# TDM and FDM MCQ || TDM and FDM Questions and Answers

1. Which of the following type of multiplexing cannot be used for analog signaling?

1. FDM
2. TDM
3. WDM
4. None of these

Explanation

Multiplexing is the process of combining multiple signals into one signal, over a shared medium.

If the analog signals are multiplexed, then it is called as analog multiplexing. Similarly, if the digital signals are multiplexed, then it is called as digital multiplexing.

Types of Multiplexing:

(1) Analog Multiplexing

• Frequency Division Multiplexing
• Wavelength Division Multiplexing

(2) Digital Multiplexing

• Time Division Multiplexing
• Synchronous TDM
• Asynchronous TDM

Digital Multiplexing:
The term digital represents discrete bits of information. Hence, the available data is in the form of frames or packets, which are discrete.

Time Division Multiplexing

• In Time Division Multiplexing (TDM), the time frame is divided into slots. This technique is used to transmit a signal over a single communication channel, by allotting one slot for each message.
• Time Division Multiplexing (TDM) can be classified into Synchronous TDM and Asynchronous TDM.

Synchronous TDM

• In Synchronous TDM, the input is connected to a frame. If there are ‘n’ number of connections, then the frame is divided into ‘n’ time slots. One slot is allocated for each input line.
• In this technique, the sampling rate is common for all signals and hence the same clock input is given. The MUX allocates the same slot to each device at all times.

Asynchronous TDM

• In Asynchronous TDM, the sampling rate is different for each of the signals and a common clock is not required.
• If the allotted device for a time slot transmits nothing and sits idle, then that slot can be allotted to another device, unlike synchronous
• This type of TDM is used in Asynchronous transfer mode networks.

Analog Multiplexing:

The signals used in analog multiplexing techniques are analog in nature. The analog signals are multiplexed according to their frequency (FDM) or wavelength (WDM).

Frequency Division Multiplexing

• In analog multiplexing, the most used technique is Frequency Division Multiplexing (FDM).
• This technique uses various frequencies to combine streams of data, for sending them on a communication medium, as a single signal.
• Example − A traditional television transmitter, which sends a number of channels through a single cable uses FDM.

Wavelength Division Multiplexing

• Wavelength Division Multiplexing (WDM) is an analog technique, in which many data streams of different wavelengths are transmitted in the light spectrum.
• If the wavelength increases, the frequency of the signal decreases.
• A prism, which can turn different wavelengths into a single line, can be used at the output of MUX and input of DEMUX.
• Example − Optical fiber communications use the WDM technique, to merge different wavelengths into a single light for communication.

2. In TDM systems, channel separation is done with the use of

1. AND gates
2. Bandpass filter
3. Differentiator circuit
4. Integrator circuit

Explanation

Time Division Multiplexing:

• Time-division multiplexing (TDM) is a method of putting multiple data streams in a single signal by separating the signal into many segments, each having a very short duration. Each individual data stream is reassembled at the receiving end based on the timing.
• In Time Division Multiplexing (TDM), the time frame is divided into slots. This technique is used to transmit a signal over a single communication channel, by allotting one slot for each message.
• For separating channels in TDM, it is necessary to use  Time slots. For this purpose AND gates are used.
• Time Division Multiplexing (TDM) can be classified into Synchronous TDM and Asynchronous TDM.

3. To transmit N signals each band limited to fm Hz by time-division multiplexing will require a minimum bandwidth of

1. fm
2. 2fm
3. N fm
4. fm/N

Explanation

For Time division multiplexing (TDM) minimum Band width is given by-

${\left( {B.W} \right)_{min}} = \frac{{{R_b}}}{2}$

Where, Rb = Nnfs

N = No. of signals

n = no. of Bit

fs = sampling frequency = 2 × fm

fm = msg signal Bandwidth

${\left( {B.W} \right)_{min}} = \frac{{Nn2{f_m}}}{2} = Nn{f_m}$

Given:

N signal with fm Bandwidth.

TDM minimum B.W = Nnfm

Assume n = 1 Bit

BWmin = Nfm

4. Which technique is based on orthogonality?

1. WCD
2. FDM
3. TDM & FDM
4. None of the mentioned

Explanation

FDM technique is based on the orthogonality of sinusoids.

Orthogonal Frequency-Division Multiplexing (OFDM) is a modulation technique for data transmission which has been known since the 1960s.

OFDM is based on splitting the data stream to be transmitted onto several orthogonal subcarriers and thereby allowing an increased symbol period. Since low-rate modulation schemes are more robust to multipath, it is more effective to transmit many low-rate streams in parallel than one single high-rate stream.

