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Roaming

Roaming

Roaming is a general term in wireless telecommunications that refers to the extending of connectivity service in a network that is different than the network with which a station is registered. The canonical example of "roaming" is for cellular phones, when you take your phone to an area where your service provider does not have coverage (eg, another country). In order for a mobile device to roam to another network, a number of processes need to be performed. The very first necessity for inter-network roaming is that your service provider must have a roaming agreement with the network to which you have moved. The details of the roaming process differ among types of wireless networks, but in general, the process resembles the following: # When the mobile is turned on or is transferred via a handover to the network, this new "visited" network sees the mobile, notices that it is not a mobile registered with its own system, and attempts to identify the home network of the mobile. If there is no roaming agreement between the two networks, maintenance of call is completely impossible, as service is then denied by the visited network. # The visited network contacts the home network and requests information about the roaming mobile. This includes the mobile's IMSI number, subscribed services, and whether or not the mobile should be allowed to roam. # If successful, the visited network begins to maintain a temporary subscriber record for the mobile. Likewise, the home network updates its information to indicate that the mobile is on the host network so that any calls made to that mobile can be correctly routed. When a call is made to a roaming mobile, the public telephone network will route the call to your service provider's network, since that is where your phone number terminates. Your home network is then responsible for re-routing the call to the host network. This will require the host network to provide a temporary phone number on its network that it will route to the mobile. Once this number is defined, the home network re-routes the incoming call to the temporary phone number, which terminates at the host network. The host network then routes that call to the mobile. In 802.11 roaming can also mean subscriber mobility or handover within the same network.

External links

- [http://www.gsmworld.com/roaming/index.shtml GSM Roaming] - International GSM Association Category:GSM Standard Category:Telephony

Wireless

Wireless is an old-fashioned term for a radio receiver, referring to its use as a wireless telegraph. The term is widely used to describe modern wireless connections such as wireless broadband internet.

History

The founding principles and inventions of wireless technology can be found in the lectures and patent record of the electrical engineer Nikola Tesla and in his 1916 deposition on the history of wireless and radio cool technology. A wireless set was the radio receiver, referring to its use as a wireless telecommunication station. The term "wireless" was widely used in the UK, long after radio was being used for other signals, such as music. For more on the history of radio transmission, please see Radio: Invention and history.

Modern usage

In modern usage, Wireless is a method of communication that uses low-powered radio waves to transmit data between devices. The term refers to communication without cables or cords, chiefly using radio frequency and infrared waves. Common uses include the various communications defined by the IrDA and the wireless networking of computers. Low-powered radio waves, such as those used in networking to transmit data between devices, are often unregulated. High powered transmission sources usually require government licenses to broadcast on a specific wavelength. This platform has historically carried voice and has grown into a large industry, carrying many thousands of broadcasts around the world. Radio waves are now increasingly being used by unregulated computer users. Optimal bandwidth routes within wireless networks requires the calculation in real-time of the best way to direct traffic. Software and hardware developers are creating smaller computer networks which form ad-hoc wireless network, with protocols such as WiFi. The IEEE 802.11 standard is for wireless, Ethernet-like LANs. The insecurities in this protocol have popularized the concept of war driving.

External links

- [http://www.tutorial-reports.com/wireless/ Wireless Networking] Details of different wireless technologies
- [http://www.must.edu.my/~dwong/book.html Wireless Internet Telecommunications] A book that explains how the wireless Internet works in a clear, simple way
- [http://www.moonblinkwifi.com/dbm_to_watt_conversion.cfm dBm to Watt Conversion Calculator] - Very useful in designing Wireless Links.
- [http://bengross.com/wireless.html Wireless Networking]
- [http://www.wireless-warrior.org Wireless Warrior]
- [http://www.itprc.com/wireless.htm ITPRC Wireless Resource Center]
- [http://mark.cabiling.free.fr/mobilemesh Mobile Mesh Networking]
- [http://www.wacklepedia.com/pdahotspots/pda_hotspots.htm PDA Hotspots] - List of websites that adjust to the small wireless PDA screen
- [http://www.airhive.net/modules.php?name=Web_Links&l_op=viewlink&cid=37 Wireless search engine categories]
- [http://www.airhive.net/modules.php?name=Encyclopedia&op=list_content&eid=3 Wireless term dictionary]
- [http://www.wlancentral.com/ WLAN Central] Wireless and WiFi news resource.

