Wireless Application Protocol

WAP is an open international standard for applications that use wireless communication. Its principal application is to enable access to the Internet from a mobile phone or PDA.

A WAP browser is to provide all of the basic services of a computer based web browser but simplified to operate within the restrictions of a mobile phone. WAP is now the protocol used for the majority of the world's mobile internet sites, known as WAP sites. The Japanese i-mode system is currently the only other major competing wireless data protocol.

Mobile internet sites, or WAP sites, are websites written in, or dynamically converted to, WML (Wireless Markup Language) and accessed via the WAP browser.

Before the introduction of WAP, service providers had extremely limited opportunities to offer interactive data services. Interactive data applications are required to support now commonplace activities such as:

• email by mobile phone
• tracking of stock market prices
• sports results
• news headlines
• music downloads

GPS Technical description

GPS satellite on test rack

System segmentation

The current GPS consists of three major segments. These are the space segment (SS), a control segment (CS), and a user segment (US).^[3]

Space segment

The space segment (SS) is composed of the orbiting GPS satellites, or Space Vehicles (SV) in GPS parlance. The GPS design calls for 24 SVs to be distributed equally among six circular orbital planes.^[4] The orbital planes are centered on the Earth, not rotating with respect to the distant stars.^[5] The six planes have approximately 55° inclination (tilt relative to Earth's equator) and are separated by 60° right ascension of the ascending node (angle along the equator from a reference point to the orbit's intersection).^[1]

Orbiting at an altitude of approximately 20,200 kilometers (12,600 miles or 10,900 nautical miles; orbital radius of 26,600 km (16,500 mi or 14,400 NM)), each SV makes two complete orbits each sidereal day, so it passes over the same location on Earth once each day. The orbits are arranged so that at least six satellites are always within line of sight from almost anywhere on Earth.^[6]

As of April 2007, there are 30 actively broadcasting satellites in the GPS constellation. The additional satellites improve the precision of GPS receiver calculations by providing redundant measurements. With the increased number of satellites, the constellation was changed to a nonuniform arrangement. Such an arrangement was shown to improve reliability and availability of the system, relative to a uniform system, when multiple satellites fail.^[7]

Control segment

The flight paths of the satellites are tracked by US Air Force monitoring stations in Hawaii, Kwajalein, Ascension Island, Diego Garcia, and Colorado Springs, Colorado, along with monitor stations operated by the National Geospatial-Intelligence Agency (NGA).^[8] The tracking information is sent to the Air Force Space Command's master control station at Schriever Air Force Base, Colorado Springs, Colorado, which is operated by the 2d Space Operations Squadron (2 SOPS) of the United States Air Force (USAF). 2 SOPS contacts each GPS satellite regularly with a navigational update (using the ground antennas at Ascension Island, Diego Garcia, Kwajalein, and Colorado Springs). These updates synchronize the atomic clocks on board the satellites to within one microsecond and adjust the ephemeris of each satellite's internal orbital model. The updates are created by a Kalman Filter which uses inputs from the ground monitoring stations, space weather information, and other various inputs.^[9]

GPS receivers come in a variety of formats, from devices integrated into cars, phones, and watches, to dedicated devices such as those shown here from manufacturers Trimble, Garmin and Leica (left to right).

User segment

The user's GPS receiver is the user segment (US) of the GPS system. In general, GPS receivers are composed of an antenna, tuned to the frequencies transmitted by the satellites, receiver-processors, and a highly-stable clock (often a crystal oscillator). They may also include a display for providing location and speed information to the user. A receiver is often described by its number of channels: this signifies how many satellites it can monitor simultaneously. Originally limited to four or five, this has progressively increased over the years so that, as of 2006, receivers typically have between twelve and twenty channels.

A typical OEM GPS receiver module, based on the SiRF Star III chipset, measuring 12 x 15mm, and used in many products.

GPS receivers may include an input for differential corrections, using the RTCM SC-104 format. This is typically in the form of a RS-232 port at 4,800 bps speed. Data is actually sent at a much lower rate, which limits the accuracy of the signal sent using RTCM. Receivers with internal DGPS receivers can outperform those using external RTCM data. As of 2006, even low-cost units commonly include WAAS receivers.

