Data Link Layer and Sub-layer, Protocol (ET: 1.5 hours)
Topic Wise Question Bank
Data Link Layer
    • 1
      Data Link Layer Data link Layer Services
      Introduction to Data Link Layer Services
      Data Link Layer
      Data Link Layer: The Data Link Layer is the 2nd layer of the OSI Model. It is also called the Link Layer. It receives a packet from the Network Layer and adds a header and trailer to create a Frame. Its main responsibility is to provide reliable node-to-node communication.
      Five Main Services of the Data Link Layer:
      1. Framing
      2. Physical Addressing
      3. Flow Control
      4. Error Control
      5. Access Control
      1. Framing: Framing is the process of dividing the stream of bits received from the Network Layer into manageable units called Frames. It helps the receiver distinguish one frame from another. The Data Link Layer adds a header and trailer to the Network Layer packet to form a frame.
      Example: Just as an envelope separates one letter from another, framing separates one data unit from another.
      2. Physical Addressing: The Data Link Layer adds the Source MAC Address and Destination MAC Address to the frame. MAC addresses are physical addresses used for communication between devices on the same network or link.
      3. Flow Control: Flow control is a speed-matching mechanism between a sender and a receiver. It prevents a fast sender from overwhelming a slow receiver. The sender controls the amount of data it sends based on the receiver's ability to process the data.
      Example: If a receiver can handle only 10 packets at a time, the sender should not continuously send 100 packets without allowing the receiver to process them.
      4. Error Control: Error control deals with errors that may occur while a frame is being transmitted. The Data Link Layer can use Error Detection and, in some systems, Error Correction mechanisms. If an error is detected, the frame may be discarded or retransmitted depending on the protocol.
      5. Access Control: Access control determines which device can use a shared communication medium at a particular time. It helps prevent or manage collisions when multiple devices want to transmit data through the same medium. This is commonly associated with Media Access Control (MAC).

    • 2
      Data Link Layer Data link SubLayer Services
      Sublayer of Data Link Layer ( Logical Link Control, MAC Sublayer)
      Data Link Layer
      Data Link Layer is the Layer 2 of the OSI Reference Model. The Data Link Layer is divided into two sublayers:
      1. Logical Link Control (LLC) Sublayer
      2. Media Access Control (MAC) Sublayer
      The two sublayers work together to provide services such as flow control, framing, physical addressing, error control, and media access control.
      Data Link Layer Structure

