#data link layer #tcp/ip protocol #computer network

1. Data Link
Layer
Made By
AKASH GHOSH(35001619025)
TAPABRATA DAS(35001619033)
BIVAS BISWAS(35001619034)

2. OSI MODEL
The open systems interconnection (OSI) model is
a conceptual model created by the International
Organization for Standardization which enables
diverse communication systems to communicate
using standard protocols.

4. DATA LINK LAYER
The Data-link layer is the second layer from the bottom in the OSI (Open
System Interconnection) network architecture model. It is responsible for
the node-to-node delivery of data. Its major role is to ensure error-free
transmission of information. DLL is also responsible to encode, decode
and organize the outgoing and incoming data. This is considered the most
complex layer of the OSI model as it hides all the underlying complexities
of the hardware from the other above layers.

5. Sub-layers of Data Link Layer:
The data link layer is further divided into two sub-layers, which
are as follows:
Logical Link Control (LLC):
This sublayer of the data link layer deals with multiplexing,
the flow of data among applications and other services,
and LLC is responsible for providing error messages and
acknowledgments as well.
Media Access Control (MAC):
MAC sublayer manages the device’s interaction,
responsible for addressing frames, and also controls
physical media access.
The data link layer receives the information in the form of
packets from the Network layer, it divides packets into
frames and sends those frames bit-by-bit to the underlying
physical layer.

6. Functions of Data Link Layer
I. Flow Control
II. Error control
III. Framing
IV. Physical Addressing
V. Access Control

7. 1.Flow
Control
• Flow control is a technique that allows two stations working at
different speeds to communicate with each other.
• It is a set of measures taken to regulate the amount of data
that a sender sends so that a fast sender does not overwhelm
a slow receiver

8. Feedback Based Flow Control
In feedback-based flow control the receiver provides feedback to the sender
about whether the receiver can receive the packets at the rate at which the
sender is sending the packets.
Rate-based Flow Control
In the rate-based flow control approach limit is put on the rate at which the
sender can send the packet to the receiver.

9. Techniques
of Flow
Control
1.Stop and
wait
2.Sliding Window Protocol
Go-Back-N
Selective Repeat
Protocol

10. Stop and Wait Protocol
• In this method, basically message or data is broken
down into various multiple frames, and then receiver
indicates its readiness to receive frame of data. When
acknowledgement is received, then only sender will
send or transfer the next frame

11. Sliding Window Protocol
• In this method, sender transmits or sends various frames or
packets before receiving any acknowledgement.
• Both the sender and receiver agree upon total number of
data frames after which acknowledgement is needed to be
transmitted
• Link Layer requires and uses this method that simply allows
sender to have more than one unacknowledged packet can
go at a time.

12. Go-Back-N
• Go – Back – N ARQ provides for sending multiple frames before
receiving the acknowledgment for the first frame. The frames are
sequentially numbered and a finite number of frames.
• If the acknowledgement of a frame is not received within a certain
period of time, all the frames present in the current window will be
transmitted.
• Here sender window size is N and receiver window size is 1.

13. Selective Repeat
• In Selective Repeat ARQ ,only the lost frames are
retransmitted .
• The receiver while keeping track of sequence numbers, buffer
the frames in memory and send negative acknowledgement
for only one frame which is missing or damaged.

14. 2.Error
Control
• When bits are transmitted over the computer network, they are
subject to get corrupted due to interference and network problems.
The corrupted bits leads to spurious data being received by the
destination and are called errors
• Data-link layer uses the techniques of error control simply to ensure
and confirm that all the data frames or packets, are transmitted or
transferred from sender to receiver with certain accuracy.

