Physical Layer

1. What is the Physical Layer?

• Bottom layer of the OSI model (Layer 1)

• Represents the actual transmission medium for data

• Converts frames into signals (bits: 1s and 0s)

• Deals with physical characteristics of transmission

• Responsible for the physical connection between devices

2. Core Responsibilities

• Transmit raw bits over a physical medium

• Define physical specifications (cables, connectors, voltages)

• Convert frames to signals and vice versa

• Manage physical channel (creation, maintenance, termination)

• Specify timing and synchronization methods

3. Key Principle

• Receives: Frames from Data Link Layer (Layer 2)

• Converts: Frames → Electrical/Light/Radio signals (bits)

• Transmits: Over physical medium

• No intelligence: Just transmits bits without understanding content

1. Medium Specification

   • Air (wireless)

   • Copper (Ethernet cables)

   • Glass (fiber optic)

   • Vacuum (theoretical)

2. Signal Types by Medium

   • Copper: Electrical pulses (voltages)

   • Fiber: Light pulses (photonics/laser)

   • Wireless: Electromagnetic waves (radio frequencies)

3. Physical Specifications

   • Pin layouts (connector types)

   • Voltages and signal levels

   • Signal timing and synchronization

   • Frequency ranges

   • Number of waves

   • Light spectrum (for fiber)

   • Data rates (bits per second)

   • Maximum transmission distances

   • Link activation/deactivation procedures

4. Channel Characteristics

   • Half Duplex: One direction at a time

   • Full Duplex: Both directions simultaneously

   • Serial: One bit at a time

   • Parallel: Multiple bits simultaneously

5. Data Context

   • Uses synchronization flags or preambles

   • Delimits transmissions (marks beginning/end)

   • Allows receiver to recognize data boundaries

1. Copper (Metallic Conductors)

• Signal Type: Electrical pulses (voltage changes)

• Examples:

  • CAT 5e, CAT 6, CAT 6a Ethernet cables

  • Coaxial cable

• Characteristics: Susceptible to electromagnetic interference (EMI)

2. Fiber Optic (Glass)

• Signal Type: Light pulses (photonics/laser)

• Examples:

  • Single-mode fiber (SMF)

  • Multi-mode fiber (MMF)

• Characteristics: Immune to EMI, longer distances, higher bandwidth

3. Wireless (Air)

• Signal Type: Electromagnetic waves (radio frequencies)

• Examples:

  • Wi-Fi (802.11)

  • Bluetooth

  • Cellular (4G, 5G)

• Characteristics: No physical cables, subject to interference

❌ WRONG: "Physical layer uses MAC addresses"

✅ CORRECT: "Physical layer only deals with bits; MAC addresses are Layer 2"

❌ WRONG: "Hub reads MAC addresses to forward data"

✅ CORRECT: "Hub is a Layer 1 device that simply repeats signals to all ports"

❌ WRONG: "Half duplex means half the speed"

✅ CORRECT: "Half duplex means communication in only one direction at a time"

❌ WRONG: "Ethernet cables can go any distance"

✅ CORRECT: "Standard Ethernet (CAT 5e/6) is limited to 100 meters"

❌ WRONG: "Fiber is always better than copper"

✅ CORRECT: "Fiber has advantages (distance, speed, EMI immunity) but is more expensive; choose based on requirements"

❌ WRONG: "Physical layer performs error correction"

✅ CORRECT: "Physical layer may define methods for error recovery, but actual error detection/correction happens at Layer 2"

❌ WRONG: "Hubs and switches are the same"

✅ CORRECT: "Hubs are Layer 1 (dumb repeaters); switches are Layer 2 (intelligent forwarding)"

❌ WRONG: "Preamble contains addressing information"

✅ CORRECT: "Preamble is for synchronization and frame delimitation, not addressing"

• Simplex: SIMple - one way only (like a TV broadcast)

• Half Duplex: HALF the time each direction (like a walkie-talkie)

• Full Duplex: FULL conversation both ways (like a phone)

