Ethernet is the most commonly used local area network (LAN) technology, but the IEEE802 committee has since defined a standard for it. The most common of which are Ethernet 2.0 and IEEE802.3. Ethernet was originally developed by Xerox and uses a co-axial cable or special grades of twisted pair wires for its LAN.
Ethernet in it self cannot make a network and need some other protocol such as TCP/IP to allow nodes to communicate.
The advantages and disadvantages are as specified below.
Ethernet uses a share-media, bus-type network topology where all nodes share a common bus. These nodes must contend for access to the network as only one node can communicate at a time. Data is then transmitted in frames which contain the MAC (media access control) source and destination address of the sending and receiving node respectively.
Ethernet uses a carrier sense, multiple access with collision control detection (CSMA/CD). Nodes monitor the bus (ether) to determine if it is busy. A node wishing to send data waits for an idle condition then transmit its message.
Collision can occur when two nodes transmit at the same time, thus nodes must monitor the cable when they transmit.
When a collision occurs, both nodes stop transmitting frames and transmit a jamming signal. This informs all nodes on the network that a collision has occurred, each of the nodes involved in the collision then waits a random period of time before attempting a re-transmission, this is called ‘back off’ .
As each node has a random delay time, then there can be a prioritization of the nodes on the network.
PACKET BROADCASTING
The method of communication in CSMA/CD networks is by broadcasting packets of data on the transmission medium. All stations are continuously ‘listening’ to the medium for packets that are addressed to them. Any station wishing to transmit a message broadcasts one or more packets (called frames in the Ethernet specification) on the medium. Each packet contains the address of the destination station, the address of the sending station and a variable-length sequence of bits representing the message to be transmitted. Data transmission proceeds at 10Mbps ( or at the higher speeds specified for 100 and 1000Mbps Ethernets) and packets vary in length between 64 and 1518 bytes, so the time to transmit a packet on a 10 Mbps Ethernet is 50-1200 microseconds, depending on its length. The MTU is specified as 1518 bytes in the IEEE standard, although there is no technical reason for any particular fixed limit except the need to limit delays caused by contention.
The address of the destination station normally refers to a single network interface. Controller hardware at each station receives a copy of every packet. It compares the destination address in each packet with a wired-in local address, ignoring packets addressed to other stations and passing those with a matching address to the local host. The destination address may also specify a broadcast or a multicast address.
Ordinarily addresses are distinguished from broadcast and multicast addresses by their higher-order bit (0 and 1, respectively). An address consisting of all 1’s is reserved for use as a broadcast address and is used when message is to be received by all of the stations on the network. This is used, for example, to implement the ARP IP address resolution protocol. Any station that receives a packet with a broadcast address will pass it on to its local host. A multicast address specifies a limited form of broadcast that is received by a group of stations whose network interfaces have been configured to receive packets with that multicast address. Not all implementations of Ethernet network interfaces can recognize multicast addresses.
The Ethernet network protocol (providing for the transmission of Ethernet packets between pairs of hosts) is implemented in the Ethernet hardware interface; protocol software is required for the transport layer and those above it.
ETHERNET PACKET LAYOUT
The packets (or more correctly, frames) transmitted by stations on the Ethernet have the following layout:
bytes: 7 1 6 6 2 46-> length->1500 4
| Preamble | S | Destination Address | Source Address | Length of data | Data for Transmit ion | Checksum |
Apart from the destination and source addresses already mentioned, frames include a fixed 8-byte prefix, a length field, a data field and a checksum. The prefix is used for hardware timing purposes and consists of a preamble of seven bytes, each containing the bit pattern 10101010 followed by a single-byte start frame delimiter (S in the diagram) with the pattern 10101011.
For details check the following websites: http://www.rad.com//1997/nettut/ethernet.html http://inews.webopedia.com/TERM/E/Ethernet.html
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