Communication Channels

A communication channel is the physical medium or electromagnetic path through which information travels from the transmitter to the receiver. The choice of channel determines the achievable data rate, attenuation characteristics, susceptibility to noise, and the practical range of a communication link. For any communication system design, understanding the properties of available channels is the starting point.

Wireline Channels

Wireline channels use metallic conductors, primarily copper, to guide electrical signals. Two main types are in use: twisted pair and coaxial cable. A twisted pair consists of two insulated copper wires twisted together, which reduces electromagnetic interference (EMI) by canceling out noise that affects both wires equally. This common-mode rejection is fundamental to its noise performance. Category 5e (Cat5e) cable supports up to 1 Gbps over 100 meters and is the standard for Ethernet LANs.

A coaxial cable has a central conductor surrounded by a dielectric insulator, a braided shield, and an outer jacket. The shielded geometry provides excellent EMI immunity and allows operation at much higher frequencies than twisted pair. Coaxial cable is used in cable television (CATV) distribution, connecting antenna to receiver, and in RF test equipment. Signal attenuation in copper cables increases with frequency approximately as the square root of frequency due to the skin effect, where current flows only in a thin surface layer of the conductor at high frequencies.

Optical Fiber Channels

Optical fiber transmits information as modulated light pulses through a glass or plastic core surrounded by cladding with a lower refractive index. The signal propagates through total internal reflection. Optical fiber offers three major advantages over copper: extremely low attenuation (0.2 dB/km for single-mode fiber versus 10 dB/100m for copper), immunity to electromagnetic interference, and very high bandwidth due to the extremely high carrier frequency (around 193 THz at 1550 nm wavelength).

Single-mode fiber (SMF) has a core diameter of 8-10 micrometers, allowing only one propagation mode, which eliminates modal dispersion and enables transmission over thousands of kilometers with amplification. Multi-mode fiber (MMF) has a core of 50 or 62.5 micrometers, supports multiple propagation modes, and is used for shorter distances within buildings and campuses. The primary impairments in optical fiber are chromatic dispersion (different wavelengths travel at slightly different speeds) and polarization mode dispersion (PMD) in SMF, and modal dispersion in MMF.

Wireless Channels

Wireless channels use free space as the propagation medium, with the signal traveling as an electromagnetic wave. The signal is not confined to any physical medium, making wireless the only option for mobile users and remote areas. The electromagnetic spectrum is divided into bands, each with different propagation characteristics. Lower frequencies (MF, HF) can diffract over terrain and propagate along the earth's surface, enabling long-range AM broadcasting. Higher frequencies (VHF, UHF) require line-of-sight but carry more bandwidth, making them suitable for television and cellular.

Microwave frequencies (1-100 GHz) are used for point-to-point terrestrial links and satellite communication. At these frequencies, the signal behaves nearly like a ray of light and requires antenna alignment. The main propagation impairment is free-space path loss, which increases with the square of the distance and the square of the frequency, as described by the Friis transmission equation. Multipath fading, where reflected copies of the signal interfere with the direct path, is a critical impairment in urban wireless environments.

Satellite Channels

Satellite communication uses a transponder in orbit to receive an uplink signal from the ground, frequency-shift it, amplify it, and retransmit it as a downlink signal to a large coverage area. The altitude of the satellite orbit determines the delay and coverage. Geostationary orbit (GEO) satellites at 35,786 km altitude have a propagation delay of approximately 270 ms one-way, which is problematic for real-time interactive applications but acceptable for broadcasting. Low Earth Orbit (LEO) satellites such as Starlink operate at 200-600 km, reducing latency to 5-20 ms at the cost of requiring a large constellation to provide continuous coverage.

Mathematical Expression

The received power in a wireless channel is described by the Friis transmission equation, which accounts for transmit power, antenna gains, wavelength, and distance. For a given link, the free-space path loss (FSPL) determines how much the signal attenuates purely due to distance, independent of any obstacles. The path loss in dB is:

FSPL (dB) = 20 log10(d) + 20 log10(f) + 20 log10(4 pi / c), which simplifies to FSPL (dB) = 20 log10(d) + 20 log10(f) - 147.55, where d is in meters and f is in Hz.