      1. Logical Link Control (LLC) Sublayer
      The Logical Link Control (LLC) is the upper sublayer of the Data Link Layer. It provides communication between the Network Layer and the lower Data Link Layer functions.
      The LLC sublayer receives the Network Layer PDU (Packet) and adds necessary control information to support reliable communication and Data Link Layer services.
      Major Responsibility of LLC
      • Provides an interface between the Network Layer and Data Link Layer.
      • Handles Flow Control.
      • Adds necessary control information to the data received from the Network Layer.
      • Helps coordinate communication between upper-layer protocols and Data Link Layer functions.
      Important Point: Among the five commonly discussed Data Link Layer services, Flow Control is associated with the LLC/DLC sublayer in this model.
      2. Media Access Control (MAC) Sublayer
      The Media Access Control (MAC) is the lower sublayer of the Data Link Layer. It is located directly above the Physical Layer and interacts directly with the physical transmission medium.
      The MAC sublayer is commonly implemented in hardware, particularly as part of the Network Interface Card (NIC).
      The MAC sublayer has two major responsibilities:
      1. Data Encapsulation
      2. Media Access Control
      2.1 Data Encapsulation
      Data encapsulation is the process of preparing the Network Layer packet for transmission over the physical medium.
      The MAC sublayer adds a Header and a Trailer to the Network Layer PDU. As a result, the Data Link Layer PDU becomes a Frame.
      Packet + MAC Header + MAC Trailer = Frame
      The MAC sublayer performs two important operations:
      • Frame Assembly: At the sender side, the MAC sublayer constructs a frame by adding the required header and trailer.
      • Frame Disassembly: At the receiver side, the MAC sublayer receives the frame and removes the Data Link Layer header and trailer to extract the packet.
      2.2 Primary Functions of MAC Sublayer
      The MAC sublayer performs the following important Data Link Layer functions:
      1. Framing — Converts the Network Layer packet into a Data Link Layer frame by adding a header and trailer.
      2. Physical Addressing / MAC Addressing — Uses MAC addresses to identify devices on the local network.
      3. Error Control — Provides mechanisms for detecting transmission errors, commonly using techniques such as FCS/CRC in many frame formats.
      4. Media Access Control — Determines how devices access and use a shared transmission medium.
      Responsibilities of LLC and MAC Sublayers
      Data Link Layer ServiceResponsible SublayerShort Explanation
      Flow ControlLLCControls the rate of data transmission between sender and receiver.
      FramingMACCreates frames from Network Layer packets.
      Physical AddressingMACUses source and destination MAC addresses.
      Error ControlMACHelps detect transmission errors in frames.
      Access ControlMACControls access to the shared transmission medium.
      LLC vs MAC Sublayer
      FeatureLLC SublayerMAC Sublayer
      Full FormLogical Link ControlMedia Access Control
      PositionUpper sublayerLower sublayer
      Located AboveNetwork Layer functionsLLC
      Located BelowMACPhysical Layer
      Main ResponsibilityFlow ControlFraming, Physical Addressing, Error Control and Media Access Control
      Physical Layer InteractionIndirectDirect
      Hardware ImplementationGenerally associated with logical functionsCommonly implemented in NIC hardware
      How Data Moves Through the Data Link Sublayers
      When a packet comes from the Network Layer, it first reaches the Data Link Layer. The LLC sublayer handles the logical link-control functions and flow-control aspects. The data then reaches the MAC sublayer, where the packet is encapsulated into a frame by adding the required header and trailer.
      The MAC sublayer also handles MAC addressing, error detection, and access to the physical medium. Finally, the frame is passed to the Physical Layer for transmission as signals over the communication medium.
      At the receiving side, the Physical Layer receives the signal and passes the received frame to the MAC sublayer. The MAC sublayer processes the frame and removes the Data Link Layer encapsulation. The resulting packet is then passed upward toward the Network Layer.
      Simple Flow:
      Network Layer → LLC → MAC → Physical Layer → Transmission Medium
      Receiver Side:
      Transmission Medium → Physical Layer → MAC → LLC → Network Layer

    • 3
      Data Link Layer DLL Protocol
      HDLC (High-Level Data Link Control), Bit Stuffing
      Data Link Layer
      HDLC (High-Level Data Link Control)
      HDLC is a bit-oriented Data Link Layer protocol used for reliable communication over data links. It was standardized by ISO and was based on IBM's SDLC (Synchronous Data Link Control).
      Bit-Oriented Protocol: In a bit-oriented protocol, a frame is treated as a sequence or collection of bits rather than a collection of characters.
      HDLC Frame Format:
      Flag | Address | Control | Information | FCS/CRC | Flag
      FieldPurpose
      FlagMarks the beginning and end of a frame. The standard flag pattern is 01111110.
      AddressIdentifies the station involved in the communication.
      ControlControls the type and operation of the HDLC frame.
      InformationCarries data received from the Network Layer. Its size is variable.
      FCS/CRCUsed for error detection.
      HDLC Frame Types
      1. I-Frame (Information Frame) — Carries user data and supports sequencing and acknowledgments.
      2. S-Frame (Supervisory Frame) — Used mainly for flow and error control.
      3. U-Frame (Unnumbered Frame) — Used for link management and other control functions.
      Control Field Identification:
      Control Field Starting BitsFrame Type
      0I-Frame
      10S-Frame
      11U-Frame
      Bit Stuffing in HDLC
      Bit Stuffing is a technique used in HDLC to prevent the data from being confused with the special Flag pattern 01111110.
      Problem: The Flag pattern contains six consecutive 1s. If a similar pattern occurs inside the actual data, the receiver might incorrectly interpret it as the end of the frame.
      Solution: The sender inserts a 0 after every sequence of five consecutive 1s in the data.
      Example:
      Original data: 01111110
      After bit stuffing: 011111010
      Here, a 0 is inserted after the fifth consecutive 1.
      At the Receiver: The receiver removes the inserted 0 whenever it detects a zero following five consecutive 1s. This process is called bit destuffing.

  • Explain briefly about the Services of Data Link Layer.
  • Explain LLC Layer and MAC Layer
  • What is HDLC?
  • Find The bit sequence after bit stuffing: 110101111101011111101011111110

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