Digital Communication and Switching - Lecture 8 & 9

Digital Communication and Switching - Lecture Notes

• Course: Digital Communication and Switching
• Semester: I AY 2024-25
• Lecture: 8 & 9 - Multiplexing
• Lecturer: Lenin Joseph (LJ)
• Date: Dec 25, 2024

Multiplexing

• Multiplexing is a set of techniques enabling simultaneous transmission of multiple signals across a single data link.
• Key techniques include Frequency-Division Multiplexing (FDM) and Time-Division Multiplexing (TDM).
• FDM divides the available bandwidth into non-overlapping frequency sub-bands, each carrying a separate signal.
• TDM transmits multiple signals over a single channel by allocating them, in turn, different time slots.

Frequency-Division Multiplexing (FDM)

• Divides the total bandwidth into non-overlapping frequency sub-bands.
• Each sub-band carries a separate signal.
• Suitable when link bandwidth exceeds combined bandwidth of signals.
• Applications include: Telemetry, FM and TV broadcasting, telephone calls via high-capacity trunk lines, satellite communication, and broadband DSL.

Time-Division Multiplexing (TDM)

• Transmits multiple signals over a single channel by assigning alternating time slots.
• Ideal when transmission medium data rate surpasses sender/receiver data requirements.
• Includes synchronous and asynchronous types.
• Applications include: Telegraphy, Digital Telephony

Synchronous TDM

• Allocates the same time slot to each device irrespective of data transmission needs.
• No addressing required.
• Inefficient: link capacity not fully utilized if not all devices have data to send.
• Data transmission in a fixed order.

Asynchronous TDM

• Flexible approach – time slots allocated only when data is present.
• Time slot allocation not fixed.
• Requires addressing to identify source of data in a frame.
• More efficient than synchronous TDM, maximizing link usage.

Wave-Division Multiplexing (WDM)

• Similar to FDM, but for light signals transmitted through fiber optic channels.
• Employs very high frequencies.

Space Division Multiplexing (SDM)

• Transmits multiple wireless signals simultaneously on a common frequency without interference.
• Achieved via multiple antenna elements forming a phased array antenna.

Multiple Access Techniques

• Channel access methods permitting multiple terminals to share a multi-point transmission medium.
• Examples include wireless networks, bus networks, ring networks, and star networks.
• Based on multiplexing methods (allowing several data streams/signals to share one channel).

Time Division Multiple Access (TDMA)

• Special form of TDM enabling sharing of a single frequency channel through allocating time slots.
• Employed in digital 2G cellular systems like GSM.
• Time slots allocated one-by-one.

Frequency Division Multiple Access (FDMA)

• Channel access scheme based on FDM for different data streams.
• Data streams allocated to specific frequency bands.
• Example – early (1G) cell phone systems, where calls were assigned to different uplink/downlink channels.

Space Division Multiple Access (SDMA)

• Channel access method for mobile communication systems to reuse cell phone frequencies in a service area.
• Reusing frequencies possible with adequate separation between cells.

Code Division Multiple Access (CDMA)

• Multiple access scheme based on spread spectrum.
• Utilizes wider radio spectrum than data rate.
• Allows multiple simultaneous signals over same carrier frequency using unique spreading codes.
• Employed in satellite systems (sharing transponders) and cellular phone systems for efficient use of spectrum compared to other methods.

Explore the fundamentals of multiplexing in Digital Communication and Switching with a focus on Frequency-Division Multiplexing (FDM) and Time-Division Multiplexing (TDM). This quiz covers essential concepts related to how multiple signals can be transmitted simultaneously over a single data link. Enhance your understanding of these crucial techniques used in various applications.