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computer Networking- CCNA part 8 of Segment 1- Data Link Layer of OSI Model

OSI Model - Data Link Layer 

Hello friends once again i am vasu birla for discussing about second and most overloaded layer of OSI model  it is Data link Layer

Data Link Layer -  It is the second layer from the bottom of OSI layer. Some Important functions of data link are following -

Framing
Error Detection
Physical Addressing
Flow Control   .

  • Framing -  in the physical layer PDU (protocol data unit) is bit  it means in the physical layer data comes in form of bit or 0,1 form.  but later these different bits is converted in the frames combination of some 0s and 1s  the reason behind the framing is  easy error detection , so framing of data takes place in the data link layer.  

  • Error Detection - When each layer of OSI pass data to other layer so it include header also ,, but the only data link layer attach header and as well as trailer . There is value in this Trailer  that is known as FCS or Frame check sequence.  FCS value is a calculated value of our frame which helps for error detection.                                                                                                                                      Now the question is how FCS helps for error detection -  When Receiver receive data , it check this frame and compare it to FCS value and if both values (frame and FCS value) it means data is 100 % correct received and it is error free  and if values are not equal it means frame is corrupted.  so this function of data link layer is known as error detection. 

  • Physical Addressing -  Physcial address is identity of your physical system or computer and it is inbuilt on NIC (Network interface card) or on LAN card  . it never changed this is fixed and unique address of your system.  example of physical address is  MAC address which is made of 48 bits or 6 byes or 12 hexadecimal numbers.  

  • Flow control -  It means when there is too much data flowing over network so it may cause of slow connetion of huge data traffic problem  this may make network shut down  for solution of this problem Data link devices control flow of data in the network.  Devices which works on data link layer is bridges and traditional switches and these devices ensure us to deliver data successfully by dividing domains (we will discuss in details about devices and their working.)
Flow control by stop and wait  using acknowledgment of frames
     
Flow control using Switch  a data link layer Device






DATA Link Layer is further divided in Two SUB layers 
  • LLC (Logical Link Control) Layer
  • MAC(Media Access Control ) Layer 

WE will discuss these two sublayer in next part.... hare krishna ,.,



#ccna #cisco  #Networking 
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