In 1999, sales of mobile phones surpassed combined sales of personal computers and automobiles. By 2010, mobile phones had replaced land-line phones in 30% of U.S. households. Smart phones, led by iPhones and Android phones, have become indispensible personal assistants. Laptop computer sales outnumber desktop computer sales, and most laptops are equipped with cellular data chipsets or USB modems. Apple's iPad has sparked the connected tablet market too. Vending machines, automobiles, mobile sensors, and many other devices include "machine to machine" cellular data modules. As a result, the number of cellular voice and data devices will soon exceed the number of people on Earth. If sheer numbers weren't enough, new uses for mobile devices are causing even faster growth in bandwidth usage. Obvious uses include video entertainment, videoconferencing, downloaded and streaming music, MMS, email, and application downloads. Facebook, Twitter, Foursquare, and many other social networking applications put further load on operator networks. Also, surprising sources of traffic have emerged, such as deliberate "miscalls". A miscall is when one subscriber calls another, but hangs up before the receiving party answers. Since operators don't charge for these uncompleted calls, subscribers are using miscalls as a free way to communicate. In
India, orders for milk are made this way. In Syria, five miscalls in a row signals the recipient to "go online" to the Internet and chat. In Bangladesh, it's estimated that up to 70% of traffic at peak times is due to miscalls. This practice isn't limited to countries with low per-capita income, and yet it places a high load on operator networks. 6 --------------------------------------------------------------------------------
There are sources of congestion based on location and time, too. Transportation clusters like airports, major highway intersections, bridges, and toll road gates all bring many people together at peak times. Also, because of home land-line replacement, many residential neighborhoods have many mobile phones in simultaneous use in mornings and evenings. Lastly, local population growth and immigration can result in too many phones for existing infrastructure. Due to long planning times, investment requirements, local government permits, and construction time, it's difficult for infrastructure to keep up with the pace of change in many developing areas, especially in growth countries.
Radio Signal Interference
Interference comes from both obvious and subtle causes. Certain materials aren't transparent to radio signals, especially durable materials used in buildings, large structures, and even automobiles. As a result there are radio shadows in which a mobile phone can't sense the signal from a base station. In addition, radio signals from adjacent channels or reflected signals can interfere with each other due to wave cancellation effects. In some cases these forms of interference primarily attenuate the signal (make it weaker). However, interference can also add noise, so that the ratio of signal to noise becomes too low for the mobile phone and the base station to understand each other.
Mobile phone networks are called "cellular" networks because they are made up of overlapping areas of coverage that are provided by base stations in fixed locations. As a mobile subscriber travels by automobile or train, he will eventually reach the limit of a base station's coverage. At that point, his mobile phone will "hand off" to a base station for the next coverage area. If signal quality is poor due to interference, or if the new base station is congested with too many mobile phones, the subscriber's connection may be lost.