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Review Questions:
1. Describe the goals of multiplexing.
It is the set of techniques that allows the simultaneous transmission of multiple signals across a single data link. As data and telecommunications use increases, so does traffic.
2. List three main multiplexing techniques mentioned in this chapter.
- Frequency-division multiplexing (FDM): It is an analog technique that can be applied when the Bandwidth of a link (in hertz) is greater than the combined bandwidths of the signals to be transmitted.
- Wavelength-division multiplexing (WDM): It is designed to use the high-data-rate capability of fiber-optic cable.
- Time-division multiplexing (TDM): It is a digital process that allows several connections to share the high bandwidth of a link Instead of sharing a portion of the bandwidth as in FDM, time is shared.
3. Distinguish between a link and a channel in multiplexing.
The word link refers to the physical path. The word channel refers to the portion of a link that carries a transmission between a given pair of lines. One link can have many (n) channels.
4. Which of the three multiplexing techniques is (are) used to combine analog signals?
Which of the three multiplexing techniques is (are) used to combine digital signals?
- FDM and WDM are used to combine analog signals, (The bandwidth is shared).
- TDM is used to combine digital signals, (The time is shared).
5. Define the analog hierarchy used by telephone companies and list different levels of
the hierarchy.
To maximize the efficiency of their infrastructure, telephone companies have traditionally multiplexed analog signals from lower-bandwidth lines onto higher-bandwidth lines. In this way, many switched or leased lines can be combined into fewer but bigger channels.
- For analog lines, FDM is used. One of these hierarchical systems used by AT&T is made up of groups, super groups , master groups, and jumbo groups.
8. Distinguish between synchronous and statistical TDM.
In synchronous TDM , each input has a reserved slot in the output frame. This can be inefficient if some input lines have no data to send. In statistical TDM, slots are 2 dynamically allocated to improve bandwidth efficiency. Only when an input line has a slot's worth of data to send is it given a slot in the output frame.
9. Define spread spectrum and its goal.
In spread spectrum , we spread the bandwidth of a signal into a larger bandwidth. Spread spectrum techniques add redundancy; they spread the original spectrum needed for each station. The expanded bandwidth allows the source to wrap its message in a protective envelope for a more secure transmission. We discussed frequency hopping spread spectrum (FHSS) and direct sequence spread spectrum (DSSS).
Exercises:
10. Assume that a voice channel occupies a bandwidth of 4 kHz. We need to multiplex 10
voice channels with guard bands of 500 Hz using FDM. Calculate the required
bandwidth.
To multiplex 10 voice channels, we need nine guard bands. The required bandwidth is then B = (4 KHz) × 10 + (500 Hz) × 9 = 44.5 KHz.
11.We need to transmit 100 digitized voice channels using a pass-band channel of 20 KHz.
What should be the ratio of bits / Hz if we use no guard band?
Here, the bandwidth allocated to each voice channel, B = 20 KHz / 100 = 200 Hz. We know that, the data rate of each digitized voice channel = 64 Kbps. Modulation technique uses the ratio is, 64 Kbps / 200 = 64,000 / 200 = 320 bits / Hz.
Thank you
Preceded by:
Dr. Eng. Samy Elmokadem
