Summary
Highlights
The physical layer is the lowest layer in the networking model, encompassing all tangible components like cables, connectors, and network interface cards (NICs). Bits (ones and zeros) travel across this layer.
Data encapsulation up to the data link layer is software-based; from the MAC sub-layer and physical layer, it's hardware-based. The physical layer addresses three main areas: physical components (what you touch), encoding (how bits are represented), and signaling (how signals are transmitted).
Encoding defines how ones and zeros are represented, such as using Manchester technique (drops for ones, rises for zeros) or 4b/5b coding. Signaling refers to how data is transmitted physically, including different frequencies for light signals in fiber optics, voltage levels in copper wires, or amplitude/frequency/phase modulation for wireless signals.
Bandwidth is the maximum data rate (like a speed limit) measured in kilobits per second. Throughput is the actual speed of data transmission, while goodput is the actual amount of 'good' (uncorrupted) data received, excluding bad or retransmitted data. Latency refers to processing and propagation delays.
Copper wires are the most common and least expensive transmission medium, using electron movement and voltage potential to represent bits. Types include unshielded twisted-pair (UTP), shielded twisted-pair (STP), and coaxial cables. Twisting in UTP reduces electromagnetic interference (EMI) between wires, but UTP is susceptible to external EMI and should be kept away from electrical machinery. Coaxial cables are designed to contain electromagnetic waves.
Category 5e is the most widely used UTP cable, supporting Gigabit Ethernet up to 100 meters. Cat 6 and 7 support 10 Gigabit Ethernet but are more expensive. RJ45 connectors are used for UTP. Proper cable termination is crucial to avoid crosstalk, which can occur if wires are unwound too much. Straight-through cables connect hosts to switches, and while crossover cables were once needed for direct device-to-device connections, modern devices use auto MDI-X to adapt.
Fiber optic cables are immune to EMI, transmit data near the speed of light, and support very long distances (up to 62.5 miles). They use LEDs or lasers for transmission. However, they are expensive to install and maintain. There are two types: single-mode fiber (thinner core, single signal, long distance, often voice) and multi-mode fiber (thicker core, multiple signals, shorter distance).
Fiber optics are used in enterprise networks, Fiber-to-the-Home (FiOS), long-haul networks, and submarine cables. Common connectors include Straight Tip (ST), Subscriber Connector (SC), and Lucent Connector (LC). Recognising these connectors is important for certifications like CCNA.