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data link protocol

In networking and communications, the transmission of a unit of data (frame, packet) from one node to another. Known as a "layer 2 protocol," the data link protocol is responsible for ensuring that the bits and bytes received are identical to the bits and bytes sent. For example, in a local network, if a message is split into 100 packets and a station receives 97, the data link protocol ensures those 97 are error free, but it is not aware of the three missing ones. TCP and other higher-layer protocols make sure all 100 are delivered (see TCP/IP). Following are major types:

Asynchronous Transmission
Originating from mechanical teletypes, each byte is treated as a separate unit with start and stop bits tacked on. It is the common form of transmission between a computer and an analog modem, and there is often only minimal error checking. See modem.

Synchronous Transmission
Developed for mainframe terminals, synchronous transmission sends contiguous blocks of data, with sending and receiving stations synchronized to each other's timing. Error checking is included. Examples are IBM's SDLC, the international HDLC and Digital Equipment's DDCMP (DECnet). See SDLC, HDLC and DDCMP.

Local Networks (LANs)
Designed for higher transmission speeds, Ethernet and Wi-Fi networks use methods that sense the presence of a carrier and include error checking (see CSMA/CD and CSMA/CA).

The Bottom Layers
In Ethernet and Wi-Fi networks, the data link layer is split into two sublayers. The Logical Link Control (LLC) is an interface to the Media Access Control (MAC) layer. The MAC is also hardware, because the functions are embedded in the transceiver chips. See IEEE 802, SNA, OSI model, Token Ring and MAC layer.

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