1. Subject Name: Computer Communication Networks Subject Code: 10EC71
Prepared By: Kala H S & Megha Gupta
Department: ECE
Date:
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2. Unit 3
MULTIPLE ACCESSES
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3. TOPICS TO BE COVERED Multiple Access Mechanism Random Access
Controlled Access
Channelization
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4. 2 Functionality-Oriented Sublayers of Data Link Layer:
Upper sublayer is responsible for data link control.
Lower sublayer is responsible for resolving access to the shared media.
If the channel is dedicated, we do not need the lower sublayer.
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5. Multiple Access Mechanism
When nodes or stations are connected and use a common link, we need a multiple-access protocol to coordinate access to the link.
These protocols regulate the contention between many transmitters which tend to use the media anywhere, anytime and independently.
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6. RANDOM ACCESS Also called as contention method.
No station is superior to another station and none is assigned the control over another.
Two features give this method its name.
Random Access: there is no scheduled time for a station to transmit. Transmission is random among the stations.
(b) Contention Method: Stations compete with one another to access the medium.
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7. ALOHA It is the earliest random access method.
It was developed at the University of Hawaii in early 1970.
It was designed for a radio (wireless) LAN, but it can be used on any shared medium.
The medium is shared between the stations.
When a station sends data, another station may attempt to do so at the same time. The data from the two stations collide and become garbled.
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8. PURE ALOHA It is the original Aloha Protocol.
Each station sends a frame whenever it has a frame to send.
Since there is only one channel to share, there is the possibility of collision between frames from different stations.
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9. PROCEDURE FOR PURE ALOHA PROTOCOL
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10. VULNERABLE TIME FOR PURE ALOHA
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11. SLOTTED ALOHA It was invented to improve the efficiency of pure ALOHA.
In slotted ALOHA we divide the time into slots and force the station to send only at the beginning of the time slot.
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12. VULNERABLE TIME FOR SLOTTED ALOHA
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13. CSMA CSMA would send a frame only if it senses no carrier.
There are two strategies for that: Non-persistent and persistent strategy.
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14. CSMA/CD
CSMA method does not specify the procedure following a collision.
Carrier sense multiple access with collision detection (CSMA/CD) augments the algorithm to handle the collision.
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.
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15. CSMA/CD FLOW DIAGRAM
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16. CSMA/CD Carrier Sense Multiple Access with Collision Avoidance.
Used in a network where collision cannot be detected. Eg: Wireless LAN
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17. contention window size is 2K-1
CSMA/CA FLOW DIAGRAM
contention window size is 2K-1
After each slot:
- If idle, continue counting
- If busy, stop counting
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18. CONTROLLED ACCESS
The stations consult one another to find which station has the right to send.
A station cannot send unless it has been authorized by other stations.
Three common methods:
Reservation
Polling
Token passing
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19. RESERVATION A station needs to make a reservation before sending data.
Time is divided into intervals. In each interval, a reservation frame precedes the data frames sent in that interval.
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20. POLLING
One device is designated as a primary station and the other devices are secondary stations.
All data exchanges are made through the primary device even when the ultimate destination is a secondary device.
The primary device controls the link; the secondary devices follow its instructions.
Primary device determines which device is allowed to use the channel at a given time.
Primary device is always the initiator of a session.
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21. POLLING and ACCESS METHOD
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22. TOKEN PASSING
The stations in a network are organized in a logical ring.
For each station, there is a predecessor and a successor.
Stations do not have to be physically connected in a ring, the ring can be a logical one.
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23. CHANNELISATION
It is a multiple-access method in which the available bandwidth of a link is shared in time, frequency, or through code, between different stations.
Three channelization protocols:
FDMA,
TDMA,
CDMA.
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24. FDMA
The bandwidth is divided into channels. Each channel is used by a single station.
FDMA is data link layer protocol (MAC sub layer) which uses FDM at the physical layer.
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25. TDMA
The entire bandwidth is just one channel. The stations share the bandwidth through time slots.
TDMA is data link layer protocol (MAC sub layer) which uses TDM at the physical layer.
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26. CDMA
All stations share the same bandwidth and the same time (no frequency division, no time division, all data transmitted
simultaneously).
One channel carries all transmissions at the same time.
Each channel is separated by code.
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27. CDMA: CHIP SEQUENCES Each station is assigned a unique chip sequence
Chip sequences are orthogonal vectors
Inner product of any pair must be zero
With N stations, sequences must have the following properties:
They are of length N
Their self inner product is always N
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28. CDMA: BIT REPRESENTATION
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29. CDMA: TRANSMISSION
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30. CDMA ENCODING
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31. CDMA DECODING
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32. Acknowledgement : MY SINCERE THANKS TO By Kala H S & Megha Gupta
THE AUTHOR PROF.BEHROUZ A FOROUZAN. BECAUSE THE ABOVE PRESENTATION MATERIALS ARE HEAVILY BORROWED FROM HIS TEXTBOOK “DATA COMMUNICATION & NETWORKING” 4TH EDITION, PUBLISHER TATA MCGRAW HILL
SURESHA V. PROFESSOR, DEPT. OF E&C, KVG COLLEGE OF ENGINEERING. SULLIA, D.K By
Kala H S & Megha Gupta
Assistant Professor
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