Cellular phone

A mobile phone, also known as a cell phone, smart phone, mobile, or hand phone, is an electronic telecommunications device with the same basic capability as a conventional fixed line telephone, but which is also entirely portable and is not required to be connected with a wire to the telephone network. Most current mobile phones connect instead to the network using a wireless radio wave transmission technology. The mobile phone communicates via a network of base stations which are in turn linked to the conventional telephone network. In addition to the standard voice function of a telephone, a mobile phone can support many additional services such as SMS for text messaging, packet switching for access to the Internet and MMS for sending and receiving photos and video. Some of the world's largest mobile phone manufacturers include Alcatel, Audiovox, Fujitsu, Kyocera (formerly the handset division of Qualcomm), LG, Motorola, NEC, Nokia, Panasonic (Matsushita Electric), Philips, Sagem, Samsung, Sanyo, Sharp, Siemens, SK Teletech, Sony Ericsson, and Toshiba. There are also specialist communication systems related to, but distinct from mobile phones, such as satellite phones and Professional Mobile Radio. Mobile phones are also distinct from cordless telephones, which generally operate only within a limited range of a specific base station.

Worldwide deployment

Radio phones have a long and varied history that stretches back to the 1950s, with hand-held cellular radio devices being available since 1983. Due to their low establishment costs and rapid deployment, mobile phone networks have since spread rapidly throughout the world, outstripping the growth of fixed telephony. In most of Europe, wealthier parts of Asia and Latin America, Australia, Canada and the US, mobile phones are now widely used, with the majority of the adult, teenage, and even child population owning one. The number of cellphone subscribers in the US has reached over 190 million. At present India and China have the largest growth rates of cellular subscribers in the world. The availability of Prepaid or pay as you go services, where the subscriber does not have to commit to a long term contract, has helped fuel this growth. The mobile phone has become ubiquitous because of the interoperability of mobile phones across different networks and countries. This is due to the equipment manufacturers all working to the same standard, particularly the GSM standard which was designed for Europe-wide interoperability. All European nations and some Asian nations chose it as their sole standard, while in Japan and South Korea another standard, CDMA, was selected.

Mobile phone culture

CDMA handset]] In less than twenty years, mobile phones have gone from being rare and expensive pieces of equipment used by businesses to a pervasive low-cost personal item. In many countries, mobile phones now outnumber land-line telephones, with most adults and many children now owning mobile phones. It is not uncommon for young adults to simply own a mobile phone instead of a land-line for their residence. In some developing countries, where there is little existing fixed-line infrastructure, the mobile phone has become widespread. With high levels of mobile telephone penetration, a mobile culture has evolved, where the phone becomes a key social tool, and people rely on their mobile phone addressbook to keep in touch with their friends. Many people keep in touch using SMS, and a whole culture of "texting" has developed from this. The commercial market in SMS's is growing. Many phones even offer Instant Messenger services to increase the simplicity and ease of texting on phones. Cellular phones in Japan, offering internet capabilities such as NTT DoCoMo's i-mode, offer text messaging via standard email. The mobile phone itself has also become a totemic and fashion object, with users decorating, customizing, and accessorizing their mobile phones to reflect their personality. This has emerged as its own industry. The sale of commercial ringtones exceeded $2.5 billion dollars in 2004 [http://digital-lifestyles.info/display_page.asp?section=distribution&id=1474]. Mobile phone etiquette has become an important issue with mobiles ringing at funerals, weddings, movies and plays. Users often speak at increased volume, with little regard for other people nearby. It has become common practice for places like bookshops, libraries, movie theatres, and houses of worship to post signs prohibiting the use of cell phones, sometimes even installing jamming equipment to prevent them. The US intercity passenger system Amtrak offers a "quiet car" where cellphone use is prohibited, much like the designated non-smoking cars in days of yore. Mobile phone use on aircraft is also prohibited. Cameraphones and videophones that can capture video and take photographs are increasingly being used to cover breaking news. Stories like the London Bombings, the Indian Ocean Tsunami and Hurricane Katrina have been reported on by cameraphone users on news sites like NowPublic and photosharing sites like Flickr. In Japan, cellular phone companies provide immediate notification of earthquakes and other natural disasters to their customers free of charge. In the event of an emergency, disaster response crews can locate trapped or injured people using the signals from their cellular phones; an interactive menu accessible through the phone's internet browser notifies the company if the user is safe or in distress.

