---

Transmission Media in Computer Networks

---

1. Classification of Transmission Media

Transmission Media represents the physical conduit that carries information in the form of electromagnetic energy from a sender to a receiver. Media are located below the physical layer and directly driven by physical layer transceivers.

---

2. Guided Media (Wired)

A. Twisted Pair Cable

A twisted pair consists of two insulated copper conductors twisted around each other in a helical pattern.

• Why Twisting?
  • If two wires run parallel, the wire closer to an external electromagnetic interference (EMI) source receives more noise, creating a differential error voltage.
  • Twisting ensures both wires spend equal distance close to and away from the noise source, causing noise to induce identical voltages on both wires. The differential receiver computes $(V_+ - V_-)$, which cancels out the common-mode noise.
• Types:
  1. UTP (Unshielded Twisted Pair): Cheap, flexible, easy to install; standard in office LANs.
  2. STP (Shielded Twisted Pair): Encased in a metal foil braid to block severe industrial EMI; bulkier and requires proper grounding.
• UTP Categories:
  • Cat 3: Up to $16\text{ MHz}$, $10\text{ Mbps}$ (early 10Base-T).
  • Cat 5e: Up to $100\text{ MHz}$, $1\text{ Gbps}$ (Gigabit Ethernet over $100\text{ m}$).
  • Cat 6 / 6a: Up to $250 / 500\text{ MHz}$, $10\text{ Gbps}$ (up to $55\text{ m} / 100\text{ m}$).
• Standard Connector: RJ-45 (8-pin modular connector).

---

B. Coaxial Cable

Carries signals of higher frequency ranges than twisted-pair cable, with a concentric layered construction:

• Standards:
  • $50\text{ }\Omega$ (Baseband): RG-58 (Thin Ethernet - 10Base2), RG-11 (Thick Ethernet - 10Base5).
  • $75\text{ }\Omega$ (Broadband): RG-59, RG-6 used for Cable Television (CATV) and cable modems (DOCSIS).
• Connectors: BNC connector, BNC T-connector, and $50\text{ }\Omega$ BNC Terminators.

---

C. Fiber Optic Cable

Transmits information as pulses of light through high-purity glass (silica) or plastic fibers.

• Physical Principle — Total Internal Reflection (TIR):
  • The core has a refractive index $n_1$, surrounded by a cladding with a lower refractive index $n_2$ ($n_1 > n_2$).
  • When light strikes the core-cladding boundary at an angle of incidence $\theta_i$ greater than the Critical Angle $\theta_c$, $100\%$ of the light reflects internally back into the core with zero refraction into the cladding:

• Fiber Types:
  1. Single-Mode Fiber (SMF):
     • Extremely thin core diameter ($8 - 10\text{ }\mu\text{m}$).
     • Light travels along a single direct path with zero modal dispersion.
     • Light Source: Laser Diodes (LD) ($1310\text{ nm}$ or $1550\text{ nm}$).
     • Used for long-distance telecommunications and ISP backbones ($> 10\text{ km}$).
  2. Multi-Mode Fiber (MMF):
     • Larger core diameter ($50\text{ }\mu\text{m}$ or $62.5\text{ }\mu\text{m}$).
     • Multiple light rays (modes) bounce at different angles, leading to modal dispersion (spreading of pulses over distance).
     • Light Source: LEDs ($850\text{ nm}$).
     • Used for local campus backbones and data centers ($< 2\text{ km}$).
• Optical Connectors: SC (Subscriber Connector), ST (Straight Tip), LC (Lucent Connector).

---

3. Comparison of Guided Media

Feature	Twisted Pair (Cat 6)	Coaxial Cable (RG-6)	Fiber Optic (SMF)
Transmission Medium	Copper wire (Electrical)	Copper wire (Electrical)	Silica Glass (Light pulses)
Data Rate	Up to $10\text{ Gbps}$	Up to $1\text{ Gbps}$	$> 100\text{ Gbps} - \text{Tbps}$
Max Segment Distance	$100\text{ meters}$	$185\text{ m} - 500\text{ m}$	Up to $40 - 80\text{ km}$ without repeater
EMI Immunity	Moderate	Good	Total immunity (non-conductive)
Attenuation	High	Moderate	Extremely low ($0.2\text{ dB/km}$)
Cost & Installation	Lowest, very easy	Moderate, moderately easy	Highest cable/transceiver cost

---

4. Unguided Media (Wireless)

Electromagnetic waves broadcast through air, space, or vacuum:

1. Radio Waves ($3\text{ kHz} - 1\text{ GHz}$):
   • Omnidirectional: Radiate in all directions from the transmitter; sender and receiver do not need to be physically aligned.
   • Penetrate walls easily (usable indoors).
   • Applications: FM/AM Radio, cordless phones, maritime radio.
2. Microwaves ($1\text{ GHz} - 300\text{ GHz}$):
   • Unidirectional (Directional): High frequency requires directional parabolic dish or horn antennas facing each other in direct Line-of-Sight (LoS).
   • Cannot penetrate obstacles; curvature of the Earth requires microwave repeaters every $40 - 50\text{ km}$.
   • Applications: Satellite communications (uplink/downlink in C, Ku, Ka bands), cellular towers (4G/5G), Wi-Fi ($2.4\text{ GHz}$, $5\text{ GHz}$, $6\text{ GHz}$).
3. Infrared ($300\text{ GHz} - 400\text{ THz}$):
   • Very high frequency waves that cannot penetrate walls or solid objects.
   • Prevents interference between adjacent rooms; provides high physical security.
   • Applications: TV remote controls, IrDA short-range data exchange.

---