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DATA LINK LAYER.pdf

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DATA LINK LAYER.pdf

  1. 1. Data Link Layer Made By AKASH GHOSH(35001619025) TAPABRATA DAS(35001619033) BIVAS BISWAS(35001619034)
  2. 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.
  3. 3. 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.
  4. 4. 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.
  5. 5. Functions of Data Link Layer I. Flow Control II. Error control III. Framing IV. Physical Addressing V. Access Control
  6. 6. 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
  7. 7. 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.
  8. 8. Techniques of Flow Control 1.Stop and wait 2.Sliding Window Protocol Go-Back-N Selective Repeat Protocol
  9. 9. 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
  10. 10. 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.
  11. 11. 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.
  12. 12. 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.
  13. 13. 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.
  14. 14. 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
  15. 15. 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.
  16. 16. 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.
  17. 17. 3.Framming • What does? – It provides a way for a sender to transmit a set of bits that are meaningful to the receiver.
  18. 18. 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.
  19. 19. 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.
  20. 20. 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.
  21. 21. 5.Access Control
  22. 22. 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
  23. 23. • What does Access control do? – It is a mechanism that controls the access of stations to the transmission broadcast link.
  24. 24. • 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.
  25. 25. Access Control Methods- • Access control methods are the methods used for providing access control. • In Data link layer we used CSMA/CD

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