Tutorials

Linux N/W System Call

Data Link Layer (DLL)

Introduction to Data Link Layers


Data Link Layer compoments

  • Data link layer divided in to two sublayers
  • upper sublayer (Data Link Control) is responsible for data link control
  • lower sublayer (Multiple access resolution ) is responsible for resolving access to the shared media.
  • If the channel is dedicated , we do not need lower sublayer.

  • In LAN:
  • upper sublayer : Responsible for flow and error control is called Logical Link Control (LLC)
  • lower sublayer : Responsible ofr multiple access resolution is called Media Access Control (MAC)

  • When node or station connected to and use common link, called multi-point or broadcast link, We need multiple access protocol to coordinate access to link.

  • Many protocols are devised to handle the shared link.
  • Categorized in to three groups
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  • Random Access :

  • Random access or contention method, no station is superior to another none is assigned control over the another.
  • No station permits or does not permit, another station send.
  • At each instance , a station that has data to send has to uses the procedure defined by protocol to make a decision on whether or not to send. This depends on the state of the medium( idle or busy).
  • Two feature give this method name:
  • First :there is no schedule time for station to transmit. Transmission is among the station. Thats why these methods are called Random Access
  • Second: No rules specify which station should send next. Station compete with one another to access medium. Thats why these method are also called contention method.

  • In random access , each station has right to the medium without being conotrolled by other station.
  • If more than one station try to send , there is an access conflict – collision – and the frame will destroyed or modified.
  • To avoid access conflict or to resolve it when it happens, each station follows the procedure.
  • When the station access the medium?
  • What can the station do if the medium busy?
  • How can the station determine the success or failure of the transmission.?
  • What can the station do if there is access conflict ?

  • The random access method provides the access techniques.

  • ALOHA : Simple procedure called Multiple Access (MA)
  • The method was improved with addition of procedure that force the station to sense the medium before transmitting.
  • This is called Carrier Sense Multiple Access (CSMA)
  • This method evolved into two parallel methods :
  • CSMA/CD : Carrier Sense Multiple Access with Collision Detection : Tells the what to do when collision detected.
  • CSMA/CA : Carrier Sense Multiple Access with Collision Avoidance: Tries to avoid collision
  • ALOHA : We are not discussing anymore.
  • Carrier Sense Multiple Access :
  • To minimize the chance of collision and therefore improve performance , CSMA Developed.
  • Sense the medium before using it. Or “Sense before transmit it” or “Listen before talk”
  • CSMA can reduce possibility of the collision, can not eliminate




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