OFDM is based on the well-known Frequency Division Multiplexing (FDM) technique. In FDM different streams of information are mapped onto separate parallel frequency channels.

5. Mobile WiMAX modulation technique is based on ________

1. OFDMA
2. TDM
3. WCDMA

Explanation

Mobile WiMAX modulation technique is based on the orthogonal frequency division multiple access (OFDM technique and particularly on the SOFDMA variant.

The frequency 1 division multiplexing (FDM) principle consists in using multiple frequencies to transmit different 3 signals in parallel. This is achieved by assigning a frequency range or subcarrier to each signal then modulating it by data. The multiple subcarriers used for transmitting different signals should be separated by guard bands to prevent interferences.

6. Frequency division multiplexing: Frequency slots::time division multiplexing:?

1. Time slots
2. Coded information
3. Pulsed information
4. Band slots

Explanation

Frequency division multiplexing: Frequency slots::time division multiplexing: time slot

The basic idea of the FDM is to divide the available bandwidth into many narrow sub-bands and to use a large number of parallel narrow-band subcarriers rather than a single wide-band carrier to transfer the information.

In time-division multiplexing (TDM), each signal occupies the entire bandwidth of the channel. However, each signal is transmitted for only a brief time. In other words, multiple signals take turns transmitting over a single channel.

7. The number of channels in a TDMA system is given by

1. $N = \dfrac{m(2B_{Tot} – B_{guard})}{B_c}$
2. $N = \dfrac{m(B_{Tot} – 2B_{guard})}{B_c}$
3. $N = \dfrac{m(B_{Tot} – 2B_{guard})}{2B_c}$
4. $N = \dfrac{2m(B_{Tot} – B_{guard})}{B_c}$

Answer.2. $N = \dfrac{m(B_{Tot} – 2B_{guard})}{B_c}$

Explanation

Time-division multiple access (TDM is a channel access method for shared-medium networks.

It allows several users to share the same frequency channel by dividing the signal into different time slots. Number of channels in TDMA system is given by-

$\rm N=\dfrac{m*(B_{tot}-2*B_{gaurd})}{B_c}$

N = number of channels

m = number of TDMA users per ratio channel

Btot = total spectrum allocation

Bguard = Guard Band

Bc = Channel bandwidth

Note:-

Number of channels in FDMA system is given by-

$\rm N=\dfrac{B_t-B_{guard}}{B_c}$

N = number of channels

Bt = total spectrum allocation

Bguard = guard band

Bc = Channel bandwidth

8. A signal is band-limited to 3.6 kHz and three other signals are band-limited to 1.2 kHz each. These signals are to be transmitted by means of time-division multiplexing. If each signal is sampled at its Nyquist rate, then the speed of the commutator (by assuming 6 samples per rotation) is

1. 864000 r.p.m.
2. 144000 r.p.m.
3. 86400 r.p.m.
4. 14400 r.p.m.

Explanation

Let the signals are m1, m2, m3, and m4

 Signal Maximum Frequency Component Sampling Frequency m1 fm1 = 3.6 kHz fs1 = 3.6 kHz m2 fm2 = 1.2 kHz fs2 = 3.6 kHz m3 fm3 = 1.2 kHz fs3 = 3.6 kHz m4 fm4 = 1.2 kHz fs4 = 3.6 kHz

The speed of the commutator will be the HCF (fs1, fs2, fs3, fs4) × 60 rpm

= HCF(7.2k, 2.4k, 2.4k, 2.4k) × 60 rpm

= 2.4 k × 60 rpm

= 144 rpm

Speed = 144,000 rpm

9. 1. The information from different measuring points is transmitted serially on the same communication channel.

2. It involves the transmission of data samples rather than continuous data transmission.

3. It is especially useful when telemetering fast-changing, high bandwidth data.

Which of the above statements are valid in respect to TDM?

1. 1, 2 and 3
2. 1 and 3 only
3. 1 and 2 only
4. 2 and 3 only

Explanation

• In TDM a single communication channel is used to transmit and receive the information from different sources serially.
• During transmission of information, it is divided into a small-small sample called the TDM frame and then transmitted over the channel.
• In TDM the whole time interval is divided into smaller time slots and each time slot are assigned to a fixed user.
• Each user can use the entire channel bandwidth during the allotted time slice, so that channel Bandwidth is utilized completely.
• In TDM, synchronization is necessary at the receiver end so fast-changing of data can create inter-symbol interference. So for a high-speed telemetry system, TDM is not the best choice.
• Hence, by considering the above points above TDM, statement 1 and 2 is valid but statement 3 is not Valid

10. Which technique provides constant delay?

1. Synchronous TDM
2. Non-synchronous TDM
3. Synchronous & Non-synchronous TDM
4. None of the mentioned