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.

                             TRANSMISSION MEDIA
                                     |
         +---------------------------+---------------------------+
         |                                                       |
  GUIDED MEDIA (Wired/Bounded)                           UNGUIDED MEDIA (Wireless)
         |                                                       |
  +------+------+-------------+                   +--------------+-------------+
  |             |             |                   |              |             |
Twisted Pair  Coaxial    Fiber Optic         Radio Waves     Microwaves    Infrared
 (UTP / STP)   Cable    (Single/Multi)      (3kHz-1GHz)     (1GHz-300GHz) (300GHz-400THz)

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.

       Insulated Copper Conductor 1
       ~~~~\        /~~~~\        /~~~~
            \      /      \      /       Twisting cancels EMI & crosstalk
       ~~~~/ \~~~~/        \~~~~/ \~~~~
       Insulated Copper Conductor 2

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:

        +-------------------------------------------------------------+
        | Outer Plastic Jacket (PVC/Teflon)                          |
        |   +-------------------------------------------------------+ |
        |   | Metallic Braided Shield (Ground / EMI shield)         | |
        |   |   +-------------------------------------------------+ | |
        |   |   | Dielectric Insulating Layer                     | | |
        |   |   |   +-------------------------------------------+ | | |
        |   |   |   | Solid Copper Core Conductor (Data signal) | | | |
        |   |   |   +-------------------------------------------+ | | |
        +---+---+---+-------------------------------------------+-+-+-+

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.

                    +------------------------------------+
                    | Outer Protective Jacket            |
                    |   +------------------------------+ |
                    |   | Kevlar Strength Fibers       | |
                    |   |   +------------------------+ | |
                    |   |   | Cladding (n2)          | | |
                    |   |   |   +------------------+ | | |
  Light Ray ======> |   |   |   | Core (n1 > n2)   | | | |
                    |   |   |   +------------------+ | | |
                    +---+---+---+--------------------+-+-+

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:

\theta_c = \arcsin\left(\frac{n_2}{n_1}\right)

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:

  3 kHz                 1 GHz                300 GHz             400 THz
    |--- Radio Waves -----|---- Microwaves -----|---- Infrared -----|
    (Omnidirectional)     (Line-of-Sight)       (Cannot penetrate walls)

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.
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 ( $2.4\text{ GHz}$ , $5\text{ GHz}$ , $6\text{ GHz}$ ).
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.

5. Key Exam Questions & Takeaways

State the condition necessary for Total Internal Reflection in optical fibers.
- The refractive index of the core ( $n_1$ ) must be strictly greater than that of the cladding ( $n_2$ ).
- The angle of incidence $\theta_i$ at the boundary must exceed the critical angle $\theta_c = \arcsin(n_2/n_1)$ .
Why are copper conductors twisted in pairs?
- Twisting balances the exposure of both conductors to ambient electromagnetic interference, allowing differential receivers to cancel out common-mode noise and prevent crosstalk between adjacent pairs.
Differentiate Single-Mode and Multi-Mode optical fibers.
- Single-Mode: Core diameter $8-10\text{ }\mu\text{m}$ , laser source, zero modal dispersion, spans tens of kilometers.
- Multi-Mode: Core diameter $50-62.5\text{ }\mu\text{m}$ , LED source, suffers modal dispersion, limited to $< 2\text{ km}$ .
Why is fiber optic immune to electromagnetic interference (EMI)?
- Optical fibers transmit dielectric light waves through silica glass rather than electrical currents through conductive metals; hence, radio waves, high-voltage lines, and magnetic fields cannot induce noise in the fiber.

Transmission Media