Analysis of CSMA The probability of success (per epoch)is Ps=g(n)e-g(n) The expected duration of an epoch is approximately T(n)~β+(1-e9) Thus the success rate per unit time is n <departure rate 8(m)le-8(m) B+1 e&(n)
Analysis of CSMA • The probability of success (per epoch) is Ps = g(n) e-g(n) • The expected duration of an epoch is approximately T(n) ~ β + (1 - e-g(n) ) • Thus the success rate per unit time is g(n)e− g(n) λ < departure rate= β +1− e− g(n) Eytan Modiano Slide 6
Maximum Throughput for CSMa The optimal value of g(n can again be obtained: g(n)≈√2月 < l+√2B Tradeoff between idle slots and time wasted on collisions High throughput when B is small Stability issues similar to aloha (less critical) Departure rate Arrival rate 入β+
Maximum Throughput for CSMA • The optimal value of g(n) can again be obtained: 1 g ( n ) ≈ 2 β λ < 1 + 2 β • Tradeoff between idle slots and time wasted on collisions • High throughput when β is small • Stability issues similar to Aloha (less critical) 1-¦2 β Arrival rate Departure rate g(n) = λβ r + nq Eytan Modiano ¦2 β Slide 7
Unslotted CSMA Slotted csMA is not practical Difficult to maintain synchronization Mini-slots are useful for understanding but not critical to the performance of CSMA Unslotted CSMA will have slightly lower throughput due to increased probability of collision Unslotted CSMa has a smaller effective value of B than slotted CSMA Essentially B becomes average instead of maximum propagation delay
Unslotted CSMA • Slotted CSMA is not practical – Difficult to maintain synchronization – Mini-slots are useful for understanding but not critical to the performance of CSMA • Unslotted CSMA will have slightly lower throughput due to increased probability of collision • Unslotted CSMA has a smaller effective value of β than slotted CSMA – Essentially β becomes average instead of maximum propagation delay Eytan Modiano Slide 8
CSMA/CD and Ethernet Two way cable wswswswswS wS CSMA with Collision Detection(CD) capability Nodes able to detect collisions Upon detection of a collision nodes stop transmission Reduce the amount of time wasted on collisions Protocol All nodes listen to transmissions on the channel When a node has a packet to send Channel idle = Transmit Channel busy = wait a random delay(binary exponential backoff) If a transmitting node detects a collision it stops transmission Waits a random delay and tries again
CSMA/CD and Ethernet Two way cable WS WS WS WS WS WS • CSMA with C ollision Detection (CD) capability – Nodes able to detect collisions – Upon detection of a collision nodes stop transmission Reduce the amount of ti m e wasted on collisions • Protocol: – All nodes listen to transmissions on the channel – When a node has a packet to send: Chann el idle = > T ransmit Channel busy => wait a random dela y (binary exponential backoff) – If a transmitting node detects a collision it stops transmission Eytan Modiano Waits a random dela y and tries again Slide 9
Time to detect collisions C= prop t wS delay a collision can occur while the signal propagates between the two nodes It would take an additional propagation delay for both users to detect the collision and stop transmitting If t is the maximum propagation delay on the cable then if a collision occurs, it can take up to 2t seconds for all nodes involved in the collision to detect and stop transmission
Time to detect collisions WS τ WS τ = prop delay • A collision can occur while the signal propagates between the two nodes • It would take an additional propagation delay for both users to detect the collision and stop transmitting • If τ is the maximum propagation delay on the cable then if a collision occurs, it can take up to 2 τ seconds for all nodes involved in the collision to detect and stop transmission Eytan Modiano Slide 10