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The link level is one of the lowest, most fundamental conceptual levels in digital communications. Basically, it's where all the logic for dealing with a data link resides. It operates in something of a hierarchy, acting as an interface, in this case, between the lowest level, the physical level that transmits the raw stream of ones and zeros, and the upper layers. In many cases, the term is used interchangeably with "link layer," which typically signifies the open systems interconnection (OSI) model's second layer, the data link layer, used in computer networking. There are numerous communication protocols that operate at the link level, but its essential functions are to prepare the data packets intended for transport as well as interpret any incoming via the data link.
There are three types of data links that a link level has to deal with: simplex, half-duplex, and full duplex. With a simplex link, the data travels in one direction, such as a broadcast network, where there is a dedicated sender and recipient, and the recipient doesn't need to send anything back to the sender. With half-duplex, the data can go both ways, but not at the same time. Full-duplex communications allow for data to travel in both directions simultaneously, which requires more effort on behalf of the link level to sort out the communications that are coming and going.
To do much of its work, the link level uses a technique known as framing. This involves tacking on an extra identifier which indicates where the frame begins or ends in the bit stream. Though other framing methods exist, in most cases, this is simply an extra bit added into the stream during specific increments. On the receiving end, the link layer synchronizes the framing bits in the stream to help separate out the frames, pull out the original packets, and pass them up the other layers as necessary. The synchronization between the sending and receiving ends is important, because if the receiving link layer happens to pick up the stream between frames, it can simply wait until the next frame begins, discarding any unusable bits that don't belong to a frame.
The OSI model's data link layer further perceives two sub-layers to the link layer. One is referred to as the logical link control (LLC), while the other is the media access control (MAC). The upper, LLC sub-layer deals with issues such as flow control and fixing errors in the transmission. Depending on the type of communication, some error correction methods may not be employed. For example, with wireless networking, the link level has the ability to request erroneous packets be sent again, which is much more rare in wired communications where the link layer only deals with detecting errors and canceling bad packets.
The lower MAC sub-layer is then responsible for identifying the physical address of the device, commonly referred to as the MAC address. It is also capable of maintaining any queuing of the data packets, as well as scheduling their delivery and ensuring the quality of the transmission. This is also where the frame synchronization takes place, as well as protocols that keep the streams from colliding.