Aloha- Basic Technique First random mac developed For radio-based communication in Hawaii (1970) Basic idea: k When you re ready transmit Receivers send acK for data Detect collisions by timing out for ACK k Recover from collision by trying after random delay k Too short> large number of collisions Too long >underutilization 2021/2/9 11
Aloha – Basic Technique First random MAC developed For radio-based communication in Hawaii (1970) Basic idea: When you’re ready, transmit Receiver’s send ACK for data Detect collisions by timing out for ACK Recover from collision by trying after random delay Too short → large number of collisions Too long → underutilization 2021/2/9 11
Pure ALOHA Simple idea Let users transmit whenever they have data to be sent Each station transmit frames arbitrarily If collision occurs the sender waits a random time and sends it again Bus channel interface The model of aloha 2021/2/9 12
Pure ALOHA Simple idea Let users transmit whenever they have data to be sent Each station transmit frames arbitrarily If collision occurs, the sender waits a random time and sends it again 2021/2/9 12 N N-1 2 1 Bus channel interface The model of ALOHA
Pure aloha collision Collision reason: two station send frame simultaneously 2021/2/9 13
Pure ALOHA Collision 2021/2/9 13 Collision reason：two station send frame simultaneously 1 2 ... N-1 N
Pure ALoHA Collision(cont'd) t1 A2 A2 A2 A3 Station A new frame Collison Station B BiBi B3 Final effect 2021/2/9 14
Pure ALOHA Collision(cont’d) 2021/2/9 14 A1 new frame B1 A2 A2 B1 collison t1 t2 B2 A2 t3 B2 t4 B3 A3 Station A Station B Final effect A1 B1 A2 B2 t t t
Performance Analysis(1) Collides with Collides with the start of the the end of the shaded frame shaded frame t +t +2t to +3t Vulnerable period for the shaded frame 2021/2/9 5
Performance Analysis(1) 2021/2/9 15 Vulnerable period for the shaded frame. Collides with the start of the shaded frame Collides with the end of the shaded frame t 0 t 0 t t + 0 t t + 2 0 t t + 3