Multiplexing: TDM, FDM, WDM, CDMA, and Packet Switching

TAKEAWAYS:

• Multiplexing combines multiple signals into a single shared medium to maximize bandwidth efficiency and reduce costs.
• TDM divides time slots (e.g., T1 lines in telephony), FDM divides frequencies (e.g., FM radio), and WDM uses light wavelengths (e.g., fiber optics).
• CDMA assigns unique codes to users (e.g., 4G/5G networks) instead of time/frequency slots.
• Packet switching (e.g., the internet) breaks data into packets, routes them dynamically, and reassembles them at the destination.
• Analog vs. digital multiplexing differs in how signals are combined (continuous vs. discrete time slots).
• Nepal’s NTC and Ncell use multiplexing for voice/data traffic; eSewa/Khalti rely on packet-switched networks for transactions.

1. Introduction to Multiplexing

Multiplexing is the technique of combining multiple signals (voice, data, video) into a single transmission medium (copper wire, fiber, radio waves) to share bandwidth efficiently. It is essential for modern telecommunications, enabling:

• Cost savings (shared infrastructure).
• Higher capacity (more users/devices on one channel).
• Efficient resource use (e.g., internet traffic, mobile networks).

Why Multiplex?

Without multiplexing, each user would need a dedicated channel, which is impractical for:

• Telephone networks (millions of calls simultaneously).
• Internet (billions of devices sharing bandwidth).
• Broadcast TV (multiple channels on one cable).

2. Types of Multiplexing

Multiplexing is classified based on how signals are separated in the shared medium. The four key types are:

A. Time Division Multiplexing (TDM)

Definition: Divides the time domain into slots, assigning each signal a fixed time interval. How it works:

1. A synchronized clock allocates time slots to each input signal.
2. Signals take turns transmitting in rapid succession.
3. At the receiver, a demultiplexer separates them using the same timing.

Visual: TDM Time Slots

timeline
    title TDM Frame Structure (4 Users)
    2023-01-01 --> | User 1 | User 2 | User 3 | User 4 | (Repeats)
    Note: Each "slice" is a time slot (e.g., 125 μs for T1 lines).

Real-World Example: NTC’s Telephone Network

• Nepal’s PSTN (Public Switched Telephone Network) uses TDM to carry 64 kbps voice channels in T1/E1 frames (24/30 channels per frame).
• Worked Example: If 4 users share a TDM line with a 1.544 Mbps (T1) rate, each gets: $$\text{Slot rate}=\frac{1.544\text{ Mbps}}{24\text{ channels}}=64\text{ kbps (voice)}.$$ This matches Nepal’s analog phone line speed.

Advantages:

• Simple to implement.
• Efficient for synchronous data (e.g., voice, video).
• Used in PDH (Plesiochronous Digital Hierarchy) and SDH (Synchronous Digital Hierarchy).

Disadvantages:

• Wastage if a user’s slot is idle (e.g., silent phone calls).
• Requires synchronization between sender/receiver.

B. Frequency Division Multiplexing (FDM)

Definition: Divides the frequency spectrum into non-overlapping bands, assigning each signal a unique frequency range. How it works:

1. Each signal is modulated to a different carrier frequency.
2. Signals are combined and transmitted simultaneously.
3. At the receiver, bandpass filters separate them.

Visual: FDM Frequency Bands

Real-World Example: FM Radio in Nepal

• Radio Sagarmatha (98.5 MHz) and FM 100 (100.0 MHz) use FDM on the same antenna.
• Worked Example: If a cable TV system has 8 channels, each with a 6 MHz bandwidth, the total spectrum needed is: $$8\times 6\text{ MHz}=48\text{ MHz}.$$ This is how NTC’s cable TV services work.

Advantages:

• No interference between channels (if filters are perfect).
• Asynchronous signals (e.g., radio broadcasts) work well.
• Used in AM/FM radio, cable TV, and early telephone systems.

Disadvantages:

• Guard bands (unused frequencies) reduce efficiency.
• Hardware complexity (filters, mixers).

C. Wavelength Division Multiplexing (WDM)

Definition: A fiber-optic version of FDM, where different wavelengths of light carry separate signals. Types:

1. Coarse WDM (CWDM): Uses 18 channels with 20 nm spacing (e.g., 1550 nm, 1570 nm).
2. Dense WDM (DWDM): Uses 80+ channels with 0.8 nm spacing (e.g., 1540 nm to 1560 nm).

Visual: DWDM Spectrum

Real-World Example: NTC’s Fiber-Optic Backbone

• Nepal’s NTC uses DWDM to carry internet traffic between Kathmandu and Pokhara.
• Worked Example: If a DWDM system has 16 channels, each at 10 Gbps, the total capacity is: $$16\times 10\text{ Gbps}=160\text{ Gbps}.$$ This powers Nepal’s high-speed internet.

Advantages:

• Massive bandwidth (terabits per second in modern systems).
• Immunity to electromagnetic interference (unlike copper).
• Used in long-haul networks (e.g., NTC, Ncell backhaul).

Disadvantages:

• Expensive (lasers, amplifiers, dispersion compensation).
• Requires precise wavelength control.

D. Code Division Multiplexing (CDMA)

Definition: Assigns a unique code to each user; all signals share the same frequency and time but are separated by codes. How it works:

1. Each user’s data is multiplied by a unique pseudorandom code (e.g., Walsh code).
2. Signals overlap in time/frequency but are orthogonal (unlike TDM/FDM).
3. At the receiver, correlation extracts the desired signal.

Visual: CDMA Code Chipping

sequenceDiagram
    participant User1 as User 1 (Code A)
    participant User2 as User 2 (Code B)
    participant Transmitter as Shared Channel
    participant Receiver as Receiver (Correlator)
    User1->>Transmitter: Data × Code A (e.g., 1, -1, 1, 1)
    User2->>Transmitter: Data × Code B (e.g., 1, 1, -1, -1)
    Transmitter-->>Receiver: Combined Signal
    Receiver->>User1: Correlate with Code A → Extract Data
    Receiver->>User2: Correlate with Code B → Extract Data
    Note: Codes are designed to be "orthogonal" (no cross-talk).

Real-World Example: Ncell’s 4G/5G Network

• Ncell uses CDMA (specifically, CDMA2000 for 3G and LTE for 4G) to handle thousands of users on the same frequency.
• Worked Example: In a CDMA system with 64 users, each gets a unique 64-bit Walsh code. If two users transmit simultaneously:
  • Their signals do not interfere because the codes are orthogonal.
  • The receiver multiplies the combined signal by the desired code to extract data.

Advantages:

• More users per channel (vs. TDM/FDM).
• Resistant to jamming (used in military communications).
• Soft handoff (seamless switching between base stations).

Disadvantages:

• Complex hardware (spread-spectrum modems).
• Higher power consumption (due to code processing).
• Interference from non-CDMA users (near-far problem).

E. Packet Switching (Statistical Multiplexing)

Definition: Breaks data into packets, which are routed dynamically through the network and reassembled at the destination. How it works:

1. Data is segmented into packets (header + payload).
2. Packets take different paths (unlike circuit switching).
3. Buffers store packets if congestion occurs.
4. Reassembly happens at the destination.

Visual: Packet Switching vs. Circuit Switching

Real-World Example: eSewa/Khalti Transactions

• When you pay via eSewa, your transaction is split into packets:
  1. DNS lookup (eSewa’s IP address).
  2. TCP handshake (connection setup).
  3. Payment data packets (routed via NTC/Ncell).
  4. Confirmation packet (reassembled at your phone).
• Worked Example: If a 1 MB file is split into 1000 packets, and packets take 10 ms, 20 ms, 15 ms to arrive, the last packet’s delay determines the perceived speed (not the average).

