Ethernet

Ethernet Technologies:

Ethernet:

Ethernet is a family of technologies that provides data-link and physical specifications for controlling access to a shared network medium. It has emerged as the dominant technology used in LAN networking.

Ethernet was originally developed by Xerox in the 1970s, and operated at 2.94Mbps. The technology was standardized as Ethernet Version 1 by a consortium of three companies - DEC, Intel, and Xerox, collectively referred to as DIX - and further refined as Ethernet II in 1982. In the mid 1980s, the Institute of Electrical and Electronic Engineers (IEEE) published a formal standard for Ethernet, defined as the IEEE 802.3 standard. The original 802.3 Ethernet operated at 10Mbps, and successfully supplanted competing LAN technologies, such as Token Ring.

Benefits:

• Simple to install and manage

• Inexpensive

• Flexible and scalable

• Easy to interoperate between vendors    

Ethernet Cabling Types:

• Coaxial cabling   

• Twisted-pair cabling

• Fiber optic cabling

Coaxial cable:

It often abbreviated as coax, consists of a single wire surrounded by insulation, a metallic shield, and a plastic sheath. The shield helps protect against electromagnetic interference (EMI), which can cause attenuation, a reduction of the strength and quality of a signal. Coax is commonly used to deploy cable television to homes and businesses.

Two types of coax were used historically in Ethernet networks:

• Thinnet

• Thicknet

Thicknet has a wider diameter and more shielding, which supports greater distances. However, it is less flexible than the smaller thinnet, and thus more difficult to work with. A vampire tap is used to physically connect devices to thicknet, while a BNC connector is used for thinnet.

Twisted-pair :

This cable consists of two or four pairs of copper wires in a plastic sheath. Wires in a pair twist around each other to reduce crosstalk, a form of EMI that occurs when the signal from one wire bleeds or interferes with a signal on another wire. Twisted-pair is the most common Ethernet cable. Twisted-pair cabling can be either shielded or unshielded. Shielded twisted pair is more resistant to external EMI.

 There are several categories of twisted-pair cable, identified by the number of twists per inch of the copper pairs:

• Category 3 or Cat3 - three twists per inch.

• Cat5 - five twists per inch.

• Cat5e - five twists per inch; pairs are also twisted around each other.

• Cat6 – six twists per inch, with improved insulation.

 An RJ45 connector is used to connect a device to a twisted-pair cable. The layout of the wires in the connector dictates the function of the cable. While coax and twisted-pair cabling carry electronic signals.

 fiber optics:

it uses light to transmit a signal. Ethernet supports two fiber specifications:

• Singlemode fiber – consists of a very small glass core, allowing only a single ray or mode of light to travel across it. This greatly reduces the attenuation and dispersion of the light signal, supporting high bandwidth over very long distances, often measured in kilometers.

• Multimode fiber – consists of a larger core, allowing multiple modes of light to traverse it. Multimode suffers from greater dispersion than single mode, resulting in shorter supported distances. Single mode fiber requires more precise electronics than multimode, and thus is significantly more expensive. Multimode fiber is often used for high-speed connectivity within a datacenter.

Communication Ethernet:

It was originally developed to support a shared media environment. This allowed two or more hosts to use the same physical network medium. There are two methods of communication on a shared physical medium:

• Half-Duplex – hosts can transmit or receive, but not simultaneously

• Full-Duplex – hosts can both transmit and receive simultaneously

Half-duplex connection:

Ethernet utilizes Carrier Sense Multiple Access with Collision Detect (CSMA/CD) to control media access. Carrier sense specifies that a host will monitor the physical link, to determine whether a carrier (or signal) is currently being transmitted. The host will only transmit a frame if the link is idle, and the Inter frame Gap has expired. If two hosts transmit a frame simultaneously, a collision will occur. This renders the collided frames unreadable. Once a collision is detected, both hosts will send a 32-bit jam sequence to ensure all transmitting hosts are aware of the collision. The collided frames are also discarded. Both devices will then wait a random amount of time before resending their respective frames, to reduce the likelihood of another collision. This is controlled by a backoff timer process. Hosts must detect a collision before a frame is finished transmitting, otherwise CSMA/CD cannot function reliably. This is accomplished using a consistent slot time, the time required to send a specific amount of data from one end of the network and then back, measured in bits. A host must continue to transmit a frame for a minimum of the slot time. In a properly configured environment, a collision should always occur within this slot time, as enough time has elapsed for the frame to have reached the far end of the network and back, and thus all devices should be aware of the transmission. The slot time effectively limits the physical length of the network – if a network segment is too long, a host may not detect a collision within the slot time period. A collision that occurs after the slot time is referred to as a late collision. For 10 and 100Mbps Ethernet, the slot time was defined as 512 bits, or 64 bytes.

Full-Duplex Communication:

Unlike half-duplex, full-duplex Ethernet supports simultaneously communication by providing separate transmit and receive paths. This effectively doubles the throughput of a network interface. Full-duplex Ethernet was formalized in IEEE 802.3x, and does not use CSMA/CD or slot times. Collisions should never occur on a functional fullduplex link. Greater distances are supported when using full-duplex over half-duplex. Full-duplex is only supported on a point-to-point connection between two devices. Thus, a bus topology using coax cable does not support full-duplex. Only a connection between two hosts or between a host and a switch supports full-duplex. A host connected to a hub is limited to half-duplex. Both hubs and half-duplex communication are mostly deprecated in modern networks.

Categories of Ethernet:

The original 802.3 Ethernet standard has evolved over time, supporting faster transmission rates, longer distances, and newer hardware technologies. These revisions or amendments are identified by the letter appended to the standard, such as 802.3u or 802.3z. Major categories of Ethernet have also been organized by their speed:

• Ethernet (10Mbps)

• Fast Ethernet (100Mbps)

• Gigabit Ethernet

• 10 Gigabit Ethernet

The physical standards for Ethernet are often labeled by their transmission rate, signaling type, and media type. For example, 100baseT represents the following:

• The first part (100) represents the transmission rate, in Mbps.

• The second part (base) indicates that it is a baseband transmission.

• The last part (T) represents the physical media type (twisted-pair).

Power over Ethernet (PoE):

Power over Ethernet (PoE) allows both data and power to be sent across the same twisted-pair cable, eliminating the need to provide separate power connections. This is especially useful in areas where installing separate power might be expensive or difficult. PoE can be used to power many devices, including:

• Voice over IP (VoIP) phones

• Security cameras

• Wireless access points

• Thin clients PoE