IR PHY =========== .. contents:: :local: :depth: 3 Overview -------- The **Infrared (IR) Physical Layer (PHY)** was one of the three physical layers defined in the original **IEEE 802.11-1997** standard, alongside **FHSS** and **DSSS**. Unlike radio-based PHYs, the IR PHY used **infrared light** in the **near-infrared band (850–950 nm)** as its transmission medium. Infrared PHY was designed for **short-range indoor wireless LANs** using diffuse (non-line-of-sight) reflection from walls and ceilings. It supported **1 Mbps** and **2 Mbps** data rates using **Pulse-Position Modulation (PPM)**. Although standardized, it was never commercialized and was later replaced by RF-based PHYs (DSSS → 802.11b, OFDM → 802.11a/g). Key Characteristics ------------------- | Parameter | Description | |------------|-------------| | Medium | Diffuse infrared light (850–950 nm) | | Transmission Range | 5–10 m (indoor only) | | Data Rates | 1 Mbps, 2 Mbps | | Modulation | 4-PPM (1 Mbps), 16-PPM (2 Mbps) | | Access Mechanism | CSMA/CA (DCF) | | Carrier Sense | Optical energy detection | | Multipath Handling | Diffuse reflection (non-directional) | | Frame Structure | Standard PLCP + PSDU | | Status | Obsolete (no commercial deployments) | Operating Principle ------------------- Infrared PHY transmits digital data using **short bursts of modulated infrared light**. Instead of using a radio carrier, the information is encoded by the **position of a light pulse in time** within each symbol period — a technique known as **Pulse-Position Modulation (PPM)**. Each symbol period is divided into equal time slots: - The **presence** of a pulse in one slot indicates the data symbol. - The **absence** of a pulse** (other slots) indicates logical zeros. This approach provides high energy efficiency and noise immunity under diffuse lighting. Infrared Transmission and Reception ----------------------------------- **Transmitter:** - Converts bits into pulse positions (4-PPM or 16-PPM). - Drives an infrared LED emitter (center wavelength ~880 nm). - Emits diffuse light across the room for non-line-of-sight coverage. **Receiver:** - Uses a **photodiode** and **optical band-pass filter** to detect pulses. - Filters out ambient light and electrical noise. - Synchronizes to pulse timing using the PLCP preamble. - Decodes the time position of pulses to recover bits. Modulation and Data Rates ------------------------- | Mode | Modulation | Bits per Symbol | Symbol Duration | Description | |------|-------------|-----------------|-----------------|--------------| | 1 Mbps | 4-PPM | 2 | 4 µs | One of 4 slots contains a light pulse | | 2 Mbps | 16-PPM | 4 | 8 µs | One of 16 slots contains a light pulse | Infrared PPM does **not use a carrier frequency** — transmission occurs by turning the LED emitter on and off according to the modulation timing. PLCP Frame Structure -------------------- The Infrared PHY uses the same **Physical Layer Convergence Procedure (PLCP)** structure as other PHYs, ensuring a consistent interface to the MAC. | Field | Description | |--------|-------------| | Preamble | Timing and synchronization for receiver | | PLCP Header | Length, rate, and service fields | | PSDU | MAC frame payload | - **Preamble** enables symbol alignment and gain control. - **PLCP Header** communicates the data rate (1 or 2 Mbps). - **PSDU** contains the MAC Protocol Data Unit (MPDU). MAC Interaction and DCF Operation --------------------------------- The MAC layer interacts with the IR PHY identically to radio PHYs. - **CSMA/CA access**: same DCF rules (DIFS, SIFS, backoff, NAV). - **ACK, RTS/CTS, and retransmission**: same as RF operation. - **Physical carrier sense**: detects infrared energy above threshold. - **Virtual carrier sense (NAV)**: uses Duration field from MAC headers. From the MAC’s perspective, the IR medium behaves like any other shared channel. Carrier Sense and Clear Channel Assessment ------------------------------------------ Infrared carrier sense relies on **optical energy detection**: - The receiver measures incoming light intensity. - If optical power > threshold → “medium busy.” - If below threshold for a full DIFS → “medium idle.” - The backoff counter runs during idle slots, identical to RF-based DCF. Because IR light cannot penetrate walls, the effective collision domain is limited to a single enclosed space. Infrared PHY Timing Parameters ------------------------------ | Parameter | 1 Mbps | 2 Mbps | |------------|--------|--------| | Symbol Duration | 4 µs | 8 µs | | Slot Time (t_slot) | 20 µs | 20 µs | | SIFS | 10 µs | 10 µs | | DIFS | 50 µs | 50 µs | | Preamble Duration | 56 µs | 56 µs | | Maximum Range | ~10 m | ~5 m | DCF timing parameters (SIFS, DIFS, EIFS) are defined in microseconds and are identical to DSSS values to maintain MAC uniformity. Advantages ----------- - Immune to **RF interference** (since it uses light, not radio). - Constrained signal propagation (no wall penetration enhances security). - Suitable for **RF-restricted environments** (e.g., hospitals, aircraft). - Low implementation cost (uses LED and photodiode components). Limitations ----------- - **Line-of-sight or reflective path required**. - **Severely limited range** (~5–10 m). - **Susceptible to ambient light** (sunlight, lamps). - **No mobility support** (receiver must remain within reflective coverage). - **Low throughput** compared to DSSS (11 Mbps) or OFDM (54 Mbps). - **No interoperability** with RF PHYs. Coexistence and Compatibility ----------------------------- - The IR PHY cannot coexist with DSSS or FHSS PHYs within