Physical media form the foundation of all network communication. Every bit of data traveling across a network ultimately moves through some physical medium, whether that's a copper wire, a glass fiber, or even the air. The properties of that medium directly determine how fast, how far, and how reliably data can travel.
This section covers the major types of physical media, the transmission characteristics that govern their performance, and the encoding techniques used to put data onto those media.
Physical Media Types and Characteristics
Types of Network Physical Media
Twisted Pair consists of two insulated copper wires twisted around each other. The twisting reduces electromagnetic interference by canceling out noise picked up by each wire. This is the most common wired medium in local area networks (Ethernet).
- Unshielded Twisted Pair (UTP) has no additional metallic shielding. It's cheaper and easier to install, but more vulnerable to external noise. Cat5e and Cat6 UTP cables are standard in most office and home networks.
- Shielded Twisted Pair (STP) wraps the wire pairs in metallic foil or braided shielding. This provides better noise immunity, which matters in environments with heavy electromagnetic interference (like factory floors).
Coaxial Cable uses a central copper conductor surrounded by insulation, a metallic shield, and an outer jacket. The concentric design gives it higher bandwidth and better noise immunity than twisted pair.
- Thinnet (10Base2) is thinner and more flexible, with a maximum segment length of 185 meters.
- Thicknet (10Base5) is thicker and more rigid, supporting segments up to 500 meters.
Coaxial cable was common in older Ethernet installations but has largely been replaced by twisted pair and fiber in modern LANs. It's still widely used for cable television and broadband internet access.
Fiber Optics transmits data as pulses of light through thin strands of glass or plastic. A fiber cable has three main layers:
- Core: the thin glass or plastic strand that carries the light signal.
- Cladding: a layer surrounding the core with a lower refractive index. This difference in refractive index causes light to reflect back into the core (total internal reflection), keeping the signal contained.
- Buffer: a protective outer coating that provides mechanical strength and shields the fiber from physical damage.
Fiber's key advantages over copper media:
- Much higher bandwidth capacity
- Immunity to electromagnetic interference (light signals aren't affected by EMI)
- Much longer transmission distances without needing signal regeneration (tens of kilometers vs. hundreds of meters for copper)
- Thinner and lighter cables
The tradeoff is that fiber is more expensive to manufacture and install, and splicing or terminating fiber requires specialized equipment.
Transmission Characteristics of Media
Three properties define how well a medium carries data: bandwidth, attenuation, and noise.
Bandwidth is the range of frequencies a medium can carry, measured in Hertz (Hz). In networking, you'll also see bandwidth expressed as a data rate in bits per second (bps). A wider frequency range allows more data to be transmitted per unit of time. For example, Cat6 UTP supports frequencies up to 250 MHz, while Cat5e supports up to 100 MHz, which is why Cat6 can achieve higher data rates.
Attenuation is the loss of signal strength as a signal travels through a medium, measured in decibels (dB). Three main factors increase attenuation:
- Distance: signal strength drops as the signal travels farther, due to resistance in copper or absorption in fiber.
- Frequency: higher-frequency signals attenuate faster because they lose energy more quickly to absorption and scattering.
- Media quality: impurities, defects, or damage in the medium disrupt signal propagation and increase loss.
Attenuation is why every medium type has a maximum cable length. Beyond that length, the signal degrades too much to be reliably decoded.
Noise is any unwanted signal that interferes with the data signal. The main types:
- Thermal noise: caused by random electron motion in conductors due to heat. It's always present and sets a baseline noise floor.
- Crosstalk: interference from adjacent wires or cables. When current flows through one wire, it creates an electromagnetic field that can induce a signal in a neighboring wire.
- Electromagnetic interference (EMI): noise from external sources like electric motors, power lines, or fluorescent lights.
Signal-to-Noise Ratio (SNR) quantifies signal quality by comparing the power of the desired signal to the power of the noise, typically expressed in decibels. A higher SNR means the signal is much stronger than the noise, which translates to fewer transmission errors. The Shannon-Hartley theorem uses SNR to calculate the theoretical maximum data rate of a channel:
where is the channel capacity in bps and is the bandwidth in Hz.
Signal Encoding and Modulation Techniques
Raw digital data (1s and 0s) can't just be "placed" on a wire. You need encoding and modulation schemes to represent that data as physical signals.
Signal Encoding converts digital data into specific signal patterns on the medium. Common line coding schemes include:
- Non-Return-to-Zero (NRZ): represents a 1 as a high voltage level and a 0 as a low voltage level. Simple to implement, but long runs of the same bit cause synchronization problems because there are no signal transitions for the receiver to lock onto.
- Manchester encoding: guarantees a transition in the middle of every bit period. A low-to-high transition represents a 1, and a high-to-low transition represents a 0 (per IEEE 802.3 convention). This solves the synchronization problem but uses twice the bandwidth of NRZ since there's always a transition per bit.
- 4B/5B encoding: maps every 4 bits of data to a 5-bit code. The 5-bit codes are chosen to ensure frequent transitions, maintaining clock synchronization and DC balance. The 25% overhead is much less than Manchester's 100% overhead, making it more bandwidth-efficient. It's used in Fast Ethernet (100BASE-TX).
Modulation varies a property of a continuous carrier signal to encode data. This is how data gets transmitted over media that carry analog signals (like radio waves or cable TV infrastructure).
- Analog modulation techniques: Amplitude Modulation (AM) varies the carrier's amplitude, Frequency Modulation (FM) varies its frequency, and Phase Modulation (PM) varies its phase.
- Digital modulation techniques map digital data onto carrier signal changes: Amplitude Shift Keying (ASK) switches between amplitude levels, Frequency Shift Keying (FSK) switches between frequencies, and Phase Shift Keying (PSK) switches between phase values.
More advanced schemes like Quadrature Amplitude Modulation (QAM) combine amplitude and phase changes to encode multiple bits per symbol, achieving higher data rates within the same bandwidth.
Performance Factors of Physical Media
Beyond the inherent properties of the medium itself, several practical factors affect real-world network performance.
Cable Length is the most straightforward constraint. Longer cables suffer more attenuation and pick up more noise. Each standard specifies maximum segment lengths for this reason (e.g., 100 meters for Cat5e/Cat6 UTP in Ethernet, 185 meters for 10Base2 coax).
Connectors and Terminations matter more than students often expect. A poorly crimped RJ-45 connector or a bad fiber splice can cause signal reflections, increased attenuation, or intermittent failures. On coaxial cable, improper termination causes signal reflections that interfere with data.
Environmental Factors affect both performance and the lifespan of the medium:
- Extreme temperatures can alter the electrical properties of copper or the optical properties of fiber.
- High humidity promotes corrosion on copper contacts, increasing resistance and signal degradation.
- EMI from nearby equipment introduces noise, which is especially problematic for unshielded copper media.
Installation and Handling practices directly impact performance:
- Maintain minimum bend radii. Bending a fiber cable too sharply can crack the core or cause light to escape through the cladding. Bending copper cables too tightly damages the conductors.
- Avoid excessive pulling tension during installation, which can stretch or break conductors.
Quality of Materials plays a role as well. Higher-grade copper (with fewer impurities) and better insulation reduce attenuation and crosstalk. Lower-quality cables may meet minimum specifications on paper but degrade faster and perform worse in challenging environments. For critical infrastructure, investing in quality cabling pays off over the life of the installation.