Unguided Transmission Media

Point a television remote at your TV and press a button. In that moment, an invisible beam of light—infrared radiation—carries digital commands across your living room. This ubiquitous technology, invisible to human eyes but essential to our daily lives, represents another category of unguided transmission media: electromagnetic radiation just below the visible light spectrum.

Infrared (IR) communication bridges the gap between radio waves and visible light. With frequencies ranging from 300 GHz to 400 THz (wavelengths from 1 mm to 750 nm), infrared offers unique characteristics that make it ideal for certain applications while completely unsuitable for others. Understanding when and why to use infrared communication is essential knowledge for network professionals designing IoT systems, short-range device links, and even next-generation free-space optical networks.

Infrared radiation occupies the electromagnetic spectrum between microwaves and visible red light. The name literally means 'below red'—referring to frequencies lower than the red light visible to human eyes. Discovered by astronomer William Herschel in 1800 while measuring heat from different colors of sunlight, infrared has since become essential to everything from thermal imaging to television remotes.

Key Physical Properties:

1. Line-of-Sight Requirement: Infrared radiation travels in straight lines and cannot penetrate opaque objects. Unlike radio waves that can diffract around corners or pass through walls, IR communication requires an unobstructed optical path between transmitter and receiver. A hand, a piece of paper, or even a thin layer of dust can block an IR signal completely.

2. Limited Range: Infrared intensity decreases with the square of distance (inverse-square law), and atmospheric absorption limits practical outdoor ranges. For direct communication, typical ranges are:

• Remote controls: 5–10 meters
• IrDA device-to-device: 0.2–1 meter (standard) to 5 meters (extended)
• Free-space optical links: 200 meters to 5+ kilometers (with high-power systems)

3. High Bandwidth Potential: The extremely high frequencies of infrared (terahertz range) theoretically allow enormous bandwidth—far exceeding any radio frequency technology. Fiber optic systems operating in the near-IR bands (1310 nm, 1550 nm) regularly achieve 100+ Gbps per wavelength.

4. Immunity to RF Interference: Infrared signals operate at frequencies millions of times higher than radio, making them immune to RF interference. This isolation allows IR to work alongside Wi-Fi, Bluetooth, and cellular signals without mutual interference—an important advantage in electromagnetically noisy environments.

5. Security Through Confinement: Because IR cannot pass through walls or other opaque barriers, signals are naturally contained within a room. This physical confinement provides inherent security—an IR communication session in a conference room cannot be intercepted from the hallway.

Infrared Emitters and Detectors:

Light-Emitting Diodes (LEDs): The most common IR emitters for communication. IR LEDs are inexpensive, reliable, and can be modulated at rates up to several hundred Mbps. They emit a relatively broad beam (typically 20–40 degrees) and operate at wavelengths between 850 nm and 950 nm. Their relatively wide emission pattern provides tolerance for device alignment.

Laser Diodes (LDs): For higher data rates and longer distances, laser diodes provide coherent, highly collimated beams. Laser-based IR systems achieve data rates in the gigabits per second and distances in kilometers (for FSO applications). However, they require precise alignment and are significantly more expensive than LEDs.

Photodiodes: IR receivers use photodiodes—semiconductor devices that generate electrical current when exposed to light. Silicon photodiodes work well for near-IR wavelengths (up to ~1100 nm), while InGaAs photodiodes are used for longer wavelengths in fiber optic communications.

Optical Filters: To reject ambient light and improve signal-to-noise ratio, IR receivers incorporate optical bandpass filters that pass only the desired IR wavelengths while blocking visible light and other IR sources (sunlight, incandescent lighting).

The Infrared Data Association (IrDA) was founded in 1993 to establish standards for infrared communication between devices. At its peak, IrDA was implemented in millions of laptops, mobile phones, PDAs, and printers, enabling wireless data transfer, printing, and device synchronization. Though largely superseded by Bluetooth and Wi-Fi, understanding IrDA provides insights into short-range wireless protocol design and the historical evolution of device connectivity.

IrDA Protocol Stack:

IrDA defines a complete protocol stack for infrared communication:

Physical Layer (IrPHY): Defines optical parameters, encoding schemes, and timing. Key specifications include:

• Wavelength: 850–900 nm
• Half-intensity angle: ±15 to ±30 degrees (cone of transmission/reception)
• Maximum range: 1 meter (standard) or 30 cm (low power)
• Minimum range: 0 cm (devices can touch)

Link Access Protocol (IrLAP): Provides device discovery, address assignment, and reliable data transmission. Key features:

• Half-duplex operation (devices alternate transmitting and receiving)
• Primary/secondary roles (similar to master/slave)
• Sliding window flow control with selective reject
• Link turnaround time specifications

Link Management Protocol (IrLMP): Multiplexes multiple logical connections over a single IrLAP link. Enables different services and applications to share the IR channel.

