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In communications, the transmission of a unit of data from one node to another (OSI layer 2). It is responsible for ensuring that the bits received are the same as the bits sent. Following are the major categories:

Asynchronous Transmission
Originating from mechanical teletype machines, asynchronous transmission treats each character as a unit with start and stop bits appended to it. It is the common form of transmission between the serial port of a computer or terminal and a modem. ASCII, or teletype, protocols provide little or no error checking. File transfer protocols, such as Zmodem and Ymodem, provide data link services and higher-level services, collectively known as transport services.

Synchronous Transmission
Developed for mainframe networks using higher speeds than teletype terminals, synchronous transmission sends contiguous blocks of data, with both sending and receiving stations synchronized to each other. Synchronous protocols include error checking. Examples are IBM's SDLC, Digital's DDCMP, and the international HDLC.

LANs
Developed for medium to high transmission speeds between stations, LANs typically use collision detection (CSMA/CD) or token passing methods for transmitting data between nodes. Common examples are Ethernet, Token Ring and FDDI.

     The IEEE 802 specification for LANs breaks the data link layer into two sublayers: the LLC (Logical Link Control) and MAC (Media Access Control). The LLC provides a common interface point to the MAC layers, which specify the access method used. The following compares the data link layer in LANs to IBM's SNA and ISO's OSI model.