Mobile phone features

See main article: Mobile phone features Mobile phones are often packed with features that offer users far more than just the capability to send text messages and make voice calls. These may include internet browsing, music (MP3) playback, personal organisers, email, watch/alarm, built-in cameras, ringtones, security measures (e.g. pin codes), SIM blocks, games, radio, Push to talk, infrared and bluetooth connectivity, and call registers.

Technology

Mobile phones and the network they operate under vary significantly from provider to provider, and even from nation to nation. However, all of them communicate over the air (through electromagnetic radio waves) with a base station (which is a special radio mast or tower designed for the purpose). The phones have a low power transceiver that is typically designed to transmit voice and data up to a few kilometers to where the tower is located. The handset constantly listens for the nearest tower with the strongest signal. Once found, the handset informs that tower of its own unique identifier, and alerts the mobile phone network that it is ready and standing-by to receive telephone calls. This cycle is then repeated as the phone roams around the network and new towers appear in the handset's range. Towers have high power radio transmitters which broadcast their presence, and relay communications to and from the mobile handsets. The tower is connected to the telephone network by a high-capacity link. The base station connects to the operator's backbone network and the wider public telephone network as well as the networks of other mobile phone operators. The dialogue between the handset and the tower is a stream of digitized audio. The technology that achieves this depends on the system which the mobile phone operator has adopted. Some technologies include AMPS for analogue, and TDMA, CDMA, GSM, GPRS and UMTS for digital communications. Each network operator also has a unique radio frequency chosen from a small number of standard frequencies allocated to each technology.

Controversy

Health controversy

See main article: Mobile phone radiation and health As with many new technologies, concerns have arisen about the effects on health from using a mobile telephone. There is a small amount of scientific evidence for an increase in certain types of rare tumors (cancer) in long-time, heavy users. More recently a pan-European study provided significant evidence of genetic damage under certain conditions. Some researchers also report the mobile phone industry has interfered with further research on health risks. So far, however, the World Health Organization Task Force on EMF effects on health has no definitive conclusion on the veracity of these allegations. (see also Electromagnetic radiation hazard). It is generally thought, however, that RF is incapable of producing any more than heating effects, as it is considered non-ionizing radiation; in other words, it lacks the energy to disrupt molecular bonds such as occurs in genetic mutations.

Driving controversy

Another controversial but perhaps more lethal health concern is the correlation with automobile accidents. Several studies have shown that motorists have a much higher risk of collisions and losing control of the vehicle while talking on the mobile telephone simultaneously with driving, even when using "hands-free" systems. One such study conducted by the television show Mythbusters concluded that use of mobile phones while driving poses the same risk as someone operating a vehicle while under the influence of alcohol. Accidents involving a driver being distracted by talking on a mobile phone have begun to be prosecuted as negligence similar to driving while intoxicated. In some jurisdictions, such as Ireland, Japan, Singapore, Brazil, Australia, Austria, the United Kingdom and France, as well as several states in the United States driving while using a mobile phone is illegal, though an exception is often made if the phone is equipped with a handsfree system. In Canada it is banned in Newfoundland (Dec 2002) with fines up to US$180.

