Physical Layer

The physical layer is responsible for moving raw bits across a network medium. It defines how bits are represented as electrical signals, light pulses, or radio waves and how devices connect physically.

The physical layer accepts frames from the data link layer and converts them into signals that can travel across the chosen medium. At the receiving end, the physical layer converts the signals back into bits.

It defines characteristics such as:

• Cable and connector types
• Pin layouts
• Voltage or light signaling
• Wireless frequencies
• Encoding methods
• Data rates
• Maximum distances

Physical layer standards are created so equipment from different manufacturers can work together. Standards define how devices connect, how signals are transmitted, and how media are built.

Physical layer technologies are affected by bandwidth needs, distance, interference, cost, and installation environment.

Bandwidth is the capacity of a medium to carry data, commonly measured in bits per second.

Related terms:

• Latency: Delay experienced while data travels from source to destination.
• Throughput: Actual measured data transfer rate.
• Goodput: Useful application data rate after overhead and retransmissions are removed.

Copper cabling uses electrical signals. It is common, affordable, and easy to install, but it can be affected by electromagnetic interference and distance limitations.

Common copper cable issues include:

• Signal weakening over distance
• Interference from motors, lights, and power cables
• Crosstalk between wire pairs
• Incorrect termination

Coaxial Cable

Coaxial cable has a central conductor, insulation, shielding, and an outer jacket. It has been used for cable internet, television systems, and some older networking technologies.

Unshielded Twisted Pair

Unshielded Twisted Pair (UTP) cable is widely used in Ethernet LANs. It contains pairs of wires twisted together to reduce interference.

Shielded Twisted Pair

Shielded Twisted Pair (STP) adds shielding to reduce interference. It may be useful in environments with high electrical noise, but it is usually more expensive and less flexible than UTP.

A straight-through Ethernet cable connects different device types, such as a PC to a switch. A crossover cable connects similar device types, such as switch to switch or PC to PC, in older environments.

Many modern devices support Auto-MDIX, which automatically adjusts transmit and receive pairs, reducing the need to choose the cable type manually.

Fiber-optic cable uses light instead of electrical signals. It supports high bandwidth and long distances and is resistant to electromagnetic interference.

Fiber is commonly used in backbone links, data centers, long-haul service provider networks, fiber-to-the-home installations, and undersea cable systems.

Diagram description: A fiber diagram commonly shows light pulses traveling through a thin glass core. Single-mode fiber is shown with a narrow straight path, while multimode fiber is shown with several light paths bouncing through the core.

Wireless communication uses radio waves instead of cables. It is convenient and flexible, but it is affected by distance, obstacles, interference, security settings, and signal strength.

Common wireless technologies include Wi-Fi, Bluetooth, cellular networks, and point-to-point wireless links.