The physical layer provides only a raw bit stream. The data link layer attempts to make the physical link reliable and provides the means to activate, maintain, and deactivate the link. The principle services provided by the data link layer to higher layers are those of error detection and error control. Thus, with a fully functional data link layer protocol, the next higher layer may assume error-free transmission over the link.
The approach taken to provide reliability is to communicate control information as well as data over the link. Data are transmitted in blocks, called frames. Each frame consists of a header, a trailer, and an optional data field. The header and trailer contain information used to manage the link.
HDLC is an ISO standard that is commonly used on computer to computer links. Normal response mode is used for point to point links and also for multipoint links. The latter are typified by a line that connects a number of terminals to a computer. There are one primary station and one or more secondary stations. The primary station is responsible for initiating activity, controlling the flow of data to and from secondary stations, recovering from errors, and logically disconnecting secondary stations. A secondary station may transmit only in response to a poll from the primary. This mode is ideally suited for a host computer supporting a number of terminals.
Asynchronous response mode is similar to the normal response mode, but a secondary station may initiate a transmission without being polled by the primary station. This mode is useful in certain types of loop configurations. Asynchronous balanced mode is used only on a point to point link. Each station assumes the role of both the primary and secondary stations. This mode is more efficient for point to point links because there is no polling over head and both stations may initiate transmission.
Address indicates the secondary station for this information for this transmission. It is needed in the case of a multipoint line; a primary may send to one of a number of secondaries and one of number secondaries may send to the primary.
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Control identifies the function of the frame. Information field contains the user data provided by the next higher layer. Cyclic redundancy check is a function of the contents of the address, control, and data fields. It is generated by the sender and again by the receiver.