Security concerns

Early mobile phones did not have much security designed in. Some problems with these models were "cloning", a variant of identity theft, and "scanning" whereby third parties in the local area could intercept and eaves drop in on calls. Analogue phones could also be listened to on some radio scanners. Although more recent digital systems (such as GSM) have attempted to address these fundamental issues, security problems continue to persist. Vulnerabilities (such as SMS spoofing) have been found in many current protocols that continue to allow the possibility of eavesdropping or cloning.

Future prospects

SMS spoofing] There is a great deal of active research and development into mobile phone technology that is currently underway. Some of the improvements that are being worked on are:
- One difficulty in adapting mobile phones to new uses is form factor. For example, ebooks may well become a distinct device, because of conflicting form-factor requirements — ebooks require large screens, while phones need to be smaller. However, this may be solved using folding e-paper or built-in projectors.
- One function that would be useful in phones is a translation function. Currently it is only available in stand-alone devices, such as Ectaco translators.
- An important area of evolution relates to the Man Machine Interface. New solutions are being developed to create new MMI more easily and let manufacturers and operators experiment new concepts. Examples of companies that are currently developing this technology are [http://www.digitalairways.com/ Digital Airways] with the Kaleido product, [http://www.e-sim.com/ e-sim], [http://www.mobile-arsenal.com.ua/ mobile arsenal], and [http://www.trigenix.com/ Qualcomm] with UIOne for the BREW environment.
- Mobile phones will include various speech technologies as they are being developed. Many phones already have rudimentary speech recognition in a form of voice dialling. However, to support more natural speech recognition and translation, a drastic improvement in the state of technology in these devices is required.
- New technologies are being explored that will utilize the Extended Internet and enable mobile phones to treat a barcode as a URL tag. Phones equipped with barcode reader-enabled cameras will be able to snap photos of barcodes and direct the user to corresponding sites on the Internet. This technology can be extended to RFID tags, or even snapped pictures of company logos. Searches can also be personalized to local areas using a GPS system built in to cell phones. Examples of companies that are currently developing this technology are [http://www.neom.com/ Neomedia] (via [http://www.paperclick.com/ Paperclick]), [http://www.mobot.com/ Mobot] and [http://www.scanbuy.com/ Scanbuy].
- Developments in miniaturised hard disks and flash drives to solve the storage space issue, therefore opening a window for phones to become portable music libraries and players similar to the iPod.
- Developments in podcast software enables mobile phones to become podcast playback devices through existing channels like [http://www.geocities.com/tvhuangsg/mmspodcast/ MMS Podcast], [http://www.geocities.com/tvhuangsg/javacast/ J2ME Podcast] and [http://www.geocities.com/tvhuangsg/mobcast/ AMR-NB Podcast].
- The emergence of integration capabilities with other unlicensed access technologies such as a WiMAX and WLAN, as well as allowing handover between traditional operator networks supporting GSM, CDMA and UMTS to unlicensed mobile networks. The new standard (UMA) has been developed for this.
- Further improvements in battery life will be required. Colour screens and additional functions put increasing demands on the device's power source, and battery developments may not proceed sufficiently fast to compensate. However, different display technologies, such as OLED displays, e-paper or retinal displays, smarter communication hardware (directional antennae, multi-mode and peer-to-peer phones) may reduce power requirements, while new power technologies such as fuel cells may provide better energy capacity.
- Speculative improvements in the future may be inspired by an English team led by James Auger and Jimmy Loizeau who in 2002 developed an implant designed to be inserted into a tooth during dental surgery. This device consists of a radio receiver and transducer, which transmits the sound via bone conduction through the jawbone into the ear. Sound is transmitted via radio waves from another device (presumably a mobile phone) and received by the implant. The implant is currently powered externally, given that no current power source is small enough to fit inside the tooth with it. In addition, the implant was only designed to receive signals, not transmit them. Directly tapping into the inner ear or the auditory nerve is already technologically feasible and will become practical as surgical methods advance.
- New technology in Japan has combined the RFID chip principle into the handset and hooked it up to a network of readers and interfaces. The system, pioneered by NTT Docomo and SonyEricsson, is called Felica and there are around 10,000 convenience stores where one can now use a phone to pay for goods just by 'swiping' it over a flat reader. By charging up a phone with pre-paid cash credits, it can act as a sophisticated mobile-phone wallet. The technology is proving popular and there are now even vending machines that accept this form of payment.
- The delivery of multimedia content including video to mobiles is beginning to become a reality with two main competing standards DMB - Digital Multimedia Broadcasting - and DVB-H - a handset version of the Digital Video Broadcasting standard. These methods avoid swamping the network by using traditional broadcasting.

