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
Wait for updates ...... happy reading