Local area networks
Cells
- Asynchronous Transfer Mode
Notes:
The first round of local area networks were designed when communications links and computing power were both expensive. It made sense to concentrate the computing on one big machine and to link quantities of terminals to the central machine through intermediate communication controllers. The IBM 3270 family of terminals and controllers typified this hub and spoke approach; there are still many systems of this kind in use.
Local networks based on the notion of every device having an equal chance of access to the communication line are called token ring networks. Ethernet takes a different approach: each device on the net determines whether the communications line is available at the instant before sending a message to another device. Ethernets operate at line speeds up to 100 million bits per second; engineers are working on still higher speeds.
The data on an Ethernet is assembled into groups of bits called packets which contain not only the message content but also the senders address, the recipients address, unique serial numbers, error-correcting codes and more. X.25 and Frame Relay are two forms of networks explicitly based on packet concepts.
Asynchronous Transfer Mode, ATM for short, is similar but distinct. All groupings of bits are exactly the same length, which makes for ease in handling at the device level, and the groupings (or cells) additionally identify the kind of data: voice, video or pure data.