Guided Media Overview: Foundations of Wired Communication

Not all cables are created equal. A Cat5e cable can stretch 100 meters but carries only 1 Gbps reliably. A Cat8 cable supports 40 Gbps but only for 30 meters. Single-mode fiber spans 100 kilometers; multimode fiber struggles past 500 meters at high speeds. These differences stem from the fundamental characteristics of each medium—physical properties that determine performance, reliability, and appropriate application.

Understanding media characteristics empowers you to select the right cable for every application, troubleshoot performance issues effectively, and design networks that meet requirements without over-engineering or under-specifying infrastructure.

Attenuation is the reduction in signal power as it travels through a medium. All transmission media exhibit attenuation—it's unavoidable physics. The key questions are: how much attenuation, over what distance, and at what frequencies?

Measuring Attenuation

Attenuation is measured in decibels (dB), a logarithmic scale:

Attenuation (dB) = 10 × log₁₀(Poutput / Pinput)

Where P is power. Since output is always less than input (signal loss), attenuation values are negative or expressed as positive loss.

Key decibel values:

• 3 dB loss = half the power (50%)
• 6 dB loss = quarter of the power (25%)
• 10 dB loss = 10% of original power
• 20 dB loss = 1% of original power
• 30 dB loss = 0.1% of original power

Frequency-Dependent Attenuation

A critical property of copper cables: attenuation increases with frequency.

This occurs due to multiple physical effects:

Skin Effect: At higher frequencies, current concentrates near the conductor surface, effectively reducing the conductor's cross-sectional area. Less area means higher resistance and greater losses.

Dielectric Loss: The cable's insulation absorbs energy at higher frequencies. Better insulation materials (lower loss tangent) reduce this effect.

Radiation Loss: Higher frequencies radiate more energy from the cable. Shielding helps but doesn't eliminate this loss.

This frequency dependence is why cable categories have frequency ratings. A Cat5e cable might have acceptable attenuation at 100 MHz but unusable attenuation at 500 MHz—making it unsuitable for 10GBASE-T even if otherwise physically intact.

Attenuation in Fiber Optics

Fiber optic attenuation works differently:

Absorption: Impurities in the glass (especially water molecules at certain wavelengths) absorb light energy. Modern 'low-water-peak' fibers minimize this.

Rayleigh Scattering: Microscopic density variations in the glass scatter light. This is the dominant loss mechanism in modern fiber and decreases with wavelength (∝ λ⁻⁴).

Bend Losses: Excessive bending causes light to escape the core. Fiber installations have minimum bend radius specifications.

Connector and Splice Losses: Each connector adds ~0.2-0.5 dB loss. Fusion splices add ~0.02-0.1 dB. Long fiber runs accumulate significant connector losses.

The wavelength dependence of fiber attenuation created the transmission 'windows' (850 nm, 1310 nm, 1550 nm) where attenuation is minimal.

Bandwidth refers to the range of frequencies that a medium can transmit with acceptable attenuation. It directly constrains maximum data rate.

Copper Cable Bandwidth

For twisted pair cables, bandwidth is specified as the maximum frequency at which the cable meets attenuation and crosstalk standards:

• Cat5e: 100 MHz
• Cat6: 250 MHz
• Cat6a: 500 MHz
• Cat7: 600 MHz
• Cat8: 2000 MHz

Higher bandwidth enables higher data rates through:

1. More signaling transitions per second — Higher frequency allows faster symbol rates
2. Lower signal distortion — Frequency components of digital signals remain intact
3. Multi-level signaling — Better high-frequency performance enables PAM-4 and beyond

*Cat6 supports 10 Gbps only to 55m due to increased crosstalk at high frequencies; Cat6a extends this to full 100m.

Fiber Optic Bandwidth

Fiber bandwidth is expressed differently depending on fiber type:

Multimode Fiber: Modal Bandwidth (MHz·km)

Multimode fibers support multiple light paths (modes) that travel at slightly different speeds, causing modal dispersion. This limits bandwidth proportionally to distance.

The MHz·km unit means: multiply bandwidth by length. OM3 at 300m provides 2000/0.3 = ~6.7 GHz effective bandwidth.

Single-Mode Fiber: Limited by Chromatic Dispersion

Single-mode fiber supports only one mode, eliminating modal dispersion. Bandwidth is limited by chromatic dispersion (different wavelengths traveling at different speeds). With typical laser sources, this allows effective bandwidths of tens of gigahertz over tens of kilometers—far exceeding copper capabilities.

Crosstalk is the unwanted coupling of signals between adjacent conductor pairs within a cable. It's a critical performance limiter in multi-pair cables like those used for Ethernet.

Types of Crosstalk

Why Crosstalk Matters

Crosstalk creates noise that must be distinguished from the actual signal. As data rates increase, the margin between signal and crosstalk noise shrinks.

Consider 1000BASE-T (Gigabit Ethernet):

• Uses all 4 pairs simultaneously
• Each pair both transmits and receives (echo cancellation)
• 250 Mbps per pair in each direction
• Crosstalk from 3 other pairs must be rejected

Now consider 10GBASE-T:

• Same 4 pairs, 2.5 Gbps per pair each direction
• Operates at much higher frequencies (up to 500 MHz)
• Crosstalk is MORE severe at higher frequencies
• Requires exceptional crosstalk performance

This is why Cat6a exists—not primarily for lower attenuation, but for dramatically better crosstalk performance at high frequencies.

Crosstalk Mitigation

Twisting: The fundamental defense. Each pair has a unique twist rate (twists per inch). This ensures that interference couples equally to both conductors and cancels differentially. Better cables have higher, more precisely controlled twist rates.

Pair Separation: Physically separating pairs reduces electromagnetic coupling. Cat6a uses internal splines or separators between pairs.