Many GPS receivers can relay position data to a PC or other device using the NMEA 0183 protocol. NMEA 2000^[10] is a newer and less widely adopted protocol. Both are proprietary and controlled by the US-based National Marine Electronics Association. References to the NMEA protocols have been compiled from public records, allowing open source tools like gpsd to read the protocol without violating intellectual property laws. Other proprietary protocols exist as well, such as the SiRF protocol. Receivers can interface with other devices using methods including a serial connection, USB or Bluetooth.

Global Positioning System

Artist's conception of GPS satellite in orbit

Civilian GPS receiver in a marine application.

The Global Positioning System (GPS) is currently the only fully functional Global Navigation Satellite System (GNSS). More than two dozen GPS satellites are in medium Earth orbit, transmitting signals allowing GPS receivers to determine the receiver's location, speed and direction.

Since the first experimental satellite was launched in 1978, GPS has become an indispensable aid to navigation around the world, and an important tool for map-making and land surveying. GPS also provides a precise time reference used in many applications including scientific study of earthquakes, and synchronization of telecommunications networks.

Developed by the United States Department of Defense, it is officially named NAVSTAR GPS (NAVigation Satellite Timing And Ranging Global Positioning System). The satellite constellation is managed by the United States Air Force 50th Space Wing. The cost of maintaining the system is approximately US$750 million per year,^[1] including the replacement of aging satellites, and research and development. Despite this fact, GPS is free for civilian use as a public good.

Push-to-Talk

Push-to-Talk (PTT), also known as "Press-to-Transmit", is a method of conversing on half-duplex communication lines, including two-way radio, using a momentary button to switch from voice reception mode to transmit mode.

Conventional two-way radios

For commercial, family and amateur two-way radios, PTT is a button that is pressed when needing to transmit with the radio on the tuned frequency or channel. While the PTT button remains unpressed (or "unkeyed"), any radio traffic that is received on the selected channel or frequency is heard through the radio's speaker. Unless the radio supports full-duplex operation, received audio is usually muted while the PTT button is pressed. Simultaneous full-duplex transmission and reception on a radio is generally not supported unless either the transmit and receive frequencies have significant separation between the two frequencies, or two different antennas are used with enough distance between them, or a cavity filter is used, due to an effect known as desensing which cancels out received transmissions.

More recently, the PTT concept has been adopted by cellphone carriers as a way to instantaneously send transmissions to other users on the system, emulating walkie-talkie communications on a mobile phone network.

Conventional two-way radios

For commercial, family and amateur two-way radios, PTT is a button that is pressed when needing to transmit with the radio on the tuned frequency or channel. While the PTT button remains unpressed (or "unkeyed"), any radio traffic that is received on the selected channel or frequency is heard through the radio's speaker. Unless the radio supports full-duplex operation, received audio is usually muted while the PTT button is pressed. Simultaneous full-duplex transmission and reception on a radio is generally not supported unless either the transmit and receive frequencies have significant separation between the two frequencies, or two different antennas are used with enough distance between them, or a cavity filter is used, due to an effect known as desensing which cancels out received transmissions.

More recently, the PTT concept has been adopted by cellphone carriers as a way to instantaneously send transmissions to other users on the system, emulating walkie-talkie communications on a mobile phone network.

Current use in mobile telephony

Traditional mobile phone networks and devices utilize full-duplex communications, allowing customers to call other persons on a mobile or land-line network and be able to simultaneously talk and hear the other party. Such communications require a connection to be started by dialing a phone number and the other party answering the call, and the connection remains active until either party ends the call or the connection is dropped due to signal loss or a network outage. Such a system does not allow for casual transmissions to be sent to other parties on the network without first dialing them up, like is allowed on two-way radios. Full-duplex operation on mobile phone networks is made possible by using separate frequencies for transmission and reception.

Mobile Push-to-Talk service, offered by some mobile carriers, adds functionality for individual half-duplex transmissions to be sent to another party on the system without needing an existing connection to be already established. Since the system is half-duplex (utilizing a single frequency), only one user can transmit by PTT at a time; the other party is unable to transmit until the transmitting user unkeys their PTT button. Currently, PTT service is supported only between parties on the same mobile carrier service, and users with different carriers will be unable to transmit to each other by PTT. However, the advancement of this service will likely bring interconnectivity of PTT traffic between different networks in the near future.

In addition to mobile handsets, the Push-to-Talk service might be complemented with fixed PC applications acting as PTT clients connected to the mobile operator via secured Internet links. Some PC clients are designed for heavy load dispatching. This is, coordinating many issues typically caused when managing large fleets from a dispatch center.

When used with GSM and CDMA networks, the PTT service commonly does not use up the regular airtime minutes that are available for general voice calls.

