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

Chapter 6: Bandwidth Utilization: Multiplexing and Spreading Dr. Clincy Lecture

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. Dr. Clincy Lecture

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) Dr. Clincy Lecture

Frequency selection in FHSS Dr. Clincy Lecture

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 Dr. Clincy Lecture

DSSS using polar NRZ encoding Dr. Clincy Lecture

Multiplexing Dr. Clincy Lecture

Dividing a link into channels – Multiplexing in general Explain this Categories of multiplexing Will also cover Statistical Time-Division Multiplexing Dr. Clincy Lecture

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) Dr. Clincy Lecture

FDM demultiplexing example On the DEMULTIPLEXING SIDE Dr. Clincy Lecture

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. Dr. Clincy Lecture

Wavelength-division multiplexing Same as FDM but instead of electrical type signals – muxing optical signals (light signals) Dr. Clincy Lecture

Time Division Multiplexing (TDM) All networking devices work off clock ticks (explain) Do “tap” analogy Explain this Dr. Clincy Lecture

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 Dr. Clincy Lecture

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 Dr. Clincy Lecture

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) Dr. Clincy Lecture

If time remains, we can review the concepts covered in lectures 1-8 Dr. Clincy Lecture