In 1973, at Xerox Corporation’s Palo Alto Research Centre (more commonly known as PARC), researcher Bob Metcalfe designed and tested the first Ethernet network. While working on a way to link Xerox’s "Alto" computer to a printer, Metcalfe developed the physical method of cabling that connected devices on the Ethernet as well as the standards that governed communication on the cable. Ethernet has since become the most popular and most widely deployed network technology in the world. Many of the issues involved with Ethernet are common to many network technologies, and understanding how Ethernet addressed these issues can provide a foundation that will improve your understanding of networking in general.<br />The Ethernet standard has grown to encompass new technologies as computer networking has matured, but the mechanics of operation for every Ethernet network today stem from Metcalfe’s original design. The original Ethernet described communication over a single cable shared by all devices on the network. Once a device attached to this cable, it had the ability to communicate with any other attached device. This allows the network to expand to accommodate new devices without requiring any modification to those devices already on the network. It is known as DIX ethernet. It uses manchester encoding .IEEE has standardized it as 802.3.<br />TYPES OF ETHERNET<br />18078454293870<br />
Figure 1- types of ethernet
Fast Ethernet was designed to compete with LAN protocols such as FDDI (Fiber Distributed Data Interface) or Fiber Channel. IEEE created Fast Ethernet under the name 802.3u. Fast Ethernet is backward-compatible with Standard Ethernet, but it can transmit data 10 times faster at a rate of 100 Mbps. The topology used by the fast ethernet is shown in the figure.
Figure 2- topology used by fast ethernet
Fast ethernet is implemented by following cables:<br />15297151033145<br /> Figure 3- cables used by fast ethernet<br />9728203458210<br />Bit rate is reduced to 10 ns from 100 ns.<br />
The need for higher data rate resulted in the design of the Gigabit Ethernet (1000 Mbps). The IEEE committee calls the standard 802.3z. All configurations of gigabit Ethernet are point to point. Point-to-point, between two computers or one computer – to –switch. It supports two different modes of operation: full duplex mode and half duplex mode. Full duplex is used when computers are connected by a switch. No collision is there and so CSMA/CD is not used.
Figure 4- (a) point to point between two computers (b) point to point between switch and computer
Half duplex is used when computers are connected by a hub. Collision in hub is possible and so CSMA/CD is required. The 802.3z committee considered a radius of 25 meters to be unacceptable and added two new features to increase the radius. Carrier Extension tells the hardware to add its own padding bits after the normal frame to extend the frame to 512 bytes. Frame Bursting allows a sender to transmit a concatenated sequence of multiple frames in a single transmission. If the total burst is less than 512 bytes, the hardware pads it again.
The cables which are used by gigabit ethernet are as follows:
The heart of the system is a switch containing a high speed back-plane and room for typically 4 to 32 plug-in cards, each containing one to eight connectors. When a station wants to transmit a frame, it outputs a frame to switch. The plug-in card checks to see if the frame is for the other station on the same card. If so, it is copied there otherwise it is sent over high speed back-plane to destination station’s card.<br />
Figure 5- switched ethernet
What happens if two machines attached to the same plug-in card transmit frames at the same time? It depends in how the card is constructed. All ports on the same card are wired together to form a local on-card LAN. Collisions on this on-card LAN are detected and handled using CSMA/CD protocol. One transmission per card is possible at any instant. All the cards can transmit in parallel. With this design each card forms its own collision domain. In other design, each input port is buffered, so incoming frames are stored in the card’s on board RAM. It allows all input ports to receive (and transmit) frame at same time. <br />