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Multiple Access Protocols: Controlling Medium Access and Avoiding Collisions

Learn about multiple access protocols used in networking to control access to a shared medium and prevent collisions between stations. Explore random access methods, ALOHA protocol, carrier sense multiple access, persistence strategies, CSMA/CD and CSMA/CA procedures, and token-passing networks. Understand frequency division multiple access (FDMA), time division multiple access (TDMA), and code division multiple access (CDMA) channelization protocols.

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Multiple Access Protocols: Controlling Medium Access and Avoiding Collisions

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  1. Chapter 13 MultipleAccess

  2. Multiple-Access Protocols • Nodes or stations are connected to or use a common link, called a multipoint or broadcast link. • Problem of controlling the access to the medium is similar to the rules of speaking in an assembly.

  3. Each station has the right to the medium without being controlled by any other station. If more than one station tries to send, there is an access conflict (collision) and the frames will be either destroyed or modified. When can the station access the medium? What can the station do if the medium is busy? How can the station determine the success or failure of the transmission? What can the station do if there is an access conflict? Evolution of random-access methods

  4. Every station that need to send a frame to another station first sends it to the base station. The base station receives the frame and relays it to the intended destination. Multiple access Acknowledgement: Expect an acknowledgement for a frame. If not received in allocated time [2*maximum propagation delay], try to send the frame again after a random amount of time. ALOHA network

  5. Procedure for ALOHA protocol

  6. Carrier sense multiple access Sense the carrier before transmit Possibility of collision still exists because of the propagation delay: when a station sends a frame, it takes a while (although very short) for the first bit to reach every station and for every station to sense it. Collision in CSMA

  7. Nonpersistent: If a station has a frame to send, it senses the line. If line is idle, the station sends the frame immediately. If line is not idle, the station waits a random period of time and then senses the line again. Chances of collision is reduced Reduces efficiency of the network Persistent: 1-Persistent: If line is idle, sends the frame immediately (with probability of 1). Chances of collision is high. P-Persistent: If line is idle, it may or may not send. It sends with probability p and refrains from sending with probability 1-p. Persistence strategies

  8. Persistence strategies

  9. If there is collision, the frame needs to be sent again. To reduce the probability of collision the second time, the station waits – it needs to back off. Waits a little the first time, more if a collision occurs again, much more if it happens a third time, and so on. CSMA/CD procedure

  10. After it finds the line idle, the station waits an IFG (Inter-frame gap) amount of time. It then waits another random amount of time. After that, it sends the frame and sets a timer. CSMA/CA Procedure

  11. A station needs to make a reservation before sending data. Time is divided into intervals. In each interval, a reservation frame precedes the data frame sent in that interval. Reservation Access Method

  12. One device is designated as a primary station and the other devices are secondary stations. All data exchange via primary station. If the primary wants to receive data, it asks the secondaries if they have anything to send; this is called as polling. If the primary device wants to send data, it tells the secondary target to get ready to receive; this is called as selecting. Select

  13. When the primary is ready to receive data, it must ask (poll) each device in turn if it has anything to send. When the first secondary is approached, it responds either with a NAK frame if it has nothing to send or with data if it does. If the response is NAK, the primary polls the next secondary. If it receives data, it sends ACK frame. Poll

  14. Station is authorized to send data when it receives a special frame called a token. Stations are arranged around a ring. When no data are being sent, a token circulates the ring. If a station needs to send data, it waits for the token. The station captures the token and sends one or more frames (as long as it has frames to send or the allocated time has not expired), and finally it releases the token to be used by next station. Token-passing Network

  15. Token-passing procedure

  16. Frequency Division Multiple Access (FDMA) The total bandwidth is divided into channels. Time Division Multiple Access (TDMA) The band is divided into one channel that is time shared Code Division Multiple Access (CDMA) One channel carries all transmission simultaneously Channelization Protocols

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