What is Ethernet – CSMA /CD – Introduction to OSI layer model – The internetworking Part3
If I go by history Ethernet was developed by XEROX to communicate between printer and a PC I might be wrong but I read it somewhere on the internet. Basically Ethernet is technology to connect various devices which are in close proximity .The devices could be printer, faxes, servers ,pc etc . All of these devices can be connected to a single link using Ethernet .which is a very good idea and its is light on the pocket too . The length of the Ethernet links are limited to few 100 meters only . So it is used to connect only local devices or devices that are few 100 meters apart .The main problem that arose while using Ethernet was collisions. The devices where sharing the same link so they were transferring data at the same time and frames were colliding so a technique called CSMA/CD (Career Sense Multiple Access with Collision Detection )was introduced .
Actually CSMA alone was introduced initially but CSMA/CD is the flavor of CSMA which is used these days. Now when a device is ready it senses the network whether there is an ongoing transmission or not. if there is no transmission is going on it starts transmitting. If there is a transmission going it waits for the channel to get free .Now if by any means a collision occurs a jam signal is sent to all the devices now the devices wait for some time to retransmit but still if collision occurs 15 times the nodes transmitting will time out and after that every node will have equal probability of transmission
The addressing in Ethernet takes place with MAC(medium access control) It is a 6 byte long or 48 bits. This address is of the physical Ethernet card or NIC(network information card ) which is installed on a system. This is called physical address as this address is programmed into the chip of network card.
The initial 24 bits are for the Vendor which identifies that the card was manufactured by which vendor.
The Ethernet frame.
The 64-bit preamble consists of alternating ones and zeros allowing the receiver to synchronise with the incoming signal. This is followed by the header consisting of a 48-bit destination address, a 48-bit source address, and a 16-bit frame type. The payload can vary in length from a minimum of 46 octets to a maximum of 1,500 octets. This is followed by a 32-bit CRC