Chapter4-1教案分析.ppt

* * The relation between S and G 1/2e = 18% 1/e = 37% Brief summary Pure ALOHA Dangerous period of collision Time length：2t Generated frame (mean)：2G Probability of no collision：P0 = e-2G Throughput：S = G P0 = G e-2G Slotted ALOHA（P254） Frame-time T is discrete interval Dangerous period of collision ：t Throughput ：S = G P0 = G e-G Comparation of pure ALOHA and slotted ALOHA Pure ALOHA: A Frame is sent at once when it is generated Collision may be occurred all the time Slotted ALOHA Frame is sent only at the very beginning of interval Frame is sent successfully once no collision is occurred at the beginning of interval (slot) Carrier Sense Protocols P255 CSMA：Carrier Sense Multiple Access Characteristic：“先听后发” Improve ALOHA Types Non-persistent CSMA Persistent CSMA 1-persistent CSMA P-persistent CSMA Non-persistent CSMA Basic idea：P256 ①A station sense channel, if no one else is sending, then it begin sending ②If the channel is already in use, it doesn’t sense it ,instead, it waits a random time and repeats①。 Advantage: Waiting a random time can reduce the probability of collision Disadvantage: longer delay (random time, maybe no data is transmitted ) Persistent CSMA (1-persistent) Basic idea：P255~256 ① A station sense channel, if no one else is sending, then it begin sending ②If channel is busy, the station waits and sense it continually, once the channel becomes idle, it begin transmitting ③If collision is occurred, waits a random time and repeats①。 Advantage: shorter delay than non-persistent problem：if 2 or more stations are wait at the same time, once the channel becomes idle, the collision is unavoidable. P-persistent CSMA Basic idea: ① A station sense channel, if no one else is sending, then it transmits with probability p, and delay one unit-time to transmit with probability (1–p) ② If channel is busy, the station waits and sense it continually, once the channel becomes idle, repeats①。 ③If a station has delayed its transmission 1 unit time, repeat①。 S