Data Link Layer Definition

The data link layer is the second layer in the OSI (open systems interconnection) seven-layer reference model. It responds to service requests from the network layer above it and issues service requests to the physical layer below it.

The data link layer is responsible for encoding bits into packets prior to transmission and then decoding the packets back into bits at the destination. Bits are the most basic unit of information in computing and communications. Packets are the fundamental unit of information transport in all modern computer networks, and increasingly in other communications networks as well.

The data link layer is also responsible for logical link control, media access control, hardware addressing, error detection and handling and defining physical layer standards. It provides reliable data transfer by transmitting packets with the necessary synchronization, error control and flow control.

The data link layer is divided into two sublayers: the media access control (MAC) layer and the logical link control (LLC) layer. The former controls how computers on the network gain access to the data and obtain permission to transmit it; the latter controls packet synchronization, flow control and error checking.

The data link layer is where most LAN (local area network) and wireless LAN technologies are defined. Among the most popular technologies and protocols generally associated with this layer are Ethernet, Token Ring, FDDI (fiber distributed data interface), ATM (asynchronous transfer mode), SLIP (serial line Internet protocol), PPP (point-to-point protocol), HDLC (high level data link control) and ADCCP (advanced data communication control procedures).

The data link layer is often implemented in software as a driver for a network interface card (NIC). Because the data link and physical layers are so closely related, many types of hardware are also associated with the data link layer. For example, NICs typically implement a specific data link layer technology, so they are often called Ethernet cards, Token Ring cards, etc. There are also several types of network interconnection devices that are said to operate at the data link layer in whole or in part, because they make decisions about what to do with data they receive by looking at data link layer packets. These devices include most bridges and switches, although switches also encompass functions performed by the network layer.

Data link layer processing is faster than network layer processing because less analysis of the packet is required.

Created October 16, 2005.
Copyright © 2005 The Linux Information Project. All Rights Reserved.