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.
|
