ch13.ppt - Chapter 12 Multiple Access Figure 12.1 Data link layer divided into two functionality-oriented sublayers Figure 12.2 Taxonomy of

ch13.ppt - Chapter 12 Multiple Access Figure 12.1 Data link...

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Chapter 12 Multiple Access Figure 12.1 Data link layer divided into two functionality-oriented sublayers Figure 12.2 Taxonomy of multiple-access protocols
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12-1 RANDOM ACCESS 12-1 RANDOM ACCESS In In random access random access or or contention contention methods, no station is superior to methods, no station is superior to another station and none is assigned the control over another. No another station and none is assigned the control over another. No station permits, or does not permit, another station to send. At each station permits, or does not permit, another station to send. At each instance, a station that has data to send uses a procedure defined by instance, a station that has data to send uses a procedure defined by the protocol to make a decision on whether or not to send. the protocol to make a decision on whether or not to send. Figure 12.3 Frames in a pure ALOHA network
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Figure 13.3 ALOHA network The earliest random-access method ,developed in 1970s Designed to be used on wireless Local area Network at 9600bps Based on Following rules Multiple Access Any station can send a frame when it has to send a frame Acknowledgment After sending a frame the station waits for an acknowledgment .If it does not receive the Ack within 2times maximum propagation delay ,It tries sending the frame again after a random amount of time
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Figure 12.4 Procedure for pure ALOHA protocol
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CSMA (Carrier Sense Multiple Access) CSMA : listen before transmit: If channel sensed idle: transmit entire frame If channel sensed busy, defer transmission Human analogy: don’t interrupt others! CSMA collisions collisions can still occur: propagation delay means two nodes may not hear each other’s transmission collision: entire packet transmission time wasted Sense before transmit The possibility of collision still exist because of the propagation delay
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Figure 12.9 Vulnerable time in CSMA
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This method has two variations I-PERSISTANT Station senses the line ,If the line is Idle station sends the frame immediately(with a
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  • Fall '16
  • SHAH EMAD
  • Time division multiple access, Code division multiple access, Channel access method, Idle station

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