1. 1 Kyung Hee University Prof. Choong Seon HONG Multiple Access

2. 2 Kyung Hee University 13.1 Random Access  MA – Multiple Access  CSMA – Carrier Sense MA  CSMA/CD – CSMA/Collision Detection  CSMA/CA – CSMA/Collision Avoidance

3. 3 Kyung Hee University Evolution of random-access methods

4. 4 Kyung Hee University Base station is central controller Base station acts as a hop Potential collisions, all incoming data is @ 407 MHz ALOHA network – Multiple Access

5. 5 Kyung Hee University Procedure for ALOHA protocol

6. 6 Kyung Hee University Collision in CSMA – Carrier Sense MA

7. 7 Kyung Hee University Persistence strategies  1- persistent  P-persistent

8. 8 Kyung Hee University CSMA/CD procedure – Collision Detection - Used in Ethernet CSMA/CD procedure – Collision Detection Usually15

9. 9 Kyung Hee University Interframe Gap CSMA/CA procedure – Collision Avoidance - Used in Wireless LAN CSMA/CA procedure – Collision Avoidance

10. 10 Kyung Hee University 13.2 Controlled Access Reservation Polling – Select and Poll Token Passing Stations consult one another to find which station has the right to send

11. 11 Kyung Hee University Reservation access method A station need to make a reservation before sending data

12. 12 Kyung Hee University Polling  If the primary want to receive data, it asks the secondaries if they have anything to send.  The secondaries are not allowed to transmit data unless asked (don’t call us - we’ll call you)

13. 13 Kyung Hee University Select

14. 14 Kyung Hee University poll

15. 15 Kyung Hee University Token-passing network  A station is authorized to send data when it receives a special frame called a token

16. 16 Kyung Hee University Token-passing procedure

17. 17 Kyung Hee University 13.3 Channelization FDMA – Frequency Division TDMA – Time Division CDMA – Code Division

18. 18 Kyung Hee University In FDMA, the bandwidth is divided into channels.  The available bandwidth is shared by all stations.  The FDMA is a data link layer protocol that uses FDM at the physical layer FDMA

19. 19 Kyung Hee University In TDMA, the bandwidth is just one channel that is timeshared. TDMA  The entire bandwidth is just one channel.  Stations share the capacity of the channel in time

20. 20 Kyung Hee University In CDMA, one channel carries all transmissions simultaneously. CDMA  Only one channel occupies the entire bandwidth of the link  All Stations can send data simultaneously; there is no time sharing.

21. 21 Kyung Hee University Chip sequences – Four Stations  CDMA is based on coding theory  Each station is assigned a code, which is a sequence of numbers called chips.  All Stations can send data simultaneously; there is no time sharing.

22. 22 Kyung Hee University Encoding Rules  When a station is idle, it sends no signal, which is represented by a 0.

23. 23 Kyung Hee University  Showing how four stations share the link during 1-bit interval.  CDMA Multiplexer Encoding Rules

24. 24 Kyung Hee University Encoding Rules  CDMA Demultiplexer

25. 25 Kyung Hee University Sequence Generation  To generate sequences, we use a Walsh table, a two- dimensional table with an equal number of rows and columns.  Each row is a sequence of chips

26. 26 Kyung Hee University Sequence Generation

27. 27 Kyung Hee University Properties of Orthogonal Sequences 1.If we multiply a sequence by -1, every element in the sequence is complemented 2.If we multiply two sequences, element by element and add the result, we get a number called the inner product. If two sequences are the same, we get N, where N is the number of sequences; if different,we get 3. Inner product of a sequence by its complement is –N. So A·B is 0.

28. 28 Kyung Hee University Example 1 Check to see if the second property about orthogonal codes holds for our CDMA example. Solution The inner product of each code by itself is N. This is shown for code C; you can prove for yourself that it holds true for the other codes. C. C =  If two sequences are different, the inner product is 0. B. C = 

29. 29 Kyung Hee University Example 2 Check to see if the third property about orthogonal codes holds for our CDMA example. Solution The inner product of each code by its complement is  N. This is shown for code C; you can prove for yourself that it holds true for the other codes. C. (  C ) =  The inner product of a code with the complement of another code is 0. B. (  C ) = 