 and five. So let's review the task of the Data Link Layer. It frames the packet SO that it can be checked for bit errors, provides various line control func- tions so that the network itself can be managed properly, provides addressing information so that the frame can be delivered appropriately; and performs error detection and (sometimes) correction. Practical Implementations A number of widely known network technologies are found at the Data Link Layer of the OSI Model. These include the access protocols used in modern local area networks, such Carrier Sense, Multiple Access with Collision Detection (CSMA/CD, used in Ethernet), and Token Ring. CSMA/CD is a protocol that relies on contention for access to the shared backbone over which all stations transmit, while Token Ring is more civ- ilized-stations take turns sharing access to the backbone. This is also the domain of frame relay and ATM technologies, which provide high- speed switching. Frame relay is a high-speed switching technology that has emerged as a good replacement for private line circuits. It offers a wide range of bandwidth and, while switched, delivers service quality that is equiva- lent to that provided by a dedicated facility. It is advantageous for the service provider to sell frame relay because it does not require the estab- lishment of a dedicated circuit, thus making more efficient use of net- work resources. The only downsides of the technology are that it relies on variable-size frames, which can lead to variable delivery delay, and requires careful engineering to ensure that proper quality of service is delivered to the customer. Frame relay offers speeds ranging from 56K to DS3, and is widely deployed internationally. ATM has become one of the most important technologies in the service provider pantheon today, because it provides the ability to deliver true, dependable, and granular quality of service over a switched-network architecture; thus, it gives service providers the ability to aggregate mul- Downloaded from Digital Enginee