitted.
• The word BASE, indicating that baseband signaling is used.
• Numbers( the 2 & 5) that refer to the coaxial cable segment length(the 185m length has been rounded up to 2, for 200)
• One or more letters of the alphabet indicating the type of medium used (F= fiber optics cable, T=copper unshielded twisted -pair)

Ethernet and the OSI Model LAN standards define the physical media and the connectors used to connect devices to media at the physical layers of the OSI Model. LAN Standard Defined;

• How to encapsulated protocol specific traffic in a way that the traffic going from one layer to the next uses the same channel. This is accomplished by two sublayers on the Data Link

1. MAC - Media Access Control (802.3) - the MAC sublayer defines how to transmit frames on the physical wire. Handles physical addressing associated with each device, network topology definiton and line discipline.
2. LLC - Logical Link Control (802.2) - the LLC sublayer is responsible for logically identifying differednt protocol types and then encapsulating them.

Naming; In order to allow local delivery of frames on the Ethernet, there must be an addressing systems, a way of naming the computers and interfaces. Every computer has a unique way of identifying itself. Each computer on a network has a:

• Physical Address
• MAC Address
• Hardware Address
• NIC Address
• Layer 2 Address
• Ethernet Address

MAC Address Format

Every device with an Ethernet Interface to the LAN must have a MAC address (48 bits in length /expressed in 12 hexadecimal digits)

Ehternet and 802.3 LANs are broadcast networks. All stations see all frames.Each station must examine every frame to determine whether that station is the desired destination.

Framing

Many types of frames are described by various standards. A single generic frame has sections called fields and each field is composed of bytes the names of the fields commonly found in a data link frame are:

Generic Frame Format

Framing[[1]]