Computer Networks
Unit 2: Physical Layer (Layer 1)
The lowest OSI layer — transmitting raw bits over physical media, signal encoding, and real-world infrastructure like submarine fiber cables.
What is the physical layer?
The Physical Layer is the lowest layer of the OSI model. It is responsible for transmitting raw bits (0s and 1s) from one device to another over a physical medium — copper wire, fiber-optic cable, or radio waves. It does not understand what the bits mean, only how to send and receive them as electrical, optical, or radio signals.
Unit of data: bit
Responsibilities
- Signal transmission — converts bits into signals (electrical, light, radio) and back.
- Bit synchronisation — keeps sender and receiver clocks aligned.
- Bit rate control — defines how many bits per second can be transmitted.
- Transmission mode — simplex (one-way), half-duplex (one direction at a time), or full-duplex (both directions simultaneously).
- Physical topology — how devices are physically arranged (bus, star, ring, mesh).
Physical media
Bits can travel through several types of media:
| Medium | Examples | Typical use |
|---|---|---|
| Copper cable | Twisted pair (Cat 5e / Cat 6), coax | Office LANs, home internet |
| Fiber optic | Single-mode, multi-mode | Data centers, long-haul |
| Wireless | Wi-Fi (radio), Bluetooth, 4G / 5G | Mobile devices, IoT |
Devices at the physical layer
- Hub — broadcasts incoming bits to all ports (mostly obsolete).
- Repeater — regenerates a weakened signal over long distances.
- Cables and connectors — RJ-45 jacks, fiber connectors (LC, SC).
- NIC (Network Interface Card) — the part that converts digital data to signals.
- Modem — modulates/demodulates signals between digital and analog.
Example — sending a single byte
When your computer sends the letter A (ASCII 01000001) over an Ethernet cable:
- The NIC takes the 8 bits.
- It converts each bit into an electrical voltage (e.g. high =
1, low =0). - The voltages travel down the cable to the next device.
- The receiving NIC reads the voltages back into bits.
The physical layer doesn’t know it’s the letter A — it just moves bits.
Real-world scenarios
Meta’s 2Africa submarine cable
To connect billions of users, Meta (Facebook) co-funds submarine fiber-optic cables that physically span continents. The 2Africa cable is one of the longest in the world, wrapping around the African continent. Every WhatsApp message, every Instagram photo, and every Facebook video call that crosses an ocean travels as pulses of light through these cables — pure physical-layer infrastructure.
Google’s private fiber backbone
Google operates one of the largest private fiber-optic backbones in the world, including dedicated submarine cables like Curie (US ↔ Chile) and Dunant (US ↔ France). This gives Google direct physical control over the medium, reducing latency between data centers and improving reliability for products like YouTube and Google Cloud.
Netflix Open Connect
Netflix ships physical Open Connect appliances (essentially specialised servers full of storage) directly to ISP data centers around the world. The video bits then travel a much shorter physical distance to your home — usually over the ISP’s local fiber — which is why Netflix loads instantly even during peak hours.
5G base stations
Telecom companies like Verizon, AT&T, and Jio install thousands of 5G antennas and small cells. Each antenna is a Physical Layer device that turns bits into radio waves and back, enabling gigabit-class wireless internet to phones.
Common physical-layer problems
- Damaged or unplugged cables.
- Faulty NIC or transceiver.
- Signal attenuation over long distances.
- Electromagnetic interference (EMI) on copper cables.
- Wi-Fi dead zones or radio interference.
When network troubleshooting, technicians always start by asking: “Is it plugged in? Is the link light on?” — that is Layer 1 thinking.