Introduction

  • The data link layer constitutes the second layer of the hierarchical OSI Model.

Features

  • Data Link Layer performs their functions in LAN mainly.
  • The Data Link layer together with physical layer provide a data link connection for reliable transfer of data bits over an imperfect physical connection, between two adjacent nodes.
  • This layer accomplishes the data transfer where sender break the input data into data frames,transmit the frames sequentially and process the acknowledgement frames sent back by the receiver.
  • Like other layers of OSI model, this layer also creates its own protocol data unit(pdu) i.e. this layer adds some control bits to the previous pdu received from network layer and convert into different protocol data unit called frames.
  • The data link layer creates and recognizes frame boundaries too.

Types

  • Data link layer is sub-divided into two parts – (a) Logical Link Control (LLC) sub-layer (b) Medium Access Control (MAC)sub-layer.

(a) Logical Link Control (LLC) sub-layer

  • The LLC is non-architecture-specific and is the same for all IEEE defined LANs.
  • It provides a reliable communication between two devices.
  • It is also involved with data flow control, error control, framing (encapsulation of network layer data packets into several small pieces of data units are called frames) and sequencing.

(b) Medium Access Control (MAC) sub-layer

  • The main function of MAC layer is Dynamic Channel/Medium Allocation (Allocation of sharing a single transmission medium  by multiple users dynamically).
  • It also generates Physical/MAC Addressing.
  • It also supports Quality of Services(QoS) regarding transmitting data.

Functions

(I) Flow Control

  • Flow control is how to keep the fast sender from overflowing a slow receiver by buffering and acknowledgement procedures.

(II) Error Control

  • Data that is either transmitted over communication channel or stored in memory is not completely error free.
  • Transmission Errors may be caused by many reasons like Signal distortion or attenuation, synchronization problems, distorted channel, etc.
  • Error control mainly includes error detection and error correction.
  • Error detection and corrections are two different but related things i.e. Error detection is the ability to detect errors of transmitting or storing data and the error correction is identification and correction of the errors.
  • Error detection always precedes error correction.
  • Both error detection and correction can be achieved by having extra/ redundant/check bits in addition to data deduce that there is an error.
Error Detection Methods

Error Correction Methods

(III) Channel Allocation

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