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A mobile phone signal (also known as reception and service) is the signal strength (measured in dBm) received by a mobile phone from a cellular network (on the downlink). Depending on various factors,such as proximity to a tower,any obstructions such as buildings or trees, etc. this signal strength will vary. Most mobile devices use a set of bars of increasing height to display the approximate strength of this received signal to the mobile phone user. Traditionally five bars are used. (see five by five)
Generally,a strong mobile phone signal is more likely in an urban area,though these areas can also have some “dead zones”, where no reception can be obtained.Cellular signals are designed to be resistant to multipath reception,which is most likely to be caused by the blocking of a direct signal path by large buildings,such as high-rise towers. By contrast,many rural or sparsely inhabited areas lack any signal or have very weak fringe reception ; many mobile phone providers are attempting to set up towers in those 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,though location of radio towers within these areas is normally prohibited or strictly regulated, and is often difficult to arrange.
In areas where signal reception would normally be strong, other factors can have an effect on reception or may cause complete failure (see RF interference) . From inside a building with thick walls or of mostly metal construction (or with dense rebar in concrete), signal attenuation may prevent a mobile phone from being used.Underground areas,such as tunnels and subway stations,will lack reception unless they are wired for cell signals.There may also be gaps where the service contours of the individual base stations (Cell towers) of the mobile provider (and/or its roaming partners) do not completely overlap.
In addition,the weather may affect the strength of a signal,due to the changes in radio propagation caused by clouds (particularly tall and dense thunderclouds which cause signal reflection), precipitation, and temperature inversions.This phenomenon,which is also common in other VHF radio bands including FM broadcasting,may also cause other anomalies,such as a person in San Diego “roaming” on a Mexican tower from just over the border in Tijuana,or someone in Detroit “roaming” on a Canadian tower located within sight across the Detroit River in Windsor,Ontario.These events may cause the user to be billed for “international” usage despite being in their own country,though mobile phone companies can program their billing systems to re-rate these as domestic usage when it occurs on a foreign cell site that is known to frequently cause such issues for their customers.
The volume of network traffic can also cause calls to be blocked or dropped due to a disaster or other mass call event which overloads the number of available radio channels in an area, or the number of telephone circuits connecting to and from the general public switched telephone network.

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