Data Link Control
In this, we will be covering one of the important concepts of the Data link layer i.e
Data Link Control.
There are two main functions of the Data link layer and these are Data Link
Control and Media Access control.
Data link control mainly deals with the design and procedure of communication
between two adjacent nodes: node-to-node communication.
Media access control is another main function of the Data Link layer which mainly
specifies how the link is shared.
Let us first cover Data link control and then we will move on to Media access control.
Functions in Data Link Control
The functions included in data link control are:
Framing
Flow and Error Control
Software Implemented protocols (that provides smooth and reliable transmission of
frames between nodes.)
Framing
In the Physical layer, data transmission means moving bits are in the form of a signal from the
source to the destination. The Physical layer also provides synchronization that mainly ensures
that the sender and the receiver use the same bit durations and timings.
The bits are packed into the frames by the data link layer; so that each frame is distinguishable
from another frame.
The Framing in the data link layer separates a message from one source to a
destination or from other messages to other destinations just by adding a sender address and
destination address; where the destination address specifies where the packet has to go and the
sender address helps the recipient to acknowledge the receipt.
Frames can be either of fixed size or of variable size. By using frames the data can be easily get
broken up into recoverable chunks and in order to check the corruption in transmission, these
chunks can be checked easily.
Problems in Framing
Given below are some of the problems caused due to the framing:
1. Detecting the start of the frame:
Whenever a frame is transmitted then every station must be able to detect this frame. Any
Station detects the frame by looking out for a special sequence of bits that are marked at
the beginning of the frame that is Starting Frame Delimiter (SFD).
2. How any station detects a frame:
Every station in the network listens to the link for the SFD pattern through the sequential
circuit. If an SFD is detected then the sequential circuit alerts the station. After that, the
Station checks the destination address in order to accept or reject the frame.
3. Detecting the end of the frame:
It is when to stop reading the frame.
Parts of a frame
Different parts of a frame are as follows:
1. Flag
A flag is used to mark the beginning and end of the frame.
2.Header
The frame header mainly contains the address of the source and the destination of the frame.
3.Trailer
The frame trailer mainly contains the error detection and error correction bits.
4.Payload Field
This field contains the message to be delivered.
Types of Framing
Framing is mainly categorized into two parts:
Fixed-size Framing
Variable-size Framing
Let us cover the above given two types one-by-one;
Fixed-size framing
In the fixed-size framing, there is no need for defining the boundaries of the frame. The size or
length of the frame itself can be used as a delimiter.
One drawback of fixed size framing is that it will suffer from Internal fragmentation if the size of
data is less than the size of the frame.
Variable-size framing
In Variable-size framing, the size of each frame is different. Thus there is a need of the way in
order to define the end of the frame and the beginning of the next.
There are two approaches used for Variable-size framing:
Character-Oriented Protocols
In the Character-Oriented protocol, data to be carried are 8-bit characters from a coding system
such as ASCII.
Now the parts of the frame in Character-Oriented Framing are as follows:
1.Frame Header
The header of the frame contains the address of the source and destination in the form of bytes.
2.Payload Field
The Payload field mainly contains the message that is to be delivered. In this case, it is a variable
sequence of data bytes.
3.Frame trailer
The trailer of the frame contains the bytes for error correction and error detection.
4.Flag
In order to separate one frame from the next an 8-bit(that is 1 byte) flag is added at the beginning
and end of the frame.
Let us take a look at the frame in Character-Oriented Protocol:
Bit-Oriented Protocols
In Bit-oriented framing mainly the data section of the frame is a sequence of bits that are to be
interpreted by the upper layer as text, graphics, audio, video, etc.
In this, there is also a need for a delimiter in order to separate one frame from the other.
Let us take a look at the frame in Bit-oriented Protocol:
