Unit II — Multiplexing & Switching Overview
Unit II covers how to efficiently share transmission media (multiplexing), route data through networks (switching), detect and correct errors in transmitted frames, and ensure reliable delivery using ARQ protocols.
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1. What is Multiplexing?
Multiplexing allows multiple signals to share a single physical transmission medium simultaneously. A MUX (Multiplexer) at the sender combines multiple signals; a DEMUX (Demultiplexer) at the receiver separates them back.
Why Multiplexing Is Essential
- Cost efficiency: One high-bandwidth link shared among many users is far cheaper than dedicated links for each
- Bandwidth utilisation: A single fibre cable can carry terabits per second — multiplexing utilises that capacity fully
- Scalability: Telephone exchanges, internet backbones, and cable TV systems serve millions of simultaneous users through multiplexing
Multiplexing Types Overview
| Type | Division Method | Signal Type | Guard Separation | Application |
|---|---|---|---|---|
| FDM | Different frequency bands | Analog | Guard bands | Radio, ADSL, cable TV |
| TDM | Different time slots | Digital | Frame sync | T1/E1 lines, SONET |
| WDM | Different light wavelengths | Optical (fibre) | Wavelength spacing | Fibre backbone, DWDM |
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2. Switching Overview
Switching routes data from source to destination through a network of interconnected switching nodes.
| Method | Dedicated Path | Store & Forward | Latency | Ideal For |
|---|---|---|---|---|
| Circuit Switching | Yes — reserved end-to-end | No (continuous stream) | Low once connected | Real-time voice/video |
| Packet Switching | No — shared dynamically | Yes (per packet) | Medium (variable queuing) | Internet, general data |
| Message Switching | No | Yes (entire message) | High | Email-like asynchronous traffic |
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3. Error Control Overview
Noise and interference corrupt bits during transmission. The Data Link layer uses:
- Error Detection: Identify that an error occurred → Parity Check, CRC, Checksum
- Error Correction: Fix without retransmission → Hamming Code, Reed-Solomon codes
- ARQ (Automatic Repeat reQuest): Detect error, signal sender to retransmit → Stop-and-Wait, Go-Back-N, Selective Repeat
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4. Key Definitions
| Term | Definition |
|---|---|
| MUX | Device combining multiple input signals onto one transmission medium |
| DEMUX | Device separating a combined signal back into individual signals |
| Guard Band | Unused frequency gap between FDM channels to prevent inter-channel interference |
| Time Slot | Fixed time interval assigned to one channel in TDM |
| Frame | Unit of data at the Data Link layer; contains header, payload, and trailer |
| Framing | Process of delimiting frame boundaries (bit stuffing, byte stuffing) |
| Switching Node | Intermediate node (switch/router) forwarding data toward destination |
Exam Tip: Know all three multiplexing types (FDM for analog, TDM for digital, WDM for fibre) and all three switching types (circuit for voice, packet for internet, message for obsolete systems). Both topics frequently appear as comparison table questions worth 5–10 marks.
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Study Deep: The Big Picture of Unit II
- Multiplexing and switching explain how millions of simultaneous phone calls and internet connections can use the same physical infrastructure.
- FDM is the reason ADSL can carry both phone calls and internet data on the same telephone line simultaneously — voice uses 0–4 kHz; DSL uses higher frequencies.
- Packet switching is why the internet is resilient: if one router fails, packets are automatically rerouted through alternate paths. This was the original design goal of ARPANET (precursor to the internet).
- Error control at the Data Link layer (CRC) is one of the most important parts of Ethernet and Wi-Fi — every Ethernet frame includes a 4-byte CRC that detects transmission errors before the frame reaches higher layers.