Networking Basics:
IEEE 802 Standards

Local Area Networks (LANs) differ, depending on which media access control (MAC) technique is used, which describes how devices share access to the LAN. A MAC technique is necessary, because every LAN uses broadcasting, which means that independent devices can start sending data at the same time. This would corrupt the data and therefore has to be prevented. There exists a set of standards to this issue laid down in the IEEE standard 802 for Local Area Networks. The standards differ at the physical layer and MAC sublayer of the OSI model, but they are compatible at the remainder of the data link layer. In standard 802.2 this part of the data link layer (Logical Link Control, provides addressing and controlling mechanisms) is described; 802.1 contains an introduction to the standards. The three major techniques for media access control are described in standard 802.3 to 802.5: CSMA/CD (Carrier Sense Multiple Access with Collision Detection), token bus and token ring.

CSMA/CD: when a device wants to send data, it first checks the channel if anyone else is sending. If it is busy, the device waits until it goes idle, otherwise it transmits immediately. If a collision happens (two or more devices begin to transmit at the same time), all involved devices stop their transmission, wait a random time and repeat the process again.

Token bus and token ring: Every device within the LAN knows about the address of the device to its "right" or "left". Then a special control data frame is passed from one device to the other. Whichever device holds this token is allowed to send data, afterwards it gives the token to its neighbour. Collisions can not occur. The difference between token bus and token ring is the physical layout of the network; the token is always passed around in a (virtual) circle.

HISTORY of IEEE

The first meeting of the IEEE Computer Society "Local Network Standards Committee", Project 802, was held in February of 1980. There was going to be one LAN standard, with speeds from 1to 20 MHz. It was divided into media or Physical layer , Media Access Control (MAC), and Higher Level Interface  The access method was similar to that for Ethernet, as well as thebus topology. By the end of 1980, a token access method was added, and a year later there were three MACs: CSMA/CD, Token Bus, and Token Ring. The unifying theme has been a common upper interface to the Logical Link Control (LLC) sublayer.