15. Types of Error
 Single bit error − In the received frame, only one bit has been
corrupted, i.e. either changed from 0 to 1 or from 1 to 0.
 Multiple bits error − In the received frame, more than one bits are
corrupted.
 Burst error − In the received frame, more than one consecutive bits are
corrupted

16. Error Detection Techniques
 Parity Check: The parity check is done by adding an extra bit, called parity bit to the
data to make a number of 1s either even in case of even parity or odd in case of odd
parity.
• In case of even parity: If a number of 1s is even then parity bit value is 0. If the number of 1s is odd
then parity bit value is 1.
• In case of odd parity: If a number of 1s is odd then parity bit value is 0. If a number of 1s is even then
parity bit value is 1.
 Check sum: Data is divided into fixed sized frames or segments. The sender adds the
segments using 1’s complement arithmetic to get the sum. It then complements the
sum to get the checksum and sends it along with the data frames. The receiver adds
the incoming segments along with the checksum using 1’s complement arithmetic to
get the sum and then complements it.If the result is zero, the received frames are
accepted; otherwise, they are discarded.
 Cyclic Redundancy Check (CRC):Cyclic Redundancy Check (CRC) involves binary
division of the data bits being sent by a predetermined divisor agreed upon by the
communicating system.
• The receiver divides the incoming data unit by the divisor. If there is no remainder, the data unit is
assumed to be correct and is accepted. Otherwise, it is understood that the data is corrupted and is
therefore rejected.

18. Error Control Techniques
Stop and Wait ARQ
 A timeout counter is maintained by the sender, which is started when a frame is sent.
 If the sender receives acknowledgment of the sent frame within time, the sender is confirmed about
successful delivery of the frame. It then transmits the next frame in queue.
 If the sender does not receive the acknowledgment within time, the sender assumes that either the
frame or its acknowledgment is lost in transit. It then retransmits the frame.
 If the sender receives a negative acknowledgment, the sender retransmits the frame.
Go-Back-N ARQ
 The sender has buffers called sending window.
 The sender sends multiple frames based upon the sending-window size. The receiver receives frames
one by one. It keeps track of incoming frame’s sequence number and sends the corresponding
acknowledgment frames.
 If the sender has received positive acknowledgment for all the frames, it sends next set of frames.
 If sender receives NACK or has not receive any ACK for a particular frame, it retransmits all the
frames after which it does not receive any positive ACK.
Selective Repeat ARQ
 Both the sender and the receiver have buffers called sending window and receiving window
respectively.
 The receiver keeps track of incoming frame’s sequence numbers, buffers the frames in memory.
 t sends ACK for all successfully received frames and sends NACK for only frames which are missing
or damaged.

19. 3.Framming
• What does?
– It provides a way for a sender to transmit a set of
bits that are meaningful to the receiver.

20. Parts of a Frame
• Frame Header − It contains the source and the
destination addresses of the frame.
• Payload field − It contains the message to be delivered.
• Trailer − It contains the error detection and error
correction bits.
• Flag − It marks the beginning and end of the frame.

21. Types of Framing
• Fixed-sized Framing
Here the size of the frame is fixed and so the
frame length acts as delimiter of the frame. Consequently,
it does not require additional boundary bits to identify the
start and end of the frame.
• Variable – Sized Framing
Here, the size of each frame to be transmitted
may be different. So additional mechanisms are kept to
mark the end of one frame and the beginning of the next
frame.

22. 4.Physical Addresing
• What does?
– If frames are to be distributed to different stations on
the network. To define the physical address of the
sender (source address) and/or receiver (destination
address) of the frame.

23. 5.Access Control

24. Types of Communication Links
• Point to Point Link
– Point to Point link is a dedicated link that exists between
the two stations.
• Broadcast Link
– Broadcast link is a common link to which multiple
stations are connected

25. • What does Access control do?
– It is a mechanism that controls the access of stations to
the transmission broadcast link.

26. • Need of Access Control
– To prevent the occurrence of collision or if the collision
occurs, to deal with it.
• Two stations A and D starts transmitting their data packets
simultaneously.
• This situation gives rise to a collision between the data packets
transmitted by them.
• Thus, to prevent the collision or to deal with it, access control is
needed.

27. Access Control Methods-
• Access control methods are the methods used for
providing access control.
• In Data link layer we used CSMA/CD