Transmission Media
Transmission media are the physical pathways that carry signals between sender and receiver. They are broadly classified into guided (wired) and unguided (wireless) media. The choice of medium affects bandwidth, noise immunity, cost, and distance.
Guided Media
Guided media provide a physical conduit. Twisted-pair cable consists of two insulated copper wires twisted together to reduce interference. Unshielded twisted pair (UTP) is common in LANs (Cat5e, Cat6). Shielded twisted pair (STP) adds metallic shielding for better noise protection.
Coaxial Cable
Coaxial cable has a central conductor surrounded by insulation, a metallic shield, and an outer jacket. It carries higher frequencies than twisted pair and is used for cable TV and older Ethernet (10Base2, 10Base5). Its shielding provides good noise immunity.
Fiber-Optic Cable
Fiber-optic cable transmits data as light pulses through a glass or plastic core. It offers extremely high bandwidth, immunity to electromagnetic interference, low attenuation, and security (difficult to tap). Single-mode fiber uses a narrow core for long distances; multimode fiber uses a wider core for shorter distances.
Unguided Media
Unguided media transport electromagnetic waves without a physical conductor. Radio waves (3 kHz to 1 GHz) are omnidirectional and used for AM/FM radio, TV, and Wi-Fi. Microwaves (1 GHz to 300 GHz) are unidirectional and used for point-to-point links and satellite communication.
Infrared and Satellite
Infrared signals are used for short-range communication (remote controls, IrDA). They cannot penetrate walls, which provides security but limits range. Satellite communication uses geostationary (GEO), medium earth orbit (MEO), or low earth orbit (LEO) satellites as relay stations in the sky.
Transmission Impairments
Signals degrade during transmission due to attenuation (loss of energy), distortion (change in signal shape), and noise (unwanted signals — thermal, induced, crosstalk, impulse). The signal-to-noise ratio (SNR) measures signal quality.
Performance Metrics
Key metrics include bandwidth (range of frequencies a medium can pass), throughput (actual data rate achieved), latency (delay from source to destination composed of propagation, transmission, queuing, and processing delays), and jitter (variation in packet delay).
Summary
Choosing the right transmission medium involves balancing bandwidth, distance, cost, and environmental factors. Fiber optics leads in performance; twisted pair dominates in cost-effectiveness for LANs; wireless provides mobility and flexibility.