question: How do Wi-Fi signals travel through walls, floors, doors etc and not get blocked/reflect?
comment: They do get blocked and reflected. They also travel through walls which reduces the signal strength. Luckily, most wifi devices can send and receive extremly small signals.
passages: When the surfaces are stationary, the frequency of the waves detected by the receiver will be equal to the transmitted frequency. However, a change in frequency will occur as a result of the Doppler principle, when a person or object is moving towards or away from the detector. Such an event initiates an alarm signal. This technology is considered obsolete by many alarm professionals, and is not actively installed.
  
  Section::::Sensor types.:Microwave detectors.
   As example for this model it is assume: a rectilinear free space with two walls, one upper and the other lower, from which two vertical bases are positioned at their ends, these are the transmitting and receiving antennas that it’s locate in such a way that their heights don’t surpass the limits of the top wall; Achieved this the structure acts as free space for its functioning similar to that of a dielectric canyon of signals propagation, since the rays transmitted from the transmitting antenna will collide each side of the upper and lower walls infinity of times (for this example up to 3 reflections) until reaching the receiving antenna. During the course of the rays for each reflection they suffer, part of the energy of the signal is dissipated in each reflection, normally after the third reflection of said ray its resulting component which is a retro-reflected ray is insignificant with a negligible energy.
   This is also a major bottleneck of the technology when based on the visible spectrum, as it is restricted to the illumination purpose and not ideally adjusted to a mobile communication purpose. Technologies that allows as roaming between various Li-Fi cells, also known as handover, may allow to seamless transition between Li-Fi.
  
  The light waves cannot penetrate walls which makes a much shorter range, though more secure from hacking, relative to Wi-Fi. Direct line of sight is not necessary for Li-Fi to transmit a signal; light reflected off the walls can achieve 70 Mbit/s.
   BULLET::::- multipath reflection along the street
  
  BULLET::::- diffraction through windows, and attenuated passage through walls, into the building
  
  BULLET::::- reflection, diffraction, and attenuated passage through internal walls, floors and ceilings within the building
  
  The combination of all these effects makes the mobile phone propagation environment highly complex, with multipath effects and extensive Rayleigh fading. For mobile phone services, these problems are tackled using:
  
  BULLET::::- rooftop or hilltop positioning of base stations
  
  BULLET::::- many base stations (usually called "cell sites"). A phone can typically see at least three, and usually as many as six at any given time.
   Passive random reflections are achieved when plane waves are subject to one or more reflective paths around an object that makes an otherwise LOS radio path into NLOS. The reflective paths might be caused by various objects that could either be metallic (very good conductors such as a steel bridge or an airplane) or relatively good conductors to plane waves such as large expanses of concrete building sides, walls etc. Sometimes this is considered a "brute force" method because, on each reflection the plane wave undergoes a transmission loss that must be compensated for by a higher output power from the transmit antenna compared to if the link had been LOS. However the technique is cheap and easy to employ and passive random reflections are widely exploited in urban areas to achieve NLOS. Communication services that use passive reflections include WiFi, WiMax, WiMAX MIMO, mobile (cellular) communications and terrestrial broadcast to urban areas.
   Specially shaped directional antennas can increase the range of a Wi-Fi transmission without a drastic increase in transmission power. High gain antenna may be of many designs, but all allow transmitting a narrow signal beam over greater distance than a non-directional antenna, often nulling out nearby interference sources. Such "WokFi" techniques typically yield gains more than 10 dB over the bare system; enough for line of sight (LOS) ranges of several kilometers and improvements in marginal locations.
  
  Section::::Increasing range in other ways.:Protocol hacking.
   Section::::Challenges and limitations.:Fresnel zone.
  
  Even if there is a direct line of sight between the transmitting and receiving antenna, reflections from objects near the path of the signal can decrease apparent signal power through phase cancellations. Whether and how much signal is lost from a reflection is determined by the location of the object in the Fresnel zone of the antennas.
  
  Section::::Two-way satellite-only communication.
   PIR sensors are commonly called simply "PIR", or sometimes "PID", for "passive infrared detector". The term "passive" refers to the fact that PIR devices do not radiate energy for detection purposes. They work entirely by detecting infrared radiation (radiant heat) emitted by or reflected from objects.
  
  Section::::Operating principles.
  
  All objects with a temperature above absolute zero emit heat energy in the form of radiation. Usually this radiation isn't visible to the human eye because it radiates at infrared wavelengths, but it can be detected by electronic devices designed for such a purpose.
  
  Section::::Construction.
   Section::::Hardware.
  
  Wi-Fi allows wireless deployment of local area networks (LANs). Also, spaces where cables cannot be run, such as outdoor areas and historical buildings, can host wireless LANs. However, building walls of certain materials, such as stone with high metal content, can block Wi-Fi signals.
  