the same BSS. - Devices operate on only **one PHY type** at a time. - The MAC layer abstracts PHY differences, but physical signals are incompatible. - No hybrid IR–RF bridging was standardized. DCF Integration and Access Example ---------------------------------- Even though it uses light instead of radio, IR follows the same contention rules. .. code-block:: none Medium idle → wait DIFS → start backoff → transmit | └─ if medium busy: freeze backoff, wait until idle again ACK and retransmission: - Receiver sends ACK after SIFS (in IR form). - Sender retries using exponential backoff if ACK is missed. Sample Transmission Diagram ---------------------------- .. code-block:: none Time → +-------------------------------------------------------------+ |<-- DIFS -->|<- backoff ->|--- IR Preamble ---|-- PSDU --| SIFS | ACK | +-------------------------------------------------------------+ Medium Idle Contention Data Transmission Interframe Control Typical Parameter Values ------------------------ | Parameter | Typical Value | |------------|----------------| | IR wavelength | 850–950 nm | | Optical power | ~10 mW (diffuse LED) | | Photodiode sensitivity | ~–40 dBm optical equivalent | | Bit error rate target | <10⁻⁵ | | Indoor coverage | Single room | Security and Environmental Aspects ---------------------------------- - Infrared signals are **confined to a room**, enhancing physical security. - Immune to RF-based eavesdropping and interference. - However, **ambient light sources** (e.g., sunlight, fluorescent lamps) can saturate the photodiode and increase bit errors. - Line-of-sight blocking (people, furniture) may cause temporary link loss. Historical Context ------------------ | Year | Event | |------|--------| | 1997 | IR PHY introduced in original IEEE 802.11 | | 1999 | 802.11b DSSS supersedes IR | | 2000+ | No IR WLAN devices in production | | 2001 | IR officially deprecated in later revisions | The IR PHY was inspired by **IrDA** (Infrared Data Association) concepts but added **contention-based medium access (CSMA/CA)** instead of point-to-point link control. Summary ------- | Concept | Description | |----------|-------------| | Medium | Diffuse infrared light (850–950 nm) | | Modulation | 4-PPM (1 Mbps), 16-PPM (2 Mbps) | | Access | CSMA/CA (DCF) identical to RF PHYs | | Carrier Sense | Optical energy detection | | Range | 5–10 m indoor only | | Advantages | No RF interference, secure, low cost | | Disadvantages | Short range, ambient-light sensitivity | | Successor | DSSS in 802.11b (RF, 11 Mbps) | References ---------- - IEEE Std **802.11-1997**, Clause **14 — Infrared PHY Specification** - IEEE Std **802.11b-1999**, Annex (PHY evolution overview) - Gast, M. *802.11 Wireless Networks: The Definitive Guide*, O’Reilly - Kavehrad & Stuckel, “Indoor Broadband Optical Wireless Communications” - IEEE 802.11 Working Group historical drafts and archives Figures ------- .. figure:: _static/ir_phy_diagram.svg :align: center :alt: Infrared PHY principle Infrared PHY transmission concept showing 4-PPM pulses, diffuse reflections, and photodiode reception. .. tab-set:: .. tab-item:: physical_rates in 802.11b DSSS PHY layer ============ ===================== ========== === ================== ========== ============ ======= ====== ============ ========= ====== ========= =============================================================================== 802.11 MCS spreading/coding Modulation BW Total-Sub-Carriers FSP Tdata=1/FSP GI symbol Bits/symbol Code rate Usable Rate Formula (Usable Rate = (Bits/Symbol ÷ Symbol Duration) × (1 / Code Rate)) ============ ===================== ========== === ================== ========== ============ ======= ====== ============ ========= ====== ========= =============================================================================== IR 4PPM (optical pulse) 4-PPM 1 N/A 1 MHz 1 µs N/A 1 µs 1 1 1 1 Mbps (1 / 1 µs) × 1 = 1 Mbps IR 16PPM (optical pulse) 16-PPM 1 N/A 2 MHz 0.5 µs N/A 0.5 µs 2 1 2 2 Mbps (2 / 0.5 µs) × 1 = 4 Mbps (but data coded as 2 Mbps) ============ ===================== ========== === ================== ========== ============ ======= ====== ============ ========= ====== ========= =============================================================================== .. tab-set:: .. tab-item:: List of channels (802.11b) ===================== ======================= ====================== ============== Channel Number Center Frequency (MHz) Frequency Range DFS Required ===================== ======================= ====================== ============== 1 2412 2401 – 2423 No 2 2417 2406 – 2428 No 3 2422 2411 – 2433 No 4 2427 2416 – 2438 No 5 2432 2421 – 2443 No 6 2437 2426 – 2448 No 7 2442 2431 – 2453 No 8 2447 2436 – 2458 No 9 2452 2441 – 2463 No 10 2457 2446 – 2468 No 11 2462 2451 – 2473 No 12 2467 2456 – 2478 No 13 2472 2461 – 2483 No 14 2484 2473 – 2495 No ===================== ======================= ====================== ============== .. tab-set:: .. tab-item:: List of Bands (802.11b) ======================= ====================== ====================== ============================================================ Band Name Frequency Range (GHz) Frequency Range (MHz) Channels ======================= ====================== ====================== ============================================================ ISM Band (Global) 2.400 – 2.4835 2400 – 2483.5 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 (12, 13, 14 vary by region) ======================= ====================== ====================== ============================================================