Tiny TP (Tiny Transport Protocol): Provides segmentation/reassembly and flow control for protocols running over IrLMP. Ensures receiver buffers aren't overwhelmed.

Object Exchange (OBEX): High-level protocol for object exchange—transferring files, contacts, and other objects between devices. OBEX was later adopted by Bluetooth for similar purposes.

IrDA Communication Procedure:

1. Device Discovery: When initiating communication, a device enters discovery mode and broadcasts discovery frames. Devices within range respond with their addresses and capabilities. Unlike Bluetooth's complex pairing process, IrDA discovery was simply 'point and shoot.'

2. Connection Establishment: The initiating device selects a target and establishes an IrLAP connection. Devices negotiate speed and link parameters to find the highest mutually supported data rate.

3. Data Transfer: Once connected, data flows through the protocol stack. Applications use IrCOMM (serial emulation), OBEX (object transfer), or other IrDA services as needed.

4. Disconnection: Either device can terminate the connection. The link is torn down gracefully, releasing resources.

IrDA Design Constraints:

• Half-Angle Cone: Devices must be within ±15 to ±30 degrees of each other's optical axis. This requires deliberate aiming.
• Range: The 1-meter maximum range meant devices had to be close together—intentionally designed for privacy but limiting convenience.
• Line-of-Sight: Any obstruction broke the link immediately, requiring users to maintain device orientation during transfer.
• Single Connection: Standard IrDA supported only one connection at a time per device.

While IrDA data communication has faded from the market, infrared remains ubiquitous in consumer electronics remote control. Nearly every television, set-top box, audio receiver, and air conditioner is controlled by infrared remotes. Understanding this technology illustrates fundamental IR communication principles and remains relevant for IoT and home automation systems.

Basic Operating Principle:

A typical IR remote control works as follows:

1. Button Press: User presses a button, triggering the microcontroller to generate a command code.

2. Modulation: The command code is modulated onto an infrared carrier wave—typically at 36–40 kHz. This carrier frequency helps the receiver distinguish intentional signals from ambient infrared noise (sunlight, incandescent lights).

3. Transmission: An IR LED emits bursts of infrared light according to the modulated signal. The LED typically peaks at 940 nm wavelength.

4. Reception: The receiving device has an IR sensor (photodiode with bandpass filter and demodulator IC) that extracts the carrier-modulated signals.

5. Decoding: A microcontroller in the receiver decodes the signal and executes the corresponding command.

Carrier Modulation:

To reject ambient infrared from the sun and artificial lighting, remote controls modulate their signals onto a carrier frequency:

• 36 kHz: Common for Sony and many Asian brands
• 38 kHz: Most widely used (preferred for new designs)
• 40 kHz: Some Philips and European devices
• 56 kHz: Some older formats

The receiver's IC (such as the popular TSOP series) is tuned to respond only to its specific carrier frequency, providing excellent noise rejection.

The NEC Protocol (Most Common):

The NEC protocol has become the de facto standard for many manufacturers due to its simplicity and reliability:

Message Format:

1. Lead-in: 9ms carrier burst + 4.5ms space (AGC and timing reference)
2. Address Byte: 8 bits identifying the device type
3. Inverted Address: 8 bits (complement of address for error checking)
4. Command Byte: 8 bits specifying the button/function
5. Inverted Command: 8 bits (complement for error checking)

Bit Encoding:

• Logical '0': 562.5μs carrier burst + 562.5μs space
• Logical '1': 562.5μs carrier burst + 1.6875ms space

Total message duration: approximately 67.5 ms

Repeat Code: When a button is held down, after the initial message, a simplified repeat code is sent every 110ms (9ms burst + 2.25ms space + 562.5μs burst). This prevents retriggering full command decoding while indicating the button remains pressed.

Modern Evolution: IR in Smart Homes

Despite predictions of obsolescence, IR remote control remains entrenched in consumer electronics. Modern smart home systems bridge the gap between legacy IR and modern networking:

IR Blasters: Smart home hubs include IR emitters that can send commands to televisions, audio equipment, and air conditioners. Users control IR devices through smartphone apps, voice assistants, or automation routines.