Shielding: Cat7 and Cat8 use individual foil shielding around each pair (S/FTP construction). This provides dramatic crosstalk improvement but adds cost, rigidity, and installation complexity.

Digital Signal Processing: Modern Ethernet PHYs (physical layer chips) use advanced DSP to cancel crosstalk mathematically. The PHY measures crosstalk patterns and subtracts them from the received signal. This is essential for 10GBASE-T operation.

External electromagnetic interference (EMI) introduces noise that degrades signal quality. Different media types exhibit dramatically different susceptibility to EMI.

Sources of EMI

Shielding Designations

Cable shielding is described using standardized naming:

ISO/IEC 11801 Notation: XX/YZZ

• XX = Overall cable shield (U=unshielded, F=foil, SF=braid+foil)
• Y = Individual pair shield (U=unshielded, F=foil, S=braid)
• ZZ = Twisted pair indicator (TP)

Common configurations:

• U/UTP — Unshielded cable, unshielded pairs (standard LAN cable)
• F/UTP — Overall foil shield, unshielded pairs
• U/FTP — No overall shield, foil-shielded pairs
• S/FTP — Braided overall shield, foil-shielded pairs (Cat7)
• SF/FTP — Braid+foil overall, foil-shielded pairs (maximum protection)

Characteristic impedance is a fundamental cable property that must be matched throughout the network to prevent signal reflections.

Understanding Characteristic Impedance

Characteristic impedance (Z₀) is the ratio of voltage to current in a traveling wave on the transmission line. It depends on the cable's physical construction:

For coaxial cable: Z₀ = (138 / √ε) × log₁₀(D/d)

Where:

• ε = dielectric constant
• D = inner diameter of outer conductor
• d = outer diameter of inner conductor

For twisted pair: Z₀ = (276 / √ε) × log₁₀(2S/d)

Where:

• S = center-to-center spacing of conductors
• d = conductor diameter

Standard impedances:

• Twisted pair Ethernet: 100 Ω (±15%)
• Coaxial RG-6: 75 Ω
• Coaxial RG-58: 50 Ω

Impedance Mismatch and Reflections

When a signal encounters an impedance mismatch (cable to connector, cable to cable, cable to equipment), part of the signal reflects back toward the source.

Reflection coefficient: Γ = (Z₂ - Z₁) / (Z₂ + Z₁)

Where Z₁ and Z₂ are the impedances on either side of the discontinuity.

Problems caused by reflections:

• Signal cancellation or reinforcement (wave interference)
• Ghost images in video transmission
• Bit errors in digital transmission
• Standing waves that cause localized high voltages
• Increased bit error rates at high speeds

Return Loss measures how well impedance is controlled:

Return Loss (dB) = -20 × log₁₀|Γ|

Higher return loss (more dB) means better impedance matching. Ethernet cables require minimum return loss of 12-20 dB depending on category and frequency.

Dispersion in optical fiber causes light pulses to spread out over distance, eventually causing inter-symbol interference where adjacent bits overlap. Multiple dispersion mechanisms exist.

Modal Dispersion (Multimode Fiber)

In multimode fiber, light travels via multiple paths (modes) that have different path lengths and travel times. A pulse that enters the fiber as a sharp edge arrives as a spread-out pulse.

Consequences:

• Sharp bit transitions become gradual
• Adjacent bits blur together at high speeds
• Limits both distance and data rate
• Worse with larger core diameter (more modes)

Chromatic Dispersion (All Fiber)

Different wavelengths of light travel at slightly different speeds in glass. Since no light source is perfectly monochromatic, chromatic dispersion affects all fiber.

Material Dispersion: The refractive index of glass varies with wavelength. Shorter wavelengths travel slower than longer wavelengths.

Waveguide Dispersion: The confinement of light in the fiber core is wavelength-dependent, causing additional velocity differences.

Chromatic dispersion is measured in ps/(nm·km) — picoseconds of pulse spreading per nanometer of source spectral width per kilometer of fiber.

Single-mode fiber has zero dispersion around 1310 nm, which is why this wavelength is used for intermediate distances. Long-haul systems at 1550 nm (lowest attenuation) use dispersion-shifted fiber or dispersion compensation.

Polarization Mode Dispersion (PMD)

Light has two polarization states that can travel at slightly different speeds due to fiber asymmetries. PMD is random and varies with temperature, stress, and vibration.

PMD is critical for very high data rates (40 Gbps+) over long distances but typically negligible in enterprise networks.

Media characteristics aren't static—they vary with environmental conditions. Understanding these variations is essential for reliable network design.

Temperature Effects

Moisture and Water

Direct contact: Water ingress damages cables over time. Copper corrodes; fiber coating can degrade. Outdoor and underground cables require water-blocking compounds or gel filling.

Humidity: High humidity can affect attenuation, particularly in poorly sealed connectors. Water vapor absorption by dielectric materials degrades performance.

Condensation: Cables entering buildings from cold exterior to warm interior may experience condensation inside the jacket. Proper sealing prevents this.

Physical Stress

Bend Radius: All cables have minimum bend radius specifications. Exceeding these:

• Copper: damages conductors, changes impedance
• Fiber: causes bend attenuation, potentially breaks glass

Tensile Load: Pulling cables exceeds tensile strength leads to conductor stretching or fiber breaks. Installation tension limits are critical.

Compression: Excessive compression (tight cable ties, heavy equipment) deforms cable geometry, affecting impedance and potentially damaging conductors or glass.

We've explored the key characteristics that define guided media performance. Let's consolidate this into actionable knowledge:

What's Next:

The next page covers installation considerations—the practical aspects of deploying guided media including cable routing, termination requirements, testing and certification, and the standards that govern professional installations.