Given:
Frequency f = 2.4 GHz = 2.4 x 10^9 Hz
Distance d = 100 m (indoor WiFi)

Why this formula applies:
Free-space path loss gives the baseline signal attenuation for any wireless link.
Actual loss will be higher due to walls and obstacles, but FSPL is the minimum.

Formula:
FSPL (dB) = 20 log10(d) + 20 log10(f) - 147.55

Substitution:
FSPL = 20 log10(100) + 20 log10(2.4 x 10^9) - 147.55

Calculation:
20 log10(100) = 20 x 2 = 40 dB
20 log10(2.4 x 10^9) = 20 x 9.38 = 187.6 dB
FSPL = 40 + 187.6 - 147.55 = 80.05 dB

Final Answer:
Free-space path loss at 100 m, 2.4 GHz = 80.05 dB
This means if TX power = 20 dBm, received signal = 20 - 80.05 = -60.05 dBm,
which is within typical WiFi receiver sensitivity of -80 to -90 dBm.

Exam Tip: In FSPL, doubling the distance adds 20 log10(2) = 6 dB of loss (not 3 dB). Doubling frequency also adds 6 dB. Both distance and frequency affect FSPL through a square law relationship, hence 20 log10 and not 10 log10.

Practical Implications of Channel Choice

The choice of communication channel fundamentally determines the system architecture. Optical fiber is the only practical choice for internet backbone links because no other medium can match its combination of extremely low attenuation, enormous bandwidth, and EMI immunity at continental and intercontinental scales. Copper twisted pair remains the most economical choice for the last hundred meters from the exchange to the home (local loop) in DSL systems, even though it is inferior to fiber.

Wireless is the only option when mobility is required or when laying physical cables is impractical. Satellite becomes the channel of choice for remote areas, maritime, and aviation applications where no terrestrial infrastructure exists. The 270 ms delay of GEO satellites is a major concern for TCP/IP performance because TCP's congestion window mechanisms do not work well with such long round-trip times, requiring specific TCP variants such as TCP Hybla for satellite links.

Mechanism Explained

• Guided channels (wireline, fiber) confine the signal to a physical medium, providing predictable loss and immunity to external interference. Unguided channels (wireless, satellite) allow mobility but introduce path loss, fading, and shared spectrum.
• Optical fiber achieves 0.2 dB/km attenuation at 1550 nm wavelength because Rayleigh scattering and material absorption are minimal at this wavelength. EDFA amplifiers are spaced every 80-120 km on long-haul fiber links.
• Free-space path loss increases with both distance and frequency. Higher frequencies suffer more path loss for the same distance, which is why lower frequency bands (700 MHz LTE) are used for wide-area coverage while higher frequency bands (2.6 GHz, 3.5 GHz) are used for capacity in dense areas.
• GEO satellite delay of 270 ms means a round-trip delay of 540 ms, which is perceived as an echo in telephony and causes TCP performance degradation. Special protocols are needed for satellite TCP optimization.
• The skin effect in copper conductors causes higher-frequency signals to travel only in a thin surface layer, increasing effective resistance and thus attenuation. This is why copper cable bandwidth-distance product is limited.

Quick Revision

• Guided media: twisted pair (Cat6: 1 Gbps/100m), coaxial (10 Gbps, CATV), optical fiber (SMF: 0.2 dB/km, Tbps).
• Unguided media: terrestrial RF, microwave LoS, satellite (GEO 35786 km delay 270 ms, LEO 200-2000 km delay 5-20 ms).
• FSPL (dB) = 20 log10(d) + 20 log10(f) - 147.55 (d in meters, f in Hz).
• Doubling distance or frequency increases FSPL by 6 dB (not 3 dB). Square law gives 20 log10, not 10 log10.
• Total internal reflection in fiber requires n_core > n_cladding. SMF eliminates modal dispersion by allowing only one mode.
• Skin effect limits copper cable at high frequencies. EDFA erbium-doped fiber amplifier extends optical fiber reach without OEO conversion.
• Exam trap: Optical fiber is NOT immune to dispersion, only to EMI. Dispersion (chromatic and modal) is the bandwidth-limiting factor in fiber, not attenuation in modern systems.