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This website is intended as a comprehensive study of the main areas of the
Data Link Layer and its role and overall importance in the Open Systems
Interconnection or OSI
model.
The Data Link Layer is situated below the Network layer and above the
Physical layer in the OSI model. It is compromised of five main areas -
Packetising, Addressing, Error Control, Flow control and Media Access
Control - three of which will be studied in detail.
The main duty of the Data Link Layer is the delivery of frames from
one node to the next. When data is moved through a LAN or a WAN, it must be
encapsulated in the appropriate packet which is defined by the protocols of
the particular LAN or WAN. The data provided is usually in the form of
addresses which enable the data to move from one hop to the next.
When data is being moved through a network, errors can sometimes
occur due to interference (electrical or thermal). These errors need to be
detected and corrected, and later we will look at the various methods for
doing this.
Another aspect of the Data Link Layer is Flow Control. Forouzan Has
the following to say about this "In most protocols, flow control is a set of
procedures that tells the sender how much data it can transmit before it
must wait for an acknowledgment from the receiver. The flow of data must
not be allowed to overwhelm the receiver"* As you can probably see, Flow
Control is essentially about ensuring that the reciever of information is
not overwhelmed and can handle the amount of data being sent to it.
Medium Access Control is the final area of the Data Link Layer that
we will be examining. When two or more devices share the same connection,
procedures need to be put in place to ensure that control of the medium is
regulated. Problems could occur if two or more devices tried to send data at
the same time. Medium Access Control (MAC) is a protocol that essentially
takes care of this. It lays out what the computer should do when it needs to
send a frame or frames.
What follows is an examination of three of the main areas of the Data Link
Layer - Error Detection and Control, Flow Control and Media Access Control.
Examples will be provided throughout, along with illustrations.
On With The Show!
*Data Communications and Networking (Third Edition) -
Behrouz A. Forouzan