1. Multiplexing and Demultiplexing

2. Question Why cannot Verizon users get an iPhone from AT&T and get it work in Verizon's network?

3. iPhone@AT&T vs. iPhone@Verizon GSM vs. CDMA – Two primary systems in cellular phones. How do they differ from each other? – Why cannot you use your GSM phone in CDMA networks?

4. Multiplexing and Demultiplexing Multiplexing: A network word for sharing – Combining information streams from multiple sources for transmission over a shared medium – Multiplexor: a method/device to implement this. – Demultiplexing: Separating a combined stream back into individual streams

5. The Basic Types of Multiplexing Four basic approaches – Frequency Division Multiplexing (FDM) Combination in the frequency domain – Time Division Multiplexing (TDM) Combination in the temporal domain – Wavelength Division Multiplexing (WDM) A form of FDM used for optical fiber – Code Division Multiplexing (CDM) Combination with pure math magic

6. Frequency Division Multiplexing FMD: Each pair of sender and receiver use a particular carrier frequency.

7. Example: FM broadcasting 101 channels between 87.8 MHz – 108.0 MHz in North America – NYC: http://www.nyradioguide.com/freqlist.htmhttp://www.nyradioguide.com/freqlist.htm – SC: http://www.statecollege.com/music/radio.phphttp://www.statecollege.com/music/radio.php Each channel is assigned a frequency band – 200KHz Each channel has a center frequency – Majic 99: 99.5 MHz

8. Frequency Division Multiplexing Advantage: A dedicated frequency channel for each pair. Limitation: Frequency interference Requiring adequate spacing between channels. Guard band

9. Use of an FM Radio Channel If a carrier uses a single frequency, why does FDM allocates blocks of frequencies? Usually divided the range of frequencies into multiple sub-channels. – Increase the data rate Each carrier for different piece of information. – Increase immunity/reliability to interference Each carrier for the same piece of information.

10. Hierarchical FDM FM signals can be further modulated to a different frequency band! Source Carrier 1 Carrier 2

11. 11 Hierarchical FDM New YorkLos Angeles Street Block Neighborhood District City For each pair of telephone numbers, knowing their path in the FDM hierarchy is good enough to determine which frequency band to use in multiplexing. Analogy: Your mailing address

12. Wavelength Division Multiplexing (WDM) FDM in optical fiber – : wavelength Prism: multiplexing and demultiplexing device – a multiplexor: combines beams of light of various wavelengths into a single beam – a demultiplexor: separate a single beam into beams with different wavelengths. f * = c (speed of light)

13. Time Division Multiplexing (TDM) A simple trick: an item from one source per unit time slot

14. Synchronous TDM

15. Unfilled Slots in Synchronous TDM

16. Statistical TDM Also called asynchronous TDM by some – Eliminating unused slots – Statistical TDM takes less time to send the same amount of data Extra overhead – ID of the receiver in each slot  MAC address (Ch. 13)

17. Inverse Multiplexing

18. Code Division Multiplexing (CDM) Unlike FDM/TDM, CDM does not rely on any physical property of signals. – Uses an interesting mathematical idea CDMA: Code Division Multiple Access – A CDM version for cellular phones

19. Orthogonal Vector Spaces Vector – (x,y), (x,y,z), (a 1, a 2, …, a n ) Dot product of two vectors a = (a 1, a 2, …, a n ) and b = (b 1, b 2, …, b n ) – Must have the same number of elements. – Multiplying the corresponding pairs and adding up the products Two vectors are said to be orthogonal if their dot product is zero – a∙b = 0 a∙b= a 1 b 1 + a 2 b 2 + … + a n b n

20. Exercise: Orthogonal or Not? (1,-1) and (1,1) – yes (1,1,1,1) and (1, -1, -1, -1) – no (0, 0) and (1, 1) – yes (-2, -1, 1) and (1, 1, 3) – yes a∙b = 0

21. Example: Two Vector CDM A sender is assigned a vector, chip sequence, that is orthogonal to all other senders’ chip sequences. – Information from this sender (digitized voice) is processed with this vector.

22. Code Division Multiplexing The first step consists of converting the binary values into vectors that use -1 to represent 0: Multiplying C 1 x V 1 and C 2 x V 2 The final signal to be sent will be the sum of the two signals

23. Code Division Multiplexing On the receiving side – Use the sender A’s vector (1, -1) – chip sequence – Treat the sequence as a vector – Compute the dot product of the vector and the chip sequence Interpreting the result as a sequence produces: (2 -2 2 -2) – In binary: (1 0 1 0)

24. Can Other Senders Extract Information? Suppose Sender B does not send anything. One receiver uses B’s chip sequence to extract information. – (1, 1) ∙ ( (1, -1), (-1, 1), (1, -1), (-1, 1) )  ( 0, 0, 0, 0) Implication: A’s information cannot be intercepted by others.

25. CDM Signals Still Require Frequency Modulation CDM combines different signals into one. FM shifts the original signal to a higher frequency band for transmission

26. Back to the Previous Questions GSM phones vs. CDMA phones – Using very different multiplexing/demultiplexing techniques – GSM: TDM (early version), TDM + FDM GSM phones in different countries – May using different frequency bands. Quad-band: 850, 900, 1800, 1900 MHz – Phones must be able to pick up a right frequency band. Impact on consumers – Natural technology monopoly/barrier – Consumers are locked into a particular system. – More versatile phones?