Computer Networks: Data Link Layer

Data Link Layer

• The Data Link Layer is the second layer in the OSI model and ensures error-free communication between directly connected devices over a specific link.
• Key functions include:
  • Providing reliable transmission of data frames between devices on the same network segment
  • Ensuring error-free transfer of data frames between devices

Nodes and Links

• Node: a device or point in a network where data can be generated, processed, or received
  • Types of nodes:
    • End Devices: devices that are the source or destination of data in a network (e.g., computers, smartphones, printers, servers)
    • Intermediate Devices: devices that facilitate communication between end devices (e.g., routers, switches, bridges)
    • Networking Equipment: devices designed for networking tasks (e.g., hubs, repeaters, access points)
• Link: a physical or logical connection between two nodes in a network
  • Types of links:
    • Physical Links: actual physical connections (e.g., Ethernet cables, fiber-optic cables) or wireless connections (e.g., Wi-Fi, radio waves)
    • Logical Links: virtual connections established over a physical medium (e.g., networking protocols)

Data Link Layer Sub-Layers

• Logical Link Control (LLC) Sub-Layer:
  • Functionality: establishes, maintains, and terminates logical links between devices
  • Handles flow control, error checking, and synchronization
  • Provides a common interface to the Network Layer
• Media Access Control (MAC) Sub-Layer:
  • Functionality: controls access to the physical network medium and manages addressing of frames
  • Assigns unique Media Access Control (MAC) addresses to each network interface card (NIC)

Error Detection and Correction

• Error detection and correction are crucial in data communication and storage systems
• Techniques used:
  • Block Coding: dividing data into fixed-size blocks and adding redundant bits (parity bits) to each block
  • Cyclic Codes: a type of linear block code with a circular structure
  • Checksum: a simple error-detection technique that adds a value (checksum) to the data being transmitted
  • Forward Error Correction (FEC): adds redundancy to transmitted data to enable error correction without retransmission

Data Link Control (DLC)

• Services and protocols provided by the Data Link Layer
• Includes framing, addressing, flow control, error detection, and access control

Data-Link Layer Protocol

• Examples of Data-Link Layer protocols:
  • High-Level Data Link Control (HDLC): a bit-oriented protocol for communication over point-to-point and multipoint links
  • Point-to-Point Protocol (PPP): a protocol for establishing and managing point-to-point connections

Point-to-Point (P2P)

• Definition: a communication scenario where data is exchanged directly between two endpoints without intermediate devices
• Characteristics:
  • Dedicated Link: point-to-point connections often involve a dedicated communication link between the sender and receiver
  • Bi-Directional or Uni-Directional: communication link may support data transmission in both directions or only one direction
• Applications:
  • Dial-up Connections: establishing a direct link between a user's computer and an internet service provider (ISP) using a modem and a phone line
  • Leased Lines: dedicated communication links leased from a service provider for secure and reliable data transfer
  • Backhaul Connections: connecting remote sites or cell towers to a central network hub for data transmission

Media Access Control (MAC)

• Definition: a sub-layer of the Data Link Layer responsible for managing access to the physical network medium and controlling the addressing of frames
• Functions:
  • Addressing: every network interface card (NIC) or network adapter has a unique MAC address
  • Framing: the MAC sub-layer frames data received from the Network Layer into frames that can be transmitted over the network medium
  • Media Access Control: implements access control mechanisms to manage how devices on a shared network medium access and transmit data

Random Access and Controlled Access

• Random Access: a type of multiple access protocol where devices contend for access to the communication medium without a predefined order
• Controlled Access: a method where devices follow a predetermined order or obtain permission before transmitting data
• Examples:
  • Carrier Sense Multiple Access with Collision Detection (CSMA/CD) used in Ethernet networks
  • Polling and Token Passing

Channelization

• Definition: dividing a communication medium into multiple logical channels, allowing different devices or signals to use separate frequencies, time slots, or code sequences
• Examples:
  • Frequency Division Multiple Access (FDMA)
  • Time Division Multiple Access (TDMA)
  • Code Division Multiple Access (CDMA)