CSE 313 Data Communication

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Presentation transcript:

CSE 313 Data Communication Lec 13 Sabbir Muhammad Saleh

CSMA/CD In CSMA/CD method, a station monitors the medium after it sends a frame to see if the transmission was successful. If so, the station is finished. If, however, there is a collision, the frame is sent again. To better understand CSMA/CD, let us look at the first bits transmitted by the two stations involved in the collision. Although each station continues to send bits in the frame until it detects the collision, we show what happens as the first bits collide. In Figure 12.11, stations A and C are involved in the collision. PG:334 Data Communications and Networking By Behrouz A.Forouzan 5th Edition.

CSMA/CD PG:335 Data Communications and Networking By Behrouz A.Forouzan 5th Edition.

CSMA/CD At time t1, station A has executed its persistence procedure and starts sending the bits of its frame. At time t2, station C has not yet sensed the first bit sent by A. Station C executes its persistence procedure and starts sending the bits in its frame, which propagate both to the left and to the right. The collision occurs sometime after time t2. Station C detects a collision at time t3 when it receives the first bit of A’s frame. Station C immediately (or after a short time, but we assume immediately) aborts transmission. Station A detects collision at time t4 when it receives the first bit of C’s frame; it also immediately aborts transmission. Looking at the figure, we see that A transmits for the duration t4 − t1; C transmits for the duration t3 − t2. PG:335 Data Communications and Networking By Behrouz A.Forouzan 5th Edition.

CSMA/CA Carrier sense multiple access with collision avoidance (CSMA/CA) was invented for wireless networks. Collisions are avoided through the use of CSMA/CA’s three strategies: the interframe space, the contention window, and acknowledgments, PG:338 Data Communications and Networking By Behrouz A.Forouzan 5th Edition.

CSMA/CA PG:340 Data Communications and Networking By Behrouz A.Forouzan 5th Edition.

CSMA/CA Figure 12.17 shows the exchange of data and control frames in time. 1. Before sending a frame, the source station senses the medium by checking the energy level at the carrier frequency. a. The channel uses a persistence strategy with backoff until the channel is idle. b. After the station is found to be idle, the station waits for a period of time called the DCF interframe space (DIFS); then the station sends a control frame called the request to send (RTS). 2. After receiving the RTS and waiting a period of time called the short interframe space (SIFS), the destination station sends a control frame, called the clear to send (CTS), to the source station. This control frame indicates that the destination station is ready to receive data. PG:340 Data Communications and Networking By Behrouz A.Forouzan 5th Edition.

CSMA/CA 3. The source station sends data after waiting an amount of time equal to SIFS. 4. The destination station, after waiting an amount of time equal to SIFS, sends an acknowledgment to show that the frame has been received. Acknowledgment is needed in this protocol because the station does not have any means to check for the successful arrival of its data at the destination. On the other hand, the lack of collision in CSMA/CD is a kind of indication to the source that data have arrived. PG:340 Data Communications and Networking By Behrouz A.Forouzan 5th Edition.