IR-to-Smart Converters: Devices like Broadlink, Sensibo, and Logitech Harmony bridge IR-controlled equipment into smart ecosystems. An 'IR blaster' in a central location can control multiple devices.

IR Learning and Databases: Cloud databases store IR codes for thousands of devices, enabling automatic configuration of smart controllers. When a device isn't in the database, learning mode can capture its signals.

Limitations Persist: IR's line-of-sight requirement means smart home IR blasters must be positioned with clear optical paths to controlled devices—often requiring multiple blasters in different locations or IR repeater systems.

While IrDA and remote controls operate at low power over short distances, Free Space Optical (FSO) communication extends infrared and visible light principles to achieve fiber-like data rates over hundreds of meters to several kilometers through open air. FSO represents a growing segment of network infrastructure for building-to-building links, cellular backhaul, and scenarios where fiber installation is impractical.

FSO System Architecture:

An FSO link consists of two optical transceiver terminals, one at each end of the communication path:

Transmitter Section:

• Laser source: Typically a semiconductor laser diode operating at 785 nm (near-IR), 850 nm, 1310 nm, or 1550 nm. Higher-power systems may use fiber-amplified lasers.
• Modulator: Encodes data onto the laser beam, typically using intensity modulation (on-off keying or more complex schemes).
• Beam-forming optics: Telescopes and lenses shape the beam for optimal propagation—narrow enough to concentrate power, but wide enough for alignment tolerance.

Receiver Section:

• Collecting optics: Large-aperture telescope or lens gathers incoming light.
• Optical filter: Rejects ambient light outside the operating wavelength.
• Photodetector: Converts light to electrical signal (APD or PIN photodiode).
• Receiver electronics: Amplifies, filters, and decodes the signal.

Pointing and Tracking: High-performance FSO systems include automatic pointing and tracking mechanisms to maintain alignment despite building sway, thermal expansion, and other movements. This can include:

• Beacon lasers for alignment reference
• Gimbal mounts with motorized adjustment
• Adaptive optics to correct atmospheric distortion

Atmospheric Effects on FSO:

FSO signals propagate through the atmosphere, encountering various phenomena that affect link quality:

Atmospheric Attenuation: Particles in the atmosphere absorb and scatter the optical beam. Attenuation varies dramatically with conditions:

• Clear air: ~0.1 dB/km
• Haze: 1–3 dB/km
• Light fog: 10–30 dB/km
• Heavy fog: 50–200+ dB/km

Fog is the primary enemy of FSO links. Dense fog can cause over 300 dB/km attenuation, making even short links impossible. This is why FSO link availability guarantees must account for local fog statistics.

Scintillation: Atmospheric turbulence causes the refractive index to fluctuate, creating scintillation—rapid intensity variations (the same effect that makes stars 'twinkle'). Scintillation worsens with:

• Longer propagation distances
• Higher temperatures and humidity
• Daytime heating (strongest near midday)
• Low elevation angles (more atmosphere traversed)

Mitigation techniques include:

• Large receiver apertures (averaging over multiple turbulence cells)
• Aperture averaging (multiple smaller apertures)
• Adaptive optics
• Multiple-beam systems with spatial diversity

Beam Wander: Turbulence also causes the entire beam to wander off-axis over time, requiring tracking systems to maintain alignment for longer links.

FSO Applications:

Building-to-Building Connectivity: The most common FSO application—connecting buildings across streets, parking lots, or campuses without trenching for fiber. Ideal for:

• Connecting buildings on the same campus
• Extending network across public rights-of-way where permits are difficult
• Rapid deployment for temporary or emergency connectivity

Cellular Backhaul: FSO provides high-capacity backhaul for 4G/5G small cells, particularly in urban areas where rooftop-to-rooftop links are feasible and fiber installation would require extensive street disruption.

Disaster Recovery: When fiber is cut or unavailable, FSO can be rapidly deployed to restore connectivity. Portable FSO systems can be set up in hours.

Dense Urban Environments: In cities where wireless spectrum is congested and fiber rights-of-way are expensive or unavailable, FSO offers an alternative high-capacity path.

Satellite and Aerospace: FSO enables high-speed inter-satellite links and satellite-to-ground communications. In the vacuum of space, atmospheric effects don't exist, making optical links highly attractive for their bandwidth density.

Although IrDA for data communication has been supplanted by Bluetooth and Wi-Fi, infrared technology continues to evolve and find new applications in modern networking. From indoor positioning to LiFi networks, infrared remains relevant in niches where its unique properties provide advantages that other technologies cannot match.

