1. © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved. © The McGraw-Hill Companies, Inc. Medium Access Control Asst. Prof. Chaiporn Jaikaeo, Ph.D. chaiporn.j@ku.ac.th http://www.cpe.ku.ac.th/~cpj Computer Engineering Department Kasetsart University, Bangkok, Thailand Adapted from the notes by Lami Kaya and lecture slides from Behrouz A. Forouzan

2. 2 Medium Access Control (MAC) How do multiple, independent computers coordinate access to a shared medium? There are three broad approaches: they can use a modified form of a multiplexing technique they can engage in a distributed algorithm for controlled access or they can use a random access strategy

3. 3 Multiple Access Protocols

4. 4 Evolution of Random Access Protocols

5. 5 ALOHA Protocol All links are wireless

6. 6 ALOHA Protocol

7. 7 CSMA CSMA – Carrier Sense Multiple Access "Listen before talk" A transmitting station makes sure no one else is using the channel

8. 8 Collision in CSMA Collision is still possible even with "carrier sensing"

9. 9 CSMA/CD Researchers at Xerox PARC created a random access protocol (1973) It is widely known as Ethernet Ethernet uses three (3) mechanisms to handle collisions: Carrier sense Collision detection Binary exponential backoff

10. 10 CSMA/CD Procedure keep sensing until channel is idle

11. 11 CSMA/CD and Wireless LANs CSMA/CD does not work as well in wireless LANs because a transmitter used in a wireless LAN has a limited range This problem is known as the hidden terminal problem

12. 12 CSMA/CA Solution for wireless LANs Try to "avoid" collision instead of detecting CSMA with Collision Avoidance

13. 13 Controlled Access Polling Reservation Token passing

14. 14 Polling Polling uses a centralized controller which cycles through stations on the network and gives each an opportunity to transmit a packet Two general polling policies: Round robin order Priority order

15. 15 Polling Example

16. 16 Reservation Access Method Often used with satellite transmission Employs a two-step process in which each round of packet transmissions is planned in advance Typically, reservation systems have a central controller

17. 17 Token Passing It is most often associated with ring topologies Although older LANs used token passing ring technology popularity has decreased, and few token passing networks remain Imagine a set of computers connected in a ring and imagine that at any instant, exactly one of the computers has received a special control message called a token When no station has any packets to send the token circulates among all stations continuously

18. 18 A Token Passing Network

19. 19 Channelization Frequency Division Multiple Access (FDMA) Time Division Multiple Access (TDMA) Code Division Multiple Access (CDMA)

20. 20 FDMA Bandwidth is divided into several bands Each channel occupies a different frequency band Used in 1G cellular systems

21. 21 TDMA Same frequency band is used, but each channel occupies different time slots Many 2G cellular systems such as GSM use a combination of TDMA and FDMA 231 MUXMUX 1 2 3 231231 1 2 3 DEMUXDEMUX Data flow

22. 22 CDMA Used in many 3G cellular systems All channels uses same frequency and time CDMA relies on orthogonal vector spaces that can be combined and separated without interference Each sender is assigned a unique binary code C i that is known as a chip sequence chip sequences are selected to be orthogonal vectors (i.e., the dot product of any two chip sequences is zero)

23. 23 Example: Chip Sequences They are orthogonal E.g., A  B = (+1)(+1)+(+1)(-1)+(+1)(+1)+(+1)(-1) = 0

24. 24 Encoding Rule Bit values and silence are encoded using the following rules Data bit 0  Data bit 1  1 Silence  0

25. 25 CDMA Multiplexor

26. 26 CDMA Demultiplexor