  Since the early 2000s, manufacturers are building wireless network adapters into most laptops. The price of chipsets for Wi-Fi continues to drop, making it an economical networking option which is included in ever more devices.
   Section::::Far-field (radiative) techniques.
  
  Far field methods achieve longer ranges, often multiple kilometer ranges, where the distance is much greater than the diameter of the device(s). High-directivity antennas or well-collimated laser light produce a beam of energy that can be made to match the shape of the receiving area. The maximum directivity for antennas is physically limited by diffraction.
  
  In general, visible light (from lasers) and microwaves (from purpose-designed antennas) are the forms of electromagnetic radiation best suited to energy transfer.
   Unlike the eye, sensors based on silicon (including CCDs and CMOS sensors) have sensitivities extending into the near-infrared. Such sensors may extend to 1000 nm. Digital cameras are usually equipped with IR-blocking filters to prevent unnatural-looking images. IR-transmitting (passing) filters, or removal of factory IR-blocking filters, are commonly used in infrared photography to "pass" infrared light and "block" visible and ultraviolet light. Such filters appear black to the eye, but are transparent when viewed with an IR sensitive device.
   Some manufacturers use proprietary spread-spectrum techniques, enabling typical operational ranges of up to 80 metres in-building. By also employing externally mounted antennas, operational ranges in excess 2000 metres (clear line-of-sight) have been achieved and several such models are sold under the Digi-Sender brand.
  
  Section::::Digital Wireless.:Wi-Fi.
   They suggest mounting the PIRs in such a way that the PIR cannot "see" out of a window. Although the wavelength of infrared radiation to which the chips are sensitive does not penetrate glass very well, a strong infrared source (such as from a vehicle headlight or sunlight) can overload the sensor and cause a false alarm. A person moving on the other side of the glass would not be "seen" by the PID. That may be good for a window facing a public sidewalk, or bad for a window in an interior partition.
   Although active components play an important role in the use of plasmonic circuitry, passive circuits are just as integral and, surprisingly, not trivial to make. Many passive elements such as prisms, lenses, and beam splitters can be implemented in a plasmonic circuit, however fabrication at the nano scale has proven difficult and has adverse effects. Significant losses can occur due to decoupling in situations where a refractive element with a different refractive index is used. However, some steps have been taken to minimize losses and maximize compactness of the photonic components. One such step relies on the use of Bragg reflectors, or mirrors composed of a succession of planes to steer a surface plasmon beam. When optimized, Bragg reflectors can reflect nearly 100% of the incoming power. Another method used to create compact photonic components relies on CPP waveguides as they have displayed strong confinement with acceptable losses less than 3 dB within telecommunication wavelengths. Maximizing loss and compactness with regards to the use of passive devices, as well as active devices, creates more potential for the use of plasmonic circuits.
   Some materials cause absorption of electromagnetic waves, preventing it from reaching the receiver, in other cases, particularly with metallic or conductive materials reflection occurs. This can cause dead zones where no reception is available. Aluminium foiled thermal isolation in modern homes can easily reduce indoor mobile signals by 10 dB frequently leading to complaints about the bad reception of long-distance rural cell signals.
  
  Section::::Properties.:Difficulties.:Multipath fading.
  
  In multipath fading two or more different routes taken by the signal, due to reflections, can cause the signal to cancel out at certain locations, and to be stronger in other places (upfade).
  
  Section::::Properties.:Difficulties.:Hidden node problem.
   According to Wi-Charge, the system transmits power using a straight, narrow beam. The beam is contained into a small spot and all the energy falls inside the receiver. Therefore, nobody is exposed to radiated energy as long as the path between the transmitter and the receiver is not crossed. If the path between transmitter and receiver is blocked, transmission stops immediately. Once line of sight is restored, charging resumes. Wi-Charge claims that this mechanism ensures that energy exposure to people, animals or unrelated objects is always below the maximum permissible exposure (MPE):"Sending energy over a distance, a beam always diverges, which is a bad thing as too little power is going in the right direction, and too much power splits and goes elsewhere. With RF, only relatively humble distances can be achieved, with a power capacity about a fraction of a Watt, limited by the maximum level of exposure allowed for safety. Shorter wavelength beams maintain their integrity better. With an infrared laser, we have a practically non-diverging beam able to deliver its entire power content onto a small receiver"
   If an object that changes a LOS link to NLOS is not a good conductor but an intermediate material, it absorbs some of the RF power incident upon it. However, if it has finite thickness the absorption is also finite and the resulting attenuation of the radio waves may be tolerable and an NLOS link may be set up using radio waves that actually pass through the material. As an example, wireless local area networks (WLANs) often use finite absorption NLOS links to communicate between a WLAN access point and WLAN client(s) in the typical office environment. The radio frequencies used, typically a few gigahertz (GHz) normally passes through a few thin office walls and partitions with tolerable attenuation. After many such walls though or after a few thick concrete or similar (non-metallic) walls the NLOS link becomes unworkable.
   Most materials of the type affecting radio wave transmission over NLOS links are intermediate: they are neither good insulators nor good conductors. Radio waves incident upon an obstruction comprising a thin intermediate material are partly reflected at both the incident and exit boundaries and partly absorbed, depending on the thickness. If the obstruction is thick enough the radio wave might be completely absorbed. Because of the absorption, these are often called lossy materials, although the degree of loss is usually extremely variable and often very dependent on the level of moisture present. They are often heterogeneous and comprise a mixture of materials with various degrees of conductor and insulator properties. Such examples are hills, valley sides, mountains (with substantial vegetation) and buildings constructed from stone, brick or concrete but without reinforced steel. The thicker they are the greater the loss. For example, a wall absorbs much less RF power from a normally incident wave than a building constructed from the same material.
   At the receiver, the optical signal is converted back to the traditional electrical signal to "appear" to the receiver as a conventional LNB.
  