Terminology

Mobile phone terms

; Brick : A large-sized early handheld mobile phone, such as the Motorola International 3200, nearly the size of a VHS video cassette, with the keypad and microphone on the narrow side. ; Candybar : A housing shape that has no hinges and resembles an oblong candy bar (US). ; Cell phone or cellular telephone : Term used currently in the United States (and in other countries as well during the 1980s) to refer to most mobile phones. It technically applies specifically to mobile phones which use a cellular network. In developing mobile phone technology, American electrical engineers saw the main technical problem as achieving a smooth handoff from one radio antenna to the next. After they gave the name "cell" to the zone covered by each antenna, it was a natural choice for them to apply the term "cellular" to both the technology and the phones that ran on it. ; Clamshell : A phone that opens up to reveal the keypad, microphone, and earpiece; these are typically more compact than other designs. Often called "flip phones". Clamshell phones became very popular in the United States after the introduction of Motorola's StarTAC in 1996. ; Handset: The term handset is used by cell-phone manufacturers to refer to a mobile phone. Also commonly used by industry insiders. ; Handy : Pronounced "Hendi", is a pseudo-anglicism, derived from the term Handy Talkie for a handheld military radio (also known as walkie-talkie), that is used in Austria and Germany for a mobile phone (rare alternative spelling: Händi). In German, the word "Handy" is derived from "handgehaltenes (or hand-held) Telefon." Similarly another pseudo-English term Hand phone is used in East and South Asian countries like South Korea, Malaysia and Singapore. ; Hands free car kit: cell phone accessory use to talk while keeping hands on the steering wheel. ; Mobile phone : A term covering cellular phones, satellite phones and any phones giving wide ranging mobility, used in most English-speaking countries. ; Mobile : Short for 'mobile phone', a term in everyday usage in some English-speaking countries such as the UK. ; Satellite phone : A mobile phone which communicates with a satellite rather than a land-based network. ; Wireless phone : A term that generally refers to a Wi-Fi VoIP phone but is sometimes used by the mobile phone industry to describe mobile phones. ; Ringtone : A song or tune that is played when a cell phone is receiving a call. ; 3G phone : A mobile phone which uses a 3G network, with greater bandwidth allowing faster data downloads and face to face video calling.

Related systems which are not cellphones

; Cordless Phone (Portable Phone) : Cordless phones are standard telephones with radio handsets. Unlike mobile phones, cordless phones use private base stations that are not shared between subscribers. The base station is connected to a land-line. ; Radio Phone : This is an term which covers radios which could connect into the telephone network. These phones may not be mobile, for example, they may require a mains power supply. ; Professional Mobile Radio : Professional mobile radio systems are very similar to cellphone systems and attempts have even been made to use TETRA, the international digital PMR standard, to implement public mobile networks, but normally PMR systems are sufficiently separate from the phone network to not really be considered phones but rather radios.