Nextel Communications (now merged into Sprint) introduced mobile Push To Talk several years ago using iDEN. The "MOTO Talk" feature by Nextel (affectionately called 'Beep-beep' or 'chirp' by teenagers) includes both on and off iDEN network walkie-talkie service for newer Motorola phone models. The off iDEN-network headset-to-headset 'Direct-Talk' feature works for a radius of up to 6 miles.

Sprint plans on implementing Qualcomm's QChat on their EV-DO Revision A network. QChat has connection times of less than a second, which brings it in line with Nextel's MOTO Talk connection speeds. This will in time replace their ReadyLink Push-to-Talk technology.

The Mobile Tornado, Motorola, Nokia, Ericsson, Siemens, Sonim, Wireless ZT, etc. versions of PTT are based on 2.5G or 3G packet-switched networks and use SIP and RTP protocols. These particular versions of PTT are called "Push to Talk over Cellular", which is abbreviated "PoC".

The Open Mobile Alliance is defining PoC as part of the IP Multimedia Subsystem, and a first version of OMA PoC standard was finalized in first half of 2005. Full fledged commercial deployments of OMA PoC are few and in between. It is very unclear whether OMA PoC will be seriously launched in the European market.

A pre-standard version of PoC is also defined by the industry consortium made up of Motorola, Nokia, Ericsson, Siemens AG, AT&T Wireless, and Cingular Wireless (ATTWS and Cingular merged in September 2004) with the aim of creating a commercial offering enabling interoperability between vendors.

Several operators are using Pre-Standard Push To Talk Server in GSM / GPRS / EDGE / CDMA / UMTS networks.

Terminal vendors has several variations of software installed on mobile terminals, so there is no 100% compatibility list available.

In Germany Talk-IP Ltd. offered Mobile Tornado's PTT solution as managed service for enterprises.

In Japan, NTT DoCoMo implemented Push-to-Talk in late 2005 with the introduction of new FOMA 902i series handsets. It's billed at 5 yen per push, and has an "unlimited" option for 1000 yen/month.

Currently Cingular Wireless and Alltel offer PTT service using the Kodiak RTX (Real Time Exchange) system to deliver PTT speeds comparable to Nextel and SouthernLinc.

In Canada the service is provided by several carriers including TELUS Mobility (Mike), Bell Mobility and Aliant Mobility (Bell 10-4). While using the service, customers do not use registered airtime minutes associated with their voice plan. The service is often offered at a discount to those customers who subscribe to a monthly airtime package. Both Bell and Aliant offer the service which allows customers to use the service in the United States without the occurrence of international roaming charges typically associated with cellular use out of country. Customers using the service in Canada have the ability to contact users across the country without the occurrence of long distance charges. The service is being promoted as a cost effective method for communication which typically runs a high cost. Offered as a solution to businesses and customers who use a great amount of long distance, PTT (Push-to-Talk) service will greatly change the way many consumers do business. The service allows a caller to simultaneously communicate with multiple users at different locations. By doing so, this eliminates multiple airtime charges associated with three-way calling. With the cellular number portability coming into effect by March 2007, the PTT (Push-to-Talk) service is expected to increase greatly with the removal of Roamer Access Numbers by August 2006. Roamer Access Numbers had given the freedom for customers to not incur incoming long distance charges by answering calls outside their local calling area if the caller first dials the Roamer Access Number.

In Slovakia, Ardaco developed SecurePTT solution for information and communication security area.

Cell Phone Signals

Signal strength

In telecommunications, and particularly in radio, signal strength transmitted signal is being received, measured, or predicted, at a reference point that is a significant distance from the transmitting antenna. It may also be referred to as received signal level or field strength. Typically, this is measured as signal electric field strength of voltage per length or signal power received by a reference antenna. Higher powered transmissions such as broadcasting use units of dB-millivolts per metre (dBmV/m). Very low-power uses such as mobile phones are most often expressed in dB-microvolts per metre (dBµV/m) or in decibels above a reference level of one milliwatt (eg -80 dBm).

In broadcasting terminology 1 mV/m is 0 dBm (a shortened dB(mV/m)), or 60 dBµ (often written dBu) and has no reference to the dB milliwatt, the more common use of dBm.

