A Star Network An ATM network is formed from a switch to which multiple computers are attached. The connection between a computer and an ATM switch consists of a pair of fibers, one carrying data in each direction. The hub does not communicate the data to any other computer other than the communicating pair. Typical connection between a computer and an ATM is at 100 Mbps therefore it often carries optical fibre. To allow the sender and receiver to co-ordinate and to ensure all computers that share a network resource have fair, prompt access, most computer networks divide data into small blocks called packets .
Computers take turns sending packets over the shared resource.
Because each packet is small, no computer experiences a long delay which waiting for access. Networks often use Time-Division Multiplexing to achieve the above. There is no universal agreement on the format of packets, Each set of hardware must agree on the format and conditions for transferring packets. Specific formats of packets determined by the hardware is referred to as frames. If the data being carried in the frame does not include all possible values, the network system can choose two unused values and use them to mark the beginning and end of a packet.