"Copper Carries Current, Fiber Flashes Light, Wireless Waves Around"

"100 meters is the magic number" - Remember this for CAT 5e, CAT 6 at standard speeds

"Hubs are DUMB - they just repeat EVERYTHING to EVERYONE"

• No intelligence

• Broadcasts to all ports

• Layer 1 device

Scenario: Installing Ethernet in an office

CORRECT Physical Layer Decisions:

- Distance from switch to workstation: 85 meters ✓

- Cable type: CAT 6 (supports Gigabit) ✓

- Connector: RJ-45 ✓

- Mode: Full duplex (modern switch) ✓

INCORRECT:

- Distance: 150 meters ✗ (exceeds 100m limit)

- Cable type: CAT 3 ✗ (too old, insufficient bandwidth)

- Mode: Half duplex on modern equipment ✗ (wastes bandwidth)

Physical Layer (Hub - Layer 1):

PC1 sends data to PC2

Hub receives on Port 1

→ Hub REPEATS signal to ALL ports (2, 3, 4, 5...)

→ All devices receive the transmission

→ Collision domain = entire network

Data Link Layer (Switch - Layer 2):

PC1 sends data to PC2

Switch receives on Port 1

→ Switch READS MAC address

→ Switch forwards ONLY to PC2's port

→ Other ports don't see the traffic

DATA LINK LAYER (Layer 2)

↓

Frame: [Header | Data | Trailer]

↓

PHYSICAL LAYER (Layer 1)

↓

Conversion to bits: 01001010110...

↓

Signal encoding:

- Copper: High voltage (1) / Low voltage (0)

- Fiber: Light on (1) / Light off (0)

- Wireless: Frequency modulation

↓

TRANSMISSION MEDIUM

↓

Travels to destination

↓

PHYSICAL LAYER (receiving end)

↓

Bits decoded: 01001010110...

↓

Passed up to Layer 2 as frame

Problem: Network connectivity issues

Physical Layer Checks:

1. ✓ Cable plugged in?

2. ✓ Link lights on?

3. ✓ Cable damaged/bent?

4. ✓ Within distance limits?

5. ✓ Correct cable type?

6. ✓ EMI sources nearby?

7. ✓ Port enabled on switch?

Tools: Cable tester, loopback plug, visual inspection

Layer 1 of the OSI model; responsible for transmitting raw bits over a physical medium

Frames

Signals (bits: 1s and 0s)

Copper (electrical), Fiber (light), Wireless (electromagnetic waves)

No, it only transmits raw bits without understanding content

Layer 1 (bottom layer of OSI model)

Physical Layer is served by/receives frames from Data Link Layer (Layer 2)

Electrical pulses (voltage changes)

CAT 5e, CAT 6, or CAT 6a (twisted pair cables)

100 meters (328 feet)

1 Gbps (Gigabit Ethernet)

1 Gbps (can do 10 Gbps up to 55 meters)

10 Gbps up to 100 meters

Susceptible to electromagnetic interference (EMI)

RJ-45

Light pulses (photonics/laser)

Glass (or sometimes plastic)

Single-mode fiber (SMF) and Multi-mode fiber (MMF)

8-10 microns

50-62.5 microns

Single-mode fiber (SMF) - up to 100+ km

Up to 2 km

Long-distance connections (WAN links, campus backbones)

Short-distance connections (LAN, within buildings)

1) Immune to EMI, 2) Longer distances, 3) Higher bandwidth

More expensive than copper; requires specialized equipment and skills

Electromagnetic waves (radio frequencies)

Air

IEEE 802.11 (Wi-Fi), Bluetooth, cellular (4G/5G)

No physical cables required; supports mobility

Subject to interference; security concerns; limited range

Communication in one direction only (e.g., radio broadcast)

Communication in both directions, but only one direction at a time (e.g., walkie-talkie)

Communication in both directions simultaneously (e.g., modern telephone, switch)

