Move the bits.
Understand the medium.

What the physical layer does

The physical layer defines how raw bits are represented and transmitted. It specifies the physical components, connectors, pinouts, cable types, radio frequencies, optical properties, and signaling or encoding methods. It does not interpret IP addresses or make forwarding decisions. Its job is to move the bit stream across the link so the receiving device can recover it.

Measure network performance

Bandwidth is the theoretical capacity of a link, commonly expressed in bits per second. Throughput is the actual rate achieved over time. Goodput measures useful application data delivered, excluding protocol overhead and retransmissions. Latency is the time required for data to travel from source to destination. A high-bandwidth link can still perform poorly when congestion, errors, or latency are high.

Copper cabling

Copper carries data as electrical signals. Common Ethernet media include unshielded twisted pair (UTP), shielded twisted pair (STP), and coaxial cable in some environments. Twisting helps reduce electromagnetic interference and crosstalk between wire pairs. Shielding can provide additional protection in electrically noisy environments, but requires appropriate grounding and installation. Copper is practical and inexpensive, but distance and interference limit its performance.

UTP pinouts and cable types

TIA/EIA-568 defines structured-cabling requirements, including the T568A and T568B conductor arrangements. A straight-through Ethernet cable uses the same wiring standard at both ends (A-to-A or B-to-B). A crossover cable uses T568A at one end and T568B at the other. Traditionally, straight-through cables connect unlike Ethernet device types and crossover cables connect like device types, but Auto-MDI/MDIX on many modern ports detects and corrects the pair assignment automatically.

A rollover cable uses a reversed pinout and is traditionally used for out-of-band console access from a computer to a router or switch. It is not an Ethernet data cable. Keep the wiring-standard distinction separate from the older device-role shortcut: the first describes the cable's pinout, while the second describes its traditional use.

Fiber-optic cabling

Fiber carries information as pulses of light through glass or plastic strands. It is resistant to electromagnetic interference and supports high bandwidth over long distances. Single-mode fiber uses a very small core and is suited to long-distance links. Multimode fiber has a larger core and is commonly used for shorter links such as within buildings or data centers. Fiber requires compatible optical transceivers and careful connector cleaning and handling.

Wireless media

Wireless networks transmit information through electromagnetic waves rather than a physical cable. Radio frequency (RF) is used by Wi-Fi and many other wireless technologies. Wireless links are affected by distance, obstacles, interference, channel overlap, antenna placement, and regulatory limits. A wireless design trades cabling convenience for a shared and more variable radio environment.

Select the medium for the environment

Media selection should account for the distance the signal must travel and the environment where the medium will be installed. Also evaluate required bandwidth, interference, physical protection, installation complexity, compatible interfaces, and total cost. Use copper where its reach and electrical environment are suitable; choose fiber for longer distances, higher capacity, or electrical isolation; choose wireless where mobility or cabling constraints justify the shared radio medium.