Mobile Phone Signal
Transcript of Mobile Phone Signal
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Mobile phone signal
From Wikipedia, the free encyclopedia
A display of bars on a mobile phone screen
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. Since cellular signals are designed to be
resistant to multipath issues, this would most likely be due to blocking by a large building such as a high-
rise. On the contrary, many rural or minimally inhabited areas lack a signal or have a very weak fringe
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, though location of
radio towers in these areas is normally prohibited or strictly regulated, and is often blocked by
mountains (where parks are often located).
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 RF interference. For
example, a building with thick walls or of mostly metal construction (or with dense rebar in concrete)
may prevent a mobile phone from being used. Underground areas, such as tunnels and subway stations,
lack reception unless they are wired for cell signals. There may also be gaps where the service contours
of the individual base stations of one's mobile carrier (and/or its roaming partners) do not completely
overlap.
Additionally, the weather may impact 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
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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 with 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.
Contents
1 Dead zones
2 Dropped calls
3 ASU
4 See also
5 References
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 or severely reduced, usually by hilly terrain, dense 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 network architecture (the locations of antennas), limitednetwork density, interference with other mobile sites, and topography. Since cell phones rely on radio
waves, which travel though the air and are easily attenuated (particularly at higher frequencies), 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.
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"). For mobile virtual network operators, the network quality depends entirely on the host network
for the particular handset in question. Some MVNOs use more than one host, which may even havedifferent technologies. (For example, different Tracfone handsets uses either CDMA and 1xRTT on
Verizon Wireless, or GSM and UMTS on AT&T Mobility.)
Dead zones can be filled-in with microcells, while picocells can handle even smaller areas without
causing interference to the larger network. Personal microcells, such as those for a home, are called
femtocells, and generally have the range of a cordless phone, but may not be usable for an MVNO
phone. A similar system can be setup to perform inmate call capture, which prevents cellphones
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smuggled into a prison from being used. These still complete calls to or from pre-authorized users such
as prison staff, while not violating laws against jamming. These systems must be carefully designed so as
to avoid capturing calls from outside the prison, which would in effect create a dead zone for any
passersby outside.
In the event of a disaster causing temporary dead zones, a cell on wheels may be brought in until thelocal telecom infrastructure can be restored. These portable units are also used where large gatherings
are expected, in order to handle the extra load.
Dropped calls
A dropped call is the common term for a wireless mobile phone call that is terminated unexpectedly as
a result of technical reasons, including presence in a dead zone. In technical circles, it is called an
abnormal release.
One reason for a dropped call is when the mobile phone moves out of range of a cellular 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. Neighbouring 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 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.
Sunspots and solar flares are rarely blamed for causing interference leading to dropped calls, as it would
take a major geomagnetic storm to cause such a disruption (except for satellite phones).
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
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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.
ASU
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.
In GSM networks, ASU is equal to the RSSI (received signal strength indicator, see TS 27.007 sub clause
8.5).
dBm = 2 × ASU - 113, ASU in the range of 0..31 and 99 (for not known or not detectable)