Multiplexing in Computer Network

What is Multiplexing?

The multiplexing technique is commonly used in telecommunications and computer networks to maximize the use of available bandwidth and optimize data transfer efficiency. There are several types of multiplexing, including time-division multiplexing (TDM), frequency-division multiplexing (FDM), and wavelength-division multiplexing (WDM). TDM divides the transmission medium into time slots, allowing multiple signals to be transmitted sequentially. FDM divides the transmission medium into frequency bands, allowing multiple signals to be transmitted simultaneously. WDM divides the transmission medium into multiple optical channels of different wavelengths, allowing multiple signals to be transmitted over a single fiber optic cable.

Multiplexing has numerous applications in modern technology, including in digital television broadcasting, internet networking, and telephone communication systems. By allowing multiple signals to be transmitted over a single transmission medium, multiplexing enables faster data transfer and improved efficiency in communication systems. Multiplexing is an essential technique that plays a crucial role in modern telecommunications and computer networks, allowing for more efficient use of bandwidth and improved data transfer speeds.

Different Ways of Multiplexing

Multiplexing is an important concept for the GATE CSE exam. it is the process of combining multiple signals or data streams into a single signal or transmission medium. There are several different methods of multiplexing, including time-division multiplexing (TDM), frequency-division multiplexing (FDM), wavelength-division multiplexing (WDM), code-division multiplexing (CDM), and space-division multiplexing (SDM).

• Frequency Division Multiple Access (FDMA)
• Time Division Multiple Access (TDMA)
• Code Divisional Multiple Access (CDMA)

Frequency Division Multiplexing

In FDMA, we divide the whole bandwidth of the channel into small segments and allot it to different users so that they can access the channel at the same time by using their allotted bandwidth.

Time Division Multiplexing

In TDMA, the whole time slot is divided among different users so that at a time only one user is accessing the channel.

Key Points

• The bandwidth requirement in TDMA and FDMA is almost the same for the same number of users.
• The TDMA system can be used to multiplex analog or digital signals, however, it is more suitable for digital signal multiplexing.
• The communication channel over which the TDMA signal is traveling should ideally have an infinite bandwidth in order to avoid signal distortion. Such channels are known as band-limited channels.

Code Division Multiplexing (CDMA)

• Instead of splitting the RF channel into sub-channels or time slots, each slot has a unique code. In the diagram, the channel is split into four code slots.

Digital Modulation Schemes

Digital Modulation Schemes are techniques used to modify digital signals to be transmitted over a communication channel. These techniques involve varying one or more of the signal's properties, such as amplitude, phase, or frequency, to carry information. There are several types of digital modulation schemes, including amplitude shift keying (ASK), frequency shift keying (FSK), phase shift keying (PSK), and quadrature amplitude modulation (QAM).

Digital Carrier Modulation: Commonly used digital modulation schemes are

• Amplitude Shift Keying (ASK)
• Frequency Shift Keying (FSK)
• Phase Shift Keying (PSK).

Amplitude Shift Keying (ASK)

• The amplitude of a high-frequency carrier is varied in accordance with digital data (0 or 1).

S(t) = A_c cos 2πf_ct; 0 ≤ t ≤ T_b

= 0; otherwise

• Bandwidth = 2 × 1/T_b= 2 × bit rate
• For digital input 1 amplitude level is high and for digital input 0 amplitude level is low.
• The signaling used is on-off signaling.

Demodulation of ASK

• For binary digit 1, A_c cos 2π f_ct × A_c cos 2πf_ct = (A²_c/2)[1 + cos 4πf_ct]

• Output of LPF = (A²_c /2)
• For binary digit 0 output of LPF = 0
• In ASK, the probability of error (P_e) is high.
• In ASK, SNR is less.

Know More: Difference between Multiplexing and Demultiplexing

Phase Shift Keying (PSK)

• In phase shift, the keying phase of the high-frequency carrier is varied in accordance with digital data 1 or 0.

• NRZ signaling is used.

S(t) = A_c cos 2πf_ct for bit 1

= – A_c cos 2pf_ct for bit 0

• The frequency of the carrier must be multiple of a bit rate.

T_b = n/f_c

F_c = nr_b

• In the case of PSK, the probability of error is less.
• In the case of PSK, SNR is high.
• Mainly used technique in wireless transmission.

Frequency Shift Keying (FSK)

• In frequency shift keying, frequency of carrier is varied in accordance with digital data (1 or 0).
• For digital data 0, we use frequency f₂ and For digital data 1 we use frequency f₁.

• NRZ signaling is used here
• VCO: The schematic diagram of VCO is given below

Bandwidth = 2Δf + 2f_m

Bandwidth = f₁ + (1/T_b) – f₂ + (1/T_b)

= f₁ – f₂ + (2/T_b); f₁ – f₂ = 2Δf

Key Points

• In the case of FSK, SNR is high but P_e is less.
• Multiplexing is difficult in FSK.

Differential Phase Shift Keying (DPSK)

In PSK it needs a complicated synchronizing circuit at the receiver, this disadvantage of PSK is removed in DPSK.

V_DPSK(t) = V(t)/V

A cos ω₀t = ± A cos ω₀t

Note: The advantage of DPSK over PSK is, DPSK does not require a coherent carrier for demodulation.

Comparison of Digital Modulation Schemes

Digital modulation schemes are used to modulate a digital signal onto an analog carrier signal for efficient transmission over a communication channel. There are several types of digital modulation schemes that differ in their complexity, data rate, and susceptibility to noise and interference.

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