  Section::::Monoblock LNBs.
   At longer distances, and with greater signal absorption, speed is usually reduced.
  
  Section::::Performance.:Transmitter power.
  
  Compared to cell phones and similar technology, Wi-Fi transmitters are low power devices. In general, the maximum amount of power that a Wi-Fi device can transmit is limited by local regulations, such as FCC Part 15 in the US. Equivalent isotropically radiated power (EIRP) in the European Union is limited to 20 dBm (100 mW).
   WiFi often extends beyond the boundaries of a home and can create coverage where it is least wanted, offering a channel through which non-residents could compromise a system and retrieve personal data. To prevent this it is usually sufficient to enforce the use of authentication, encryption, or VPN that requires a password for network connectivity.
  
  However new Wi-Fi standards working at 60 GHz, such as 802.11ad, enable confidence that the LAN will not trespass physical barriers, as at such frequencies a simple wall would attenuate the signal considerably.
  
  Section::::Common issues and concerns.:Electrical grid noise.
   First, AMC or HIS surfaces are designed to have an allotted set of frequencies over which electromagnetic surface waves and currents will not be allowed to propagate. These materials are then both beneficial and practical as antenna ground planes, small flat signal processing filters, or filters as part of waveguide structures. For example, AMC surfaces as antenna ground planes are able to effectively attenuate undesirable wave fluctuations, or undulations, while producing good radiation patterns. This is because the material can suppress surface wave propagation within the prescribed range of forbidden frequencies.
   BULLET::::- Building Automation, Access Control, Smart Energy . DASH7's signal propagation characteristics allow it to penetrate walls, windows, doors, and other substances that serve as impediments to other technologies operating at 2.45 GHz, for example. For smart energy and building automation applications, DASH7 networks can be deployed with far less infrastructure than competing technologies and at far lower total cost of ownership.
   Passive Wi-Fi
  
  Passive Wi-Fi is a refinement of Wi-Fi technology that uses passive reflection to reduce energy consumption.
  
  Section::::Wi-Fi energy use.
  
  Wi-Fi use can account for up to 60 percent of a smartphone’s energy consumption. When not connected to a network, Wi-Fi consumes energy because the device constantly searches for a signal.
  
  Section::::Backscattering.
  
  The technique communicates via backscattering, reflecting incoming radio waves sent from a separate device. The technique is similar to contactless RFID chip cards although unlike such cards, the new technique does not require a special device to read the signal.
   If formula_8 the material is a good conductor or a poor insulator and substantially "reflects" the radio waves that are incident upon it with almost the same power. Therefore, virtually no RF power is absorbed by the material itself and virtually none is transmitted, even if it is very thin. All metals are good conductors and there are of course many examples that cause significant reflections of radio waves in the urban environment, for example bridges, metal clad buildings, storage warehouses, aircraft and electrical power transmission towers or pylons.
  
presuppositions: Wifi signals don't get blocked or interfered by walls, floors, doors, and other blockages. Wi-Fi signals don't get reflected or blocked.
corrections: Wifi signals are blocked and reflected by doors, floors, etc., but the signals are extremely small which manage to reach the devices regardless of the block. Wi-Fi signals do get blocked, reflected, and suffer reduction in strength when going through walls.
raw_presuppositions: Wifi signals don't get blocked or interfered by walls, floors, doors, and other blockages. Wi-Fi signals don't get reflected or blocked.
raw_corrections: Wifi signals are blocked and reflected by doors, floors, etc., but the signals are extremely small which manage to reach the devices regardless of the block. Wi-Fi signals do get blocked, reflected, and suffer reduction in strength when going through walls.