1. Dr. Clincy Professor of CS
Chapter 5 and 6 Handout #5
Dr. Clincy
Professor of CS
Test on February 12th
Your test will cover lectures 1 – 8 and will be 75 minutes
You should use a calculator
You can view the handouts via your laptop or you can print them - you shouldn’t use the browser
Dr. Clincy
Lecture

2. Chapter 6: Bandwidth Utilization: Multiplexing and Spreading
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Lecture

3. Multiplexing & Spreading (Physical Layer Issues)
Up to this point, you have learning about translating “data” into a “signal” – so that the “signal” can travel across the transport
It would be very efficient use of the transport’s bandwidth if multiple signals could travel on the transport at the same time ?
Also, it would be great if we could protect against eavesdropping
That efficiency can be achieved by multiplexing; privacy and anti-jamming can be achieved by spreading.
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4. SPREAD SPECTRUM
In spread spectrum (SS), we combine signals from different sources to fit into a larger bandwidth, but our goals are to prevent eavesdropping and jamming. To achieve these goals, spread spectrum techniques add redundancy.
Typically used for wireless applications – privacy outweighs efficiency in this case
Frequency Hopping Spread Spectrum (FHSS) Direct Sequence Spread Spectrum Synchronous (DSSS)
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5. Frequency selection in FHSS
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6. DSSS – Direct Sequence Spread Spectrum
Each bit sent by the Tx is replaced with a set of bits called a “chip code”
For the time it takes to send the original single bit, it now will take more time to send the chip code
Therefore, the data rate must be N times the original data rate, where N is the # of bits of the chip code
Also, the bandwidth for the chip code should N times greater than the original bit stream’s BW
Example of original bits being transmitted as 6-bit chip codes
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7. DSSS using polar NRZ encoding
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8. Multiplexing
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9. Dividing a link into channels – Multiplexing in general
Explain this
Categories of multiplexing
Will also cover Statistical Time-Division Multiplexing
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10. Frequency-division multiplexing
Divide the link’s bandwidth into separate channels (guardband separating each channel)
Recall from the Modulation Lectures that – being able to modulate around different “carrier frequencies” was important to being able to adjust the modulated signal into a particular “band” (bandpass signal)
On the MULTIPLEXING SIDE
Resultant modulated signals are combined into a single composite signal
Signals modulate different carrier frequencies (based on amplitude in this case)
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11. FDM demultiplexing example
On the DEMULTIPLEXING SIDE
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12. Example
Assume that a voice channel occupies a bandwidth of 4 kHz. We need to combine three voice channels into a link with a bandwidth of 12 kHz, from 20 to 32 kHz. Show the configuration, using the frequency domain. Assume there are no guard bands.
Solution
We shift (modulate) each of the three voice channels to different bandwidth. We use the 20- to 24-kHz bandwidth for the first channel, the 24- to 28-kHz bandwidth for the second channel, and the 28- to 32-kHz bandwidth for the third one. Then we combine them into a single composite signal.
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13. Wavelength-division multiplexing
Same as FDM but instead of electrical type signals – muxing optical signals (light signals)
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14. Time Division Multiplexing (TDM)
All networking devices work off clock ticks (explain)
Do “tap” analogy
Explain this
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15. Synchronous time-division multiplexing
Given n connections needing to be muxed, each frame is divided into n parts (for each slot)
Also notice that the time duration before muxing is 1/3 of the time duration after muxing
In this case, each frame is divided into 3 time slots
For synchronous TDM, the Tx and Rx must be in synch for the Rx to “pull out” of the frame the correct set of data (called interleaving)
For synchronous TDM, the data rate of the output link must be n times the data rate of the connection to guarantee the flow of data
In keeping the mux and demux in synch, synch bits (framing bits) are added at the beginning of each frame
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16. Suppose the input data rates are different ?
Multilevel multiplexing
When input data rates are multiple of others – can be combined to make equal – for example, the two 20 kbps links could be muxed together as a 40 kbps link
Multi-slot multiplexing
Allocate more than 1 time slot in a frame to a single input – for example, the 50 kbps line gets 2 slots, while the 25 kbps lines get 1 slot each
Pulse Stuffing
Make the highest input data rate the dominate rate and then add dummy bits (stuffing) to the other input lines
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17. Statistical TDM
For STATISTICAL TDM - Time slots are dynamically allocated based on previous history
Slots are reserved – could be wasted slots
Slots are allocated to Input Lines with data only – no wasted slots – because of this, the address of the Rx has to be carried with the data
The address needs to be n bits to define N output lines – with n = log2N (ie. need 5-bit address for 32 output lines)
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18. If time remains, we can review the concepts covered in lectures 1-8
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