Random Access Protocols

Random Access Protocols

1. ALOHA

   • The earliest random access method
     • Pure ALOHA
       • The original ALOHA protocol is pure ALOHA
       • Each station sends a frame whenever it has a frame to be sent.
       • There are potential collisions in this arrangement and the data from the two station collide and become garbled
       • After transmittinga frame,the sender waits foran ACK for an amount of time If no ACK was received, the sender will resends that frame after it waits for a random amount of time If station fails to receive an ACK afer repeated transmissions, it gives upe.
       • The vulnerable Time: 2*Frame transmission time
       • Throughput : G*e^(-2G)
         • Student name:Nawal Alamri
           • ID: 2190005382
       • Maximum Throughput: 18.4%
       • Collision Status : It does not reduce the total number of collisions to half
     • Slotted ALOHA
       • Time is divided into slots equal to a frame transmission time (Tfr) seconds.
       • The data at the beginning of any random time slot If a station misses the beginning of a slot, it has to wait until the beginning of the next time slot.
       • A station informs all stations about the start of each slot.
       • The vulnerable Time: Frame transmission time
       • Throughput : G*e^-2
       • Maximum Throughput : 36.8%

2. CSMA

   • carrier sense multiple access
     • To minimize the chance of collision and increase the performance
   • each station first listen to the medium before sending.
   • The vulnerable Time: propagation Time Tp
   • 1-Persistent
     • It is simple and straightforward
       • station idle sends the frame immediately
         • has the highest chance of collision
         • Same
           • if not idle, waits for a random amount of time then senses the line again
             • it reduces the chance of collision and the efficiency
   • Nonpersistent
     • A station that has a frame to send senses the line.
   • p -Persistent
     • It is used if the channel has time slots with a slot duration equal to or greater than the maximum propagation time.
       • It reduces the chance of collision and improves efficiency
     • After the station finds the line idle it follows these steps
       • With probability p, the station sends its frame.
         • This step
       • With probability q=1−p,the station waits for the beginning of the next time slot and checks the line again.
         • If the line is idle, it goes to step 1
         • If the line is busy, it acts as though a collision has occurred and uses the backoff procedure.
   • CSMA/CD
     • carrier sense multiple access with collision detection
       • augments the algorithm to handle the collision
     • a station monitors the medium after it sends to see if the transmission was successful.
   • CSMA/CA
     • Carrier sense multiple access with collision avoidance
     • Collisions are avoided through the use of CSMA/CA’s three strategies:
       • The interframe space
       • The contention window.
       • Acknowledgments

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