Terms in other countries

For a list of what mobile phones are called in other countries around the world, see the following article: Mobile phone terms across the world

External links

- [http://electronics.howstuffworks.com/cell-phone.htm How Cell Phones Work] (How Stuff Works)
- [http://www.howardforums.com/ Howard Forums] Forums for cell phone professionals
- [http://www.cellphonehacks.com/ CellPhoneHacks] Mobile phone forum for beginners to experts
- [http://cellwatch.blogspot.com/ Cell Watch] Cell phone trends
- [http://www.isracast.com/tech_news/250705_tech.htm Cell Phone Radiation May cause visual damage] - a possible link between microwave radiation, and different kinds of damage to the visual system was found by a team of researchers from the Technion. Category:Wireless communications Category:Embedded systems Category:Telephony Category:Consumer electronics Category:Mobile telephony standards ko:휴대 전화 ja:携帯電話 zh-min-nan:Hêng-tōng tiān-oē

Handoff

In telecommunication, the term handoff refers to the process of transferring an ongoing call or data session from one channel connected to the core network to another. In satellite communications it is the process of transferring satellite control responsibility from one earth station to another without loss or interruption of service. The actual process of transferring the call is referred to as a handover. In telecommunication there are two reasons why a handover might be conducted. If the user has moved out of range from one base station and can get a better connection from a stronger transmitter or if one base station is full the connection can be transferred to another nearby base station. The most basic form of handover is that used in most 1G and 2G systems where a phone call in progress is redirected from one cell transmitter and receiver and frequency pair to another cell transmitter and receiver using a different frequency pair without interrupting the call. If the terminal can only be connected to one base station and therefore needs to drop the connection for a brief period of time before being connected to the other, stronger transmitter, this is referred to as a hard handover. In CDMA systems the user can be connected to several base stations simultaneously, combining the data from all transmitters in range into one signal using a RAKE receiver. The set of base stations the terminal is currently connected to is referred to as the active set. A soft handover happens when there are several base stations in the active set and the terminal drops one of these to add a new one. In W-CDMA there is a special case called softer handover where several connections in the active set point to the same base station. The softer handover happens when one of these connections is dropped for another from the same base station. There are also inter system handovers where a connection is transferred from one access technology to another, e.g. a call being transferred from GSM to W-CDMA.

External links

- [http://www.eventhelix.com/RealtimeMantra/Telecom/GSM_Handover_Call_Flow.pdf Intra MSC GSM Handover Call Flow]
- [http://www.eventhelix.com/RealtimeMantra/Telecom/GSM_Inter_MSC_Handover_Call_Flow.pdf Inter MSC GSM Handover Call Flow] Category:GSM Standard

Handoff

External links

802.11

IEEE 802.11 or Wi-Fi denotes a set of Wireless LAN/WLAN standards developed by working group 11 of the IEEE LAN/MAN Standards Committee (IEEE 802). The term is also used to refer to the original 802.11, which is now sometimes called "802.11legacy." IEEE 802 IEEE 802 The 802.11 family currently includes six over-the-air modulation techniques that all use the same protocol, the most popular (and prolific) techniques are those defined by the a, b, and g amendments to the original standard; security was originally included, and was later enhanced via the 802.11i amendment. Other standards in the family (c–f, h–j, n) are service enhancement and extensions, or corrections to previous specifications. 802.11b was the first widely accepted wireless networking standard, followed (somewhat counterintuitively) by 802.11a and 802.11g. 802.11b and 802.11g standards use the 2.4 gigahertz (GHz) band, operating under Part 15 of the FCC Rules and Regulations. The 802.11a standard uses the 5 GHz band. Operating in the 2.4 gigahertz frequency band, 802.11b and 802.11g equipment can incur interference from microwave ovens, cordless phones, and other appliances using the same 2.4 GHz band. It is a common misconception that 802.11a and g operate in an unlicensed portion of the radio frequency spectrum. Unlicensed (legal) operation of 802.11 a & g is covered under Part 15 of the FCC Rules and Regulations. Frequencies used by channels one (1) through six (6) (802.11b) fall within the range of the 2.4 gigahertz Amateur Radio band. Licensed amateur radio operators may operate 802.11b devices under Part 97 of the FCC Rules and Regulations.