Half duplex

Full duplex

No, it means communication in only one direction at a time

Hub, walkie-talkie, legacy Ethernet with collisions

Modern Ethernet switch, telephone

Yes, but they will negotiate to half duplex (lowest common denominator)

Sending data one bit at a time over a single channel

Sending multiple bits simultaneously over multiple channels

Serial transmission

Ethernet, USB, Serial cables

Old printer cables (parallel port), internal computer buses

Layer 1 (Physical Layer)

Repeats/broadcasts signals to all ports (no intelligence)

Layer 1 (Physical Layer)

Amplifies or regenerates signals to extend transmission distance

A device that converts signals between different media types (e.g., copper to fiber)

Layer 1 (Physical Layer)

A device that transmits and receives signals (combination of transmitter and receiver)

No, it's a "dumb" device with no intelligence; it just repeats signals

It repeats the signal to ALL other ports

They create collisions; all devices share bandwidth; create a single collision domain

No, only Layer 2 devices (switches) can read MAC addresses

Ethernet standards (including Physical Layer specifications)

Structured cabling standards for commercial buildings

A standard for serial communication (older standard for serial ports)

Cable damage, wrong cable type, distance too long, EMI, unplugged cables, faulty ports

Cable tester or continuity tester

Testing if a network port is functioning properly

Physical Layer: Is the cable plugged in? Are link lights on?

Signal loss/degradation over distance

Resistance in the cable; increases with distance

By regenerating/amplifying the signal to extend transmission distance

Electrical noise that can disrupt signals in copper cables

Fiber optic cable

Interference between adjacent wire pairs in a cable

LEDs on network devices that indicate physical connection status

Layer 1 (Physical Layer)

Converting data into signals suitable for transmission over the medium

The actual process of transmitting the encoded data as physical signals

Different voltage levels (e.g., high voltage = 1, low voltage = 0)

Presence or absence of light (light on = 1, light off = 0)

Frequency or phase modulation of radio waves

The maximum data transfer rate supported by the physical medium

The time delay between sending and receiving data

Cable type, distance, interference, hardware capabilities

Fiber optic typically has much higher bandwidth

The actual data transfer rate achieved (often less than bandwidth due to overhead)

Fiber optic (copper is limited to 100 meters)

Yes, it's within the 100-meter limit

Fiber optic (immune to EMI from machinery)

Layer 1 (Physical Layer) - check cable, port, connections

CAT 6a

Almost never in modern networks; hubs are obsolete (switches are better in every way)

Single-mode: longer distance (100+ km), smaller core; Multi-mode: shorter distance (2 km), larger core

Upgrade to full-duplex capable devices (replace hubs with switches)

Yes, but performance will be limited by the slowest component

Signal attenuation causes errors, packet loss, or complete connection failure

Hubs create a single collision domain; all devices share bandwidth and compete for transmission

Exceeds 100-meter maximum distance for copper Ethernet; signal attenuation

Layer 2 (Data Link) - Physical Layer is working if link lights are on

Cable limitation (need CAT 6a minimum); NIC limitation; duplex mismatch

Electromagnetic interference (EMI) from lights affecting copper cables

Use a repeater, switch, or convert to fiber optic

Device can send or receive, but not both simultaneously; possibly connected to a hub or old device

Layer 2 issues: VLAN configuration, port configuration, cable type (straight-through vs. crossover)

1. "At what layer does a hub operate?" → Layer 1

2. "What is the maximum distance for CAT 6 Ethernet?" → 100 meters

3. "Which cable type is immune to EMI?" → Fiber optic

4. "What is full duplex?" → Both directions simultaneously

5. "What does the Physical Layer convert frames into?" → Bits/signals

• "Hub" → Think Layer 1, no intelligence, collision domain

• "100 meters" → Standard Ethernet distance limit

• "EMI/Interference" → Problem with copper, use fiber

• "Distance exceeds 100m" → Need fiber or repeater

• "Link lights off" → Layer 1 problem

• "Collisions" → Half duplex or hub issue