Optical Wireless Communication (OWC) and LiFi:

The concept of using light for high-speed data communication has been renewed under the banner of Optical Wireless Communication (OWC) and LiFi (Light Fidelity). While LiFi often uses visible light (LED lighting fixtures as access points), many implementations use infrared for uplink (device to access point) and for systems where visible light modulation would be distracting.

Key OWC/LiFi Characteristics:

• Data rates up to multi-gigabit per second
• Uses existing lighting infrastructure (with modifications)
• Does not interfere with RF systems—ideal for hospitals, aircraft, and industrial environments
• Highly secure: signals don't pass through walls
• Supports high density: light cells can be very small

Standards development is progressing through IEEE 802.11bb (LiFi as a Wi-Fi extension), promising integration with existing networking infrastructure.

Infrared in Sensing and Indoor Positioning:

Infrared is increasingly used for non-communication purposes that support networking:

Infrared Indoor Positioning Systems (IPS): Some indoor positioning systems use IR beacons or ceiling-mounted IR detectors to track objects with higher precision than Wi-Fi or Bluetooth-based systems. The line-of-sight requirement guarantees the tracked device is in the same room as the detector.

Occupancy and Presence Detection: PIR (Passive Infrared) sensors detect human presence by sensing body heat. These sensors are widely used in smart buildings for occupancy-based HVAC and lighting control, feeding data into building management and IoT networks.

Time-of-Flight (ToF) Depth Sensing: Modern smartphones and devices include IR-based depth sensors (Face ID, hand tracking) using structured light or ToF principles. These enable augmented reality applications and new forms of human-computer interaction.

Infrared in Industrial and Specialized Networks:

Industrial Automation: In environments with extreme electromagnetic interference (near motors, welding equipment, power systems), infrared communication provides reliable data links that are immune to EMI. Some industrial safety light curtains also integrate IR communication for status reporting.

Medical Environments: Infrared is preferred in some medical settings where RF interference with sensitive equipment is a concern. Patient monitoring devices, infusion pumps, and other equipment can communicate via IR without affecting nearby RF-sensitive instruments.

Automotive: Modern vehicles include IR-based systems for driver monitoring (gaze detection, drowsiness alerts), gesture control interfaces, and night vision enhancement. While not traditional 'networking,' these systems increasingly integrate with vehicle networks.

Data Centers: Emerging research explores IR for ultra-high-speed interconnects within data centers, potentially replacing short-reach fiber or copper for rack-to-rack or even chip-to-chip communication. The ability to route beams through free space without cables could revolutionize data center design.

Understanding where infrared fits among wireless technologies helps network professionals choose the right tool for each application. Each technology has distinct strengths and weaknesses based on their underlying physics.

When to Choose Infrared:

1. RF-Prohibited Environments: Hospitals, aircraft, research facilities, or industrial settings where RF emissions are restricted or could cause interference. IR provides connectivity without RF.

2. High-Security Requirements: Conference rooms, government facilities, or financial trading floors where signal leakage could enable eavesdropping. IR signals don't pass through walls.

3. FSO Backhaul/Point-to-Point: When fiber isn't available and microwave spectrum is congested or unavailable. FSO provides multi-gigabit connectivity without spectrum licensing.

4. Legacy Device Control: Integrating smart home automation with IR-controlled TVs, HVAC, and audio equipment. IR remains the universal language of consumer electronics.

5. Specialized Industrial Applications: Environments with extreme EMI where RF reliability is compromised. IR provides deterministic, interference-free communication.

When Other Technologies Excel:

• Bluetooth: Best for personal-area networking, audio streaming, peripherals, low-power IoT
• Wi-Fi: Best for general-purpose networking, video streaming, internet access
• Cellular: Best for wide-area mobility, outdoor coverage, carrier-managed services
• LoRa/Zigbee: Best for low-power, long-range IoT sensing

In practice, many deployments use multiple technologies together—Wi-Fi for primary connectivity, IR blasters for device control, Bluetooth for wearables, and cellular for backup or mobile access.

Infrared communication fills a unique niche in the wireless landscape—operating at frequencies far above radio but sharing many networking challenges with other unguided media. From the IrDA standards that connected 1990s devices to the FSO links carrying gigabits across cityscapes today, infrared continues to evolve and find new applications.

What's Next:

In the next page, we'll explore satellite communication—extending wireless connectivity beyond the atmosphere to orbit and back. We'll examine satellite orbital mechanics, earth station design, LEO/MEO/GEO systems, and the new era of satellite internet constellations like Starlink that are transforming global connectivity.