Introduction to Multiplexing

Introduction to Multiplexing: Sharing Communication Resources

Multiplexing is the technique of combining multiple signals or data streams into a single shared communication channel. Without multiplexing, every phone call would need its own dedicated cable from your house to the person you are calling — an impossibly expensive infrastructure. Multiplexing enables thousands of conversations to share a single optical fiber, millions of devices to share a wireless spectrum, and billions of internet packets to traverse shared network links.

Why Multiplexing Is Essential

Think of it this way: a highway has multiple lanes, allowing many cars to travel simultaneously without building a separate road for each car. Similarly, a communication channel (fiber, cable, radio spectrum) has far more capacity than a single user needs. Multiplexing divides this capacity among many users efficiently.

The economic argument: A single fiber optic cable costs the same whether it carries 1 phone call or 100,000. By multiplexing 100,000 calls onto one fiber, the cost per call drops by a factor of 100,000. This is why long-distance calls went from dollars per minute in the 1970s to essentially free today.

The capacity argument: A fiber optic cable has bandwidth of several THz (terahertz) — enough for millions of voice calls. Wasting this capacity on a single call would be like reserving an entire airport runway for one bicycle.

Fundamental Types of Multiplexing

Multiplexing divides shared resources along different dimensions:

1. Frequency Division Multiplexing (FDM)

• Each user gets a different frequency band
• All users transmit simultaneously but at different frequencies
• Like radio stations — each on its own frequency
• Used in: AM/FM radio, cable TV, ADSL, 4G/5G (OFDMA)

2. Time Division Multiplexing (TDM)

• Each user gets exclusive access for a brief time slot
• Users take turns rapidly (round-robin)
• Like a conversation where people speak one at a time in rotation
• Used in: telephone networks (E1/T1), GSM cellular, TDMA

3. Code Division Multiplexing (CDM)

• Each user gets a unique spreading code
• All users transmit simultaneously on the same frequency at the same time
• Separated by mathematical orthogonality of codes
• Like a room where everyone speaks different languages simultaneously
• Used in: 3G CDMA, GPS, military communication

4. Wavelength Division Multiplexing (WDM)

• Each signal uses a different wavelength (color) of light
• Multiple wavelengths travel through the same fiber simultaneously
• FDM applied to optical frequencies
• Used in: Long-haul fiber optic networks, submarine cables

5. Space Division Multiplexing (SDM)

• Multiple parallel spatial channels (separate fibers, antenna beams)
• Used in: MIMO wireless, multi-core fiber, sectored antennas

Multiplexing vs. Multiple Access

These terms are related but subtly different:

Multiplexing: Combining signals from a single source for transmission (done at one location — a multiplexer)

Multiple Access: Allowing multiple independent users to share a channel (done across distributed locations — a protocol)

The technical mechanisms are the same; the difference is administrative (centralized vs. distributed control).

The Multiplexer and Demultiplexer

Multiplexer (MUX): Combines N input channels into one output channel

• Located at the transmitting end
• Assigns resources (frequency, time slot, code) to each input
• Produces a composite signal for transmission

Demultiplexer (DEMUX): Separates the composite signal back into N individual channels

• Located at the receiving end
• Applies inverse operation (filtering, time-gating, correlation)
• Routes each recovered signal to the correct output

Synchronous vs. Statistical Multiplexing

Synchronous (fixed allocation):

• Each user gets a fixed allocation regardless of whether they have data to send
• Simple but wasteful if users are bursty (idle slots go unused)
• Example: E1 TDM — each of 30 channels gets one slot per frame

Statistical (dynamic allocation):

• Resources allocated on demand — active users get capacity, idle users get none
• More efficient for bursty data (internet traffic is highly bursty)
• Requires buffering and may introduce variable delay
• Example: Ethernet, WiFi, packet-switched networks

Statistical multiplexing improves efficiency dramatically: if each user is active only 10% of the time, statistical multiplexing serves 10× more users with the same capacity.

Key Performance Metrics

Multiplexing gain: Number of users that can share the channel (N)

Spectral efficiency: Total data rate per unit bandwidth (bits/s/Hz)

Fairness: Whether all users get equal access or some are prioritized

Crosstalk: Interference between multiplexed channels (ideally zero)

Overhead: Capacity lost to guard bands, guard times, or synchronization

The Evolution of Multiplexing in Telephony

The telephone network illustrates multiplexing evolution:

Each generation multiplied capacity by 10-1000× while reducing cost per channel.

Modern Multiplexing: OFDM

Orthogonal Frequency Division Multiplexing (OFDM) combines FDM with digital signal processing:

• Divides wideband channel into thousands of narrow subcarriers
• Each subcarrier carries a low-rate QAM signal
• Subcarriers are orthogonal (overlap without interfering)
• Eliminates guard bands — 100% spectral efficiency!
• Used in: WiFi, 4G LTE, 5G NR, DVB-T, DSL

OFDM is the dominant multiplexing technique in modern broadband communication.

Key Takeaways

1. Multiplexing enables multiple users to share a single communication channel, making modern telecommunications economically viable.
2. The four fundamental dimensions for multiplexing are frequency (FDM), time (TDM), code (CDM), and wavelength/space (WDM/SDM).
3. Statistical multiplexing outperforms fixed allocation for bursty traffic by dynamically assigning resources to active users only.
4. Guard bands/times between channels prevent crosstalk but reduce usable capacity — OFDM eliminates this waste.
5. Multiplexing capacity has increased by factors of millions over a century, driving down communication cost per user.
6. Modern systems combine multiple multiplexing types: OFDMA (frequency + time) in 5G, WDM + TDM in fiber, MIMO + OFDM in WiFi.