Physical Layer Functionalities

🧭

On this page — click any heading below to jump straight to it. ⬅️ Back to P-II.II. Computer Communications & Networks

---

OSI Layer: Physical Layer (L1)

Definition

The Physical Layer (OSI Layer 1) is responsible for the actual transmission of raw bits over a physical medium — the lowest layer, dealing purely with hardware, electrical/optical/radio signals, and physical connections, with no awareness of addressing, framing, or error control (those begin at the Data Link layer).

Core Functions

1. Bit-to-Signal Encoding

Represents binary 0s/1s as physical signals appropriate to the medium:

• Digital-to-digital encoding — e.g., NRZ (Non-Return-to-Zero), Manchester encoding (each bit has a guaranteed transition, aiding clock synchronisation), Differential Manchester.
• Digital-to-analogue modulation — e.g., ASK (Amplitude Shift Keying), FSK (Frequency Shift Keying), PSK (Phase Shift Keying) — used when digital data must travel over an analogue channel (e.g., traditional modems).
• Analogue-to-digital conversion — e.g., PCM (Pulse Code Modulation), used to digitise analogue voice for transmission over digital telephone networks.

2. Physical Topology

Defines how devices are physically connected (bus, star, ring, mesh) — see the dedicated Network Topologies page for full classification and trade-offs.

3. Transmission Mode (Direction of Flow)

• Simplex — data flows in one direction only (e.g., a keyboard to a computer, or a broadcast TV signal).
• Half-duplex — data flows both ways, but only one direction at a time (e.g., a walkie-talkie).
• Full-duplex — data flows both ways simultaneously (e.g., a telephone call, modern Ethernet).

4. Multiplexing (sharing one physical medium among multiple signals)

• FDM (Frequency Division Multiplexing) — the available bandwidth is split into non-overlapping frequency bands, each carrying one signal continuously; classic analogue technique (e.g., radio/TV broadcast, older telephone trunking).
• TDM (Time Division Multiplexing) — the full bandwidth is used by each signal, but only during its own dedicated, rotating time slot; the standard digital technique (e.g., T1/E1 telephone trunks).
  • Synchronous TDM — fixed time slots assigned regardless of whether a source has data to send (can waste slots).
  • Statistical (Asynchronous) TDM — slots are dynamically assigned only to sources that actually have data, improving efficiency.
• WDM (Wavelength Division Multiplexing) — the fibre-optic analogue of FDM: multiple signals are carried simultaneously on the same fibre using different light wavelengths (colours).
• CDM (Code Division Multiplexing) — every signal is transmitted across the full bandwidth simultaneously, but each is distinguished using a unique spreading code (basis of CDMA mobile technology).

5. Bit Synchronisation

Ensures the sender's and receiver's clocks agree on where one bit ends and the next begins — achieved through self-clocking encodings (e.g., Manchester) or a separate synchronisation signal/preamble.

Comparison of Multiplexing Types

Scope Boundary (a common exam trap)

• No addressing, no error detection, and no flow control happen at this layer — those are Data Link (L2) and above responsibilities.
• The Physical layer only guarantees that a stream of bits arrives as some signal; whether that signal is correctly interpreted, framed, and error-checked is entirely the job of higher layers.

CSS Exam Angle

• Keep the "functions" list crisp and well-organised (encoding, topology, transmission mode, multiplexing, synchronisation) — a "define and list functions" style answer with sub-classification of multiplexing types is usually sufficient for full marks.
• Explicitly state that error detection and addressing are not physical-layer functions — a common conflation examiners specifically test for.
• Naming all four multiplexing types (FDM, TDM, WDM, CDM) with one example each is a strong differentiator versus only mentioning TDM/FDM.

Example

The choice between copper Ethernet cable and fibre optic for a new office building is a Physical-layer decision, directly affecting achievable data rate and distance; a long-haul national fibre backbone (e.g., PTCL's) uses WDM to carry many independent data streams as different light wavelengths on the very same physical fibre strand.

Likely Questions

1. Discuss the functions of the physical layer in the OSI model.
2. Differentiate FDM, TDM, WDM, and CDM with one example of each.
3. Distinguish simplex, half-duplex, and full-duplex transmission modes.
4. Why is error detection not considered a physical-layer function?

📜 Past-Paper Questions & Structured Answers

Question wording is paraphrased. Answer emphasis is a study guide, not an official FPSC marking scheme. Papers reviewed: 2016–2026; MCQ coverage is limited to available sheets.

2017 · Paper-II · Q3(b) · 6 marks

Question (paraphrased): Find transmission time for 1,000,000 bytes over 200 Kbps.

Answer focus: Given values → convert units → formula → substitution → result and meaning.

Given: size L = 1,000,000 bytes; rate R = 200,000 bits/s (decimal K).

Convert: L = 1,000,000 × 8 = 8,000,000 bits.

Formula: transmission delay = L/R.

Calculation: 8,000,000 / 200,000 = 40 seconds.

This is the time to put all bits onto the link. It excludes propagation, processing, queuing and additional protocol overhead. It is not the total end-to-end delay.

2020 · Paper-II · Q5(a) · 8 marks

Question (paraphrased): Explain signal encoding and the main techniques used in data communication.

Answer focus: Definition + four data/signal combinations + examples and trade-offs.

Definition: encoding represents information as a signal suitable for transmission and recoverable at the receiver.

Selection: consider bandwidth, noise tolerance, synchronisation, DC content and complexity. For example, a speech signal can be sampled and PCM-encoded, then line-coded for a digital link. Encoding does not itself guarantee error-free delivery; error detection and recovery are separate functions.

2023 · Paper-II · Q7(a) · related image-transmission numerical · 10 marks

Question (paraphrased): Calculate transmission minutes for a 1024×1024, 256-level image at 56K and 750K, with start/stop framing.

Answer focus: Image bits → framing overhead → both rates → minutes; state the paper’s rate convention.

1. Image representation: 256 intensity levels require log₂256 = 8 bits per pixel. Pixels = 1024 × 1024 = 1,048,576; raw information = 8,388,608 bits.

2. Framing: each 8-bit byte is transmitted with one start and one stop bit. Therefore transmitted bits = 1,048,576 × 10 = 10,485,760 bits.

3. At 56K: t = 10,485,760 / 56,000 = 187.2457 s = 3.1208 minutes.

4. At 750K: t = 10,485,760 / 750,000 = 13.9810 s = 0.2330 minutes.

Interpretation: the faster link takes about 1/13.39 of the time. Start/stop bits add 25% to the raw payload size. No compression or other overhead is assumed.

Terminology correction: baud means symbols/s, not necessarily bits/s. The question uses baud as a bit rate; the calculations follow that stated convention, equivalent to assuming one bit per symbol.

2024 · Paper-II · Q1 item 6 · 1 mark

Question: Best encoding choice among the listed binary/decimal/Excess-3 options?

Answer: B — binary, the intended choice because two logical states are straightforward to represent and distinguish. This does not mean binary always needs fewer written digits, or that multilevel signalling cannot transmit data. Encoding efficiency depends on the representation and signalling scheme. Paper