Some examples

• 100 dBµ or 100 mV/m: blanketing interference occurs
• 60 dBµ or 1 mV/m: the edge of a radio station's protected area
• 40 dBµ or 100 µV/m: the minimum strength at which a station can be received

Contents

Formula

The electic field strength can be calculated from the effective radiated power, ERP, of the antenna and it's distance, d ^[1](here, based on a resistance of 50Ω):

Where E is in volts per metre, and d is in metres.

Cell Phone Signals

Although there are cell phone base station tower networks across many nations globally, there are still many areas within those nations that do not have good reception. Some rural areas are unlikely ever to be effectively covered since the cost of erecting a cell tower is too high for only a few customers. Even in high reception areas it is often found that basements and the interiors of large buildings have poor reception.

Weak signal strength can also be caused by destructive interference of the signals from local towers in urban areas, or by the construction materials used in some buildings causing rapid attenuation of signal strength. Large buildings such as warehouses, hospitals and factories often have no useable signal further than a few metres from the outside walls.

This is particularly true for the networks which operate at higher frequency since these are attenuated more rapidly by intervening obstacles, although they are able to use reflection and diffraction to circumvent obstacles.

Cell phones in the U.S. operate at around 800MHz and PCS phones at 1900MHz: classified as UHF and low energy microwaves respectively. This has lead to the rapid growth in the home cellular repeater market. The more advanced models now typically include an external directional antenna and an amplifier (usually operating at 55db gain) - which is generally enough to turn a very weak signal into a clear one over the local area (from around a thousand square feet to over twenty thousand).

Mobile communication studies

Since 2002, many books have been written on the social impact of mobile phones:

• Agar, Jon, Constant Touch: A Global History of the Mobile Phone, 2004
• Glotz, Peter & Bertsch, Stefan, eds. Thumb Culture: The Meaning of Mobile Phones for Society, 2005
• Katz, James E. & Aakhus, Mark, eds. Perpetual Contact: Mobile Communication, Private Talk, Public Performance, 2002
• Kavoori, Anandam & Arceneaux, Noah, eds. The Cell Phone Reader: Essays in Social Transformation, 2006
• Ling, Rich, The mobile connection 2004 [4]
• Ling, Rich and Pedersen, Per, eds. Mobile Communications: Renegotiation of the Social Sphere 2005
• Nyíri, Kristóf, ed. Mobile Communication: Essays on Cognition and Community, 2003
• Nyíri, Kristóf, ed. Mobile Learning: Essays on Philosophy, Psychology and Education, 2003
• Nyíri, Kristóf, ed. Mobile Democracy: Essays on Society, Self and Politics, 2003
• Nyíri, Kristóf, ed. A Sense of Place: The Global and the Local in Mobile Communication, 2005
• Nyíri, Kristóf, ed. Mobile Understanding: The Epistemology of Ubiquitous Communication, 2006
• Levinson, Paul, Cellphone: The Story of the World's Most Mobile Medium, and How It Has Transformed Everything! 2004
• Rheingold, Howard, Smart Mobs: The Next Social Revolution, 2002

Cellular frequencies

Mobile phone tower

Cell Phone tower located in Lynnwood, WA

Mobile phones and the network they operate under vary significantly from provider to provider, and nation to nation. However, all of them communicate through electromagnetic radiowaves with a cell site base station, the antennas of which are usually mounted on a tower, pole, or building.

The phones have a low-power transceiver that transmits voice and data to the nearest cell sites, usually not more than 5 to 8 miles (approximately 8 to 13 kilometres) away. When the mobile phone or data device is turned on, it registers with the mobile telephone exchange, or switch, with its unique identifiers, and will then be alerted by the mobile switch when there is an incoming telephone call. The handset constantly listens for the strongest signal being received from the surrounding base stations. As the user moves around the network, the mobile device will "handoff" to various cell sites during calls, or while waiting (idle) between calls it will reselect cell sites.

Cell sites have relatively low-power (often only one or two watts) radio transmitters which broadcast their presence and relay communications between the mobile handsets and the switch. The switch in turn connects the call to another subscriber of the same wireless service provider or to the public telephone network, which includes the networks of other wireless carriers. Many of these sites are camouflaged to blend with existing environments, particularly in high-scenery areas.

The dialogue between the handset and the cell site is a stream of digital data that includes digitized audio (except for the first generation analog networks). The technology that achieves this depends on the system which the mobile phone operator has adopted. Some technologies include AMPS for analog, and D-AMPS, CDMA2000, GSM, GPRS, EV-DO, and UMTS for digital communications. Each network operator has a unique radio frequency band.