A mobile phone signal (or reception) is the strength (measured in dBm) of the connection to the mobile phone with its network. Depending on various factors, such as proximity to a tower, obstructions such as buildings or trees, etc, the signal may vary. Most mobile devices use a set of bars of varying heights to display the strength of the signal where the device is located. Traditionally five bars are used; see five by five.
Generally, a stronger mobile phone signal is even harder to obtain in an urban area, though urban areas do have some "dead zones" where a reception can be obtained. On the contrary, many rural or minimally inhabited areas lack a signal or have a very weak reception, but 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 receptions.
In an area where the signal 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 a reception. Additionally, the weather and volume of network traffic may impact the strength.