2. Introduction
• Data link layer is responsible for something called Framing, which is the division of a stream of bits
from the network layer into manageable units (called frames).
• Each frame consists of the sender’s address and a destination address. The destination address
defines where the packet is to go and the sender’s address helps the recipient acknowledge the
receipt.
• Frames could be fixed size or variable size.
• In fixed-size framing, there is no need for defining the boundaries of the frames as the size itself can
be used to define the end of the frame and the beginning of the next frame.
• But, in variable-size framing, we need a way to define the end of the frame and the beginning of the
next frame.
3. Introduction
• To separate one frame from the next, an 8-bit (or 1-byte) flag is
added at the beginning and the end of a frame.
• But the problem with that is, any pattern used for the flag could
also be part of the information. So, there are two ways to
overcome this problem:
1. Using Byte stuffing (or character stuffing)
2. Using Bit stuffing
4. Comparison
Feature Byte stuffing Bit stuffing
Definition
A technique used to prevent the occurrence of a specific byte in a
data stream by adding an extra byte.
A technique used to prevent the occurrence of a specific bit
sequence in a data stream by inserting an additional bit.
Concept Adds an extra byte to the data when a special byte is found. Adds an extra bit to the data when a special bit pattern is found.
Special byte/bit Flag byte (01111110) Flag bit pattern (01111110)
Insertion Extra byte is inserted after each occurrence of the flag byte. Extra bit is inserted after each occurrence of the flag bit pattern.
Removal The extra byte is removed at the receiver end. The extra bit is removed at the receiver end.
5. Comparison
Feature Byte stuffing Bit stuffing
Efficiency Less efficient due to the addition of an extra byte. More efficient as only an extra bit is added.
Applications Used in protocols like PPP and HDLC. Used in protocols like Ethernet and Token Ring.
Objective
To ensure data integrity and prevent data loss due to errors caused by
the presence of a specific byte in the data stream.
To ensure data integrity and prevent data loss due to errors
caused by the presence of a specific bit sequence in the data
stream.
How it works
The specific byte is replaced with a unique escape byte, followed by a
second byte that indicates the original byte’s value.
The specific bit sequence is replaced with a unique bit sequence
that indicates the original bit sequence’s value.
6. Byte stuffing
• It is a byte (usually escape character(ESC)), which has a predefined bit pattern is added to
the data section of the frame when there is a character with the same pattern as the flag.
• Whenever the receiver encounters the ESC character, it removes it from the data section
and treats the next character as data, not a flag.
• But the problem arises when the text contains one or more escape characters followed by
a flag.
• To solve this problem, the escape characters that are part of the text are marked by
another escape character i.e., if the escape character is part of the text, an extra one is
added to show that the second one is part of the text.
8. Bit stuffing
• Mostly flag is a special 8-bit pattern “01111110” used to define the beginning and the end of the
frame.
• Problem with the flag is the same as that was in the case of byte stuffing.
• So, in this protocol what we do is, if we encounter 0 and five consecutive 1 bits, an extra 0 is added
after these bits.
• This extra stuffed bit is removed from the data by the receiver.
• The extra bit is added after one 0 followed by five 1 bits regardless of the value of the next bit.
• Also, as the sender side always knows which sequence is data and which is flag it will only add this
extra bit in the data sequence, not in the flag sequence.
10. • Byte stuffing: It is used to convert a message format of a sequence of
bytes that may contain reserved values into another byte sequence that
does not contain reserved values. It is also known as character-oriented
framing. Here, a special byte is stuffed before flag and esc also that special
byte is escape(ESC).
• Bit stuffing: It is used for inserting one or more non-information bits into
a message to be transmitted, to break message sequence for
synchronization. It is also known as bit-oriented framing. Here,0 bit stuffed
after five consecutive 1 bits.i.e extra bit is added after five consecutive
ones.
