The design issue of the Data Link Layer in the OSI Reference Model is

Concept

In the OSI reference model each layer owns a distinct set of design issues — the problems that layer alone is responsible for solving. The physical layer's design issues concern the signal itself: how a single bit is represented on the medium, and how the two ends stay in step while those bits flow. The data link layer is handed that raw, undifferentiated stream of bits and must turn it into a dependable link between two directly connected machines, so its design issues are delimiting the stream into discrete units, link addressing, error control and flow control.

Application

Of the four functions offered here, framing is the one that appears on the data link layer's list. Framing is the mechanism that decides where one unit of bits ends and the next begins, so the receiver can lift out whole, self-contained units instead of consuming an endless stream. The classical framing methods are a byte count carried in a header field, flag bytes with byte stuffing, a flag bit pattern with bit stuffing, and physical-layer coding violations.

Cross-check

Each remaining function is the recognised design issue of a different layer:

• Representation of bits — encoding a logical 1 or 0 as a voltage level, a current, or a light pulse through line codes such as NRZ, RZ or Manchester — is settled by the physical layer, because it is a property of the signal on the medium rather than of the link.

• Synchronization of bits — keeping the receiver's sampling instant aligned with the arriving signal, whether by a separate clock line or by recovering the clock from transitions in the encoding — is likewise a physical layer concern.

• Connection control — setting up, maintaining and releasing a logical association between two communicating parties — is standardly listed among the network and transport layer design issues; between two adjacent nodes the link itself already exists, so there is no such association to negotiate.

The data link layer design issue among the options offered is therefore framing.