Advantages:

• Efficient bandwidth use (only transmits when data is present).
• Fault tolerance (lost packets are retransmitted).
• Supports variable-bit-rate services (e.g., video calls, web browsing).

Disadvantages:

• Delay and jitter (packets may arrive late or out of order).
• Complexity (routing, congestion control, error handling).

3. Comparison of Multiplexing Techniques

4. Hybrid Multiplexing Systems

In real networks, multiple multiplexing techniques are combined:

1. TDM + FDM:
   • Used in cable TV (FDM for channels, TDM for digital signals within a channel).
2. WDM + TDM:
   • DWDM systems use TDM within each wavelength (e.g., 10G TDM per λ).
3. CDMA + FDM:
   • 4G LTE uses FDM for carriers and CDMA for user separation.

Example: Ncell’s 4G Network

• FDM: Different frequency bands (e.g., 1800 MHz, 2600 MHz).
• CDMA: OFDM (Orthogonal FDM) for user separation.
• TDM: Time slots for uplink/downlink.

5. Multiplexing in Nepal’s Telecommunications

Case Study: Kathmandu Traffic as a TDM Analogy Imagine Kathmandu’s ring road as a TDM system:

• Each lane = a time slot.
• Cars = signals.
• If only one car uses a lane at a time (like TDM), traffic flows smoothly.
• But if multiple cars share a lane (like FDM), collisions (interference) occur.
• Solution: Use multiple lanes (FDM) or dynamic lane switching (packet switching) like Kathmandu’s smart traffic lights.

6. Exam Tip: How to Score Full Marks

1. Define clearly: Always start with a one-sentence definition (e.g., "TDM is a multiplexing technique that divides the time domain into slots...").
2. Draw diagrams: For TDM/FDM/WDM/CDMA, sketch a time/frequency/wavelength/code diagram. Use Mermaid blocks for processes.
3. Compare in tables: The comparison table is a high-mark question. Memorize the key differences (efficiency, applications, synchronization).
4. Real-world links: Connect to NTC, Ncell, eSewa, or NEPSE in every answer. Example:

   "Like Ncell’s 4G network uses CDMA to allow multiple users on the same frequency, TDM assigns fixed time slots to each user in a T1 line."

5. Worked examples: Solve numerical problems (e.g., "If a DWDM system has 16 channels at 10 Gbps each, calculate total capacity").
6. Advantages/disadvantages: Always list 2 pros and 2 cons for each technique.
7. Hybrid systems: Mention TDM+FDM or WDM+TDM in answers about modern networks.

Common Mistakes to Avoid:

• Confusing TDM (time slots) with FDM (frequency bands).
• Forgetting guard bands in FDM or orthogonality in CDMA.
• Ignoring packet switching in internet-related questions.

In the Real World

1. eSewa/Khalti Transactions

   • Packet Switching (TCP/IP): When you transfer money, your request is split into packets, routed via NTC/Ncell’s IP network, and reassembled at the bank’s server.
   • Why it matters: Without packet switching, online payments would fail if one packet was lost.

2. Ncell’s 4G/5G Network

   • CDMA (OFDM): Your phone shares the same 2600 MHz frequency with thousands of others, but unique codes prevent interference.
   • Why it matters: Enables high-speed data for YouTube, WhatsApp, and mobile banking.

3. NTC’s Fiber-Optic Backbone

   • DWDM: Carries 160 Gbps by combining 16 wavelengths, each at 10 Gbps.
   • Why it matters: Powers Nepal’s internet, including eSewa, Daraz, and online classes.

4. FM Radio in Nepal

   • FDM: Radio Sagarmatha (98.5 MHz) and FM 100 (100.0 MHz) broadcast simultaneously without overlapping.
   • Why it matters: Lets you listen to multiple stations on one antenna.

5. NEPSE Stock Market

   • Packet Switching: Stock prices are sent as real-time packets to brokers’ terminals.
   • Why it matters: Ensures low-latency updates for traders.

Visual Summary: Multiplexing Techniques