Mobile phone signal
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A mobile phone signal (or reception) is the signal strength (measured in dBm) received by the mobile phone from the cellular network (on the downlink). Depending on various factors, such as proximity to a tower, obstructions such as buildings or trees, etc., the signal strength will vary. Most mobile devices use a set of bars of increasing height to display the approximate strength of the received signal to the mobile phone user. Traditionally five bars are used; see five by five.
Generally, a stronger mobile phone signal is easy to obtain in an urban area, though urban areas do have some "dead zones" where no reception can be obtained. On the contrary, many rural or minimally inhabited areas lack a signal or have a very weak reception; many mobile phone providers are attempting to set up towers in parts of these areas most likely to be occupied by users, such as along major highways. Even some national parks and other popular tourist destinations away from urban areas now have cell phone reception.
In an area where signal reception would normally be strong, certain other factors may have an effect on the reception, thereby making it either stronger or weaker, or may cause complete interference. For example, a building with thick walls may prevent a mobile phone from being used. Many underground areas, such as tunnels and subway stations, lack reception. Additionally, the weather and volume of network traffic may impact the strength.
Dead zones 
Areas where mobile phones cannot transmit to a nearby mobile site, base station, or repeater are known as dead zones. In these areas, the mobile phone is said to be in a state of outage. Dead zones are usually areas where mobile phone service is not available because the signal between the handset and mobile site antennas is blocked, usually by hilly terrain, excessive foliage, or physical distance.
A number of factors can create dead zones which may exist even in locations in which a wireless carrier offers coverage, due to limitations in cellular/Mobile network architecture (the locations of antennas), limited network density, interference with other mobile sites, and topography. Since cell phones rely on radio waves, and radio waves travel though the air and are easily attenuated, mobile phones may be unreliable at times. Like other radio transmissions, mobile phone calls can be interrupted by large buildings, terrain, trees, or other objects between the phone and the nearest base station antennas.
Many wireless service providers work continually to improve and upgrade their networks in order to minimize dropped calls, access failures, and dead zones (which they call "coverage holes" or "no-service areas").
Dropped calls 
One reason for a dropped call is when the mobile phone moves out of range of a wireless network. An active call cannot usually be maintained across a different company's network (as calls cannot be re-routed over the traditional phone network while in progress), resulting in the termination of the call once a signal cannot be maintained between the phone and the original network.
Another common reason is when a phone is taken into an area where wireless communication is unavailable, interrupted, interfered with, or jammed. From the network's perspective, this is the same as the mobile moving out of the coverage area.
Occasionally, calls are dropped upon handoff between cells within the same provider's network. This may be due to an imbalance of traffic between the two cell sites' areas of coverage. If the new cell site is at capacity, it cannot accept the additional traffic of the call trying to "hand in." It may also be due to the network configuration not being set up properly, such that one cell site is not "aware" of the cell to which the phone is trying to handoff. If the phone cannot find an alternative cell to which to move that can take over the call, the call is lost.
Co-channel and Adjacent channel interference can also be responsible for dropped calls in a wireless network. Neighbour cells with the same frequencies interfere with each other, deteriorating the quality of service and producing dropped calls. Transmission problems are also a common cause of dropped calls. Another problem may be a faulty transceiver (XCVR) inside the base station.
Calls can also be dropped if a mobile phone at the other end of the call loses battery power and stops transmitting abruptly.
Sun spots and solar flares are rarely blamed for causing interference leading to dropped calls.
Experiencing too many dropped calls is one of the most common customer complaints received by wireless service providers. They have attempted to address the complaint in various ways, including expansion of their home network coverage, increased cell capacity, and offering refunds for individual dropped calls.
Various signal booster systems are manufactured to reduce problems due to dropped calls and dead zones. Many options, such as wireless units and antennas, are intended to aid in strengthening weak signals.
Arbitrary Strength Unit (ASU) is an integer value proportional to the received signal strength measured by the mobile phone.
It is possible to calculate the real signal strength measured in dBm (and thereby power in Watts) by a formula. However, there are different formulas for 2G and 3G networks.
- dBm = 2 × ASU - 113, ASU in the range of 0..31 and 99 (for not known or not detectable)
- dBm = ASU - 116, ASU in the range of -5..91 and 255 (for not known or not detectable)
It is widely disbelieved[by whom?] that ASU = "Active Set update". The Active Set Update is a signalling message used in handover procedures of UMTS and CDMA mobile telephony standards. On Android phones, the acronym ASU has nothing to do with Active Set Update. It has not been declared precisely by Google developers.
See also 
- Cell phone tower
- Subscriber Identity Module
- Cellular repeater
- Missed call (intentionally dropped calls)