Protocols

802.11 legacy

The original version of the standard IEEE 802.11 released in 1997 specifies two raw data rates of 1 and 2 mega bits per second (Mbit/s) to be transmitted via infrared (IR) signals or in the Industrial Scientific Medical frequency band at 2.4 GHz. IR remains a part of the standard but has no actual implementations. The original standard also defines Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) as the media access method. A significant percentage of the available raw channel capacity is sacrificed (via the CSMA/CA mechanisms) in order to improve the reliability of data transmissions under diverse and adverse environmental conditions. At least five different, somewhat-interoperable, commercial products appeared using the original specification, from companies like Alvarion (PRO.11 and BreezeAccess-II), Netwave Technologies (AirSurfer Plus and AirSurfer Pro), Symbol Technologies (Spectrum24), and Proxim (OpenAir). A weakness of this original specification was that it offered so many choices that interoperability was sometimes challenging to realize. It is really more of a "meta-specification" than a rigid specification, allowing individual product vendors the flexibility to differentiate their products. Legacy 802.11 was rapidly supplemented (and popularized) by 802.11b. However, 802.11 as a standard has been essentially supplanted by the other standards (802.11a/b/g). Widespread adoption of 802.11 networks only occured after 802.11b was ratified and as a result few networks ran on the 802.11 standard.

802.11b

The 802.11b amendment to the original standard was ratified in 1999. 802.11b has a maximum raw data rate of 11 Mbit/s and uses the same CSMA/CA media access method defined in the original standard. Due to the CSMA/CA protocol overhead, in practice the maximum 802.11b throughput that an application can achieve is about 5.9 Mbit/s over TCP and 7.1 Mbit/s over UDP. 802.11b products appeared on the market very quickly, since 802.11b is a direct extension of the DSSS (Direct-sequence spread spectrum) modulation technique defined in the original standard. Technically, the 802.11b standard uses Complementary code keying (CCK) as its modulation technique, which is a variation on CDMA. Hence, chipsets and products were easily upgraded to support the 802.11b enhancements. The dramatic increase in throughput of 802.11b (compared to the original standard) along with substantial price reductions led to the rapid acceptance of 802.11b as the definitive wireless LAN technology. 802.11b is usually used in a point-to-multipoint configuration, wherein an access point communicates via an omni-directional antenna with one or more clients that are located in a coverage area around the access point. With high-gain external antennas, the protocol can also be used in fixed point-to-point arrangements, typically at ranges up to eight kilometers (km) although some report success at ranges up to 80–120 km where line of sight can be established. This is usually done in place of costly leased lines or very cumbersome microwave communications equipment. 802.11b cards can operate at 11 Mbit/s, but will scale back to 5.5, then 2, then 1 Mbit/s (a.k.a Adaptive Rate Selection), if signal quality becomes an issue. Since the lower data rates use less complex and more redundant methods of encoding the data, they are less susceptible to corruption due to interference and signal attenuation. Extensions have been made to the 802.11b protocol (e.g., channel bonding and burst transmission techniques) in order to increase speed to 22, 33, and 44 Mbit/s, but the extensions are proprietary and have not been endorsed by the IEEE. Many companies call enhanced versions "802.11b+". These extensions have been largely obviated by the development of 802.11g, which has data rates up to 54 Mbit/s and is backwards-compatible with 802.11b.

Channels and international compatibility

802.11b and 802.11g divide the spectrum into 14 overlapping, staggered channels whose center frequencies are 5 megahertz (MHz) apart. It is a common misconception that channels 1, 6 and 11 (and, if available in the regulatory domain, channel 14) do not overlap and those channels (or other sets with similar gaps) can be used such that multiple networks can operate in close proximity without interfering with each other, but this statement is somewhat over-simplified. The 802.11b and 802.11g standards do not specify the width of a channel; rather, they specify the center frequency of the channel and a spectral mask for that channel. The spectral mask for 802.11b requires that the signal be attenuated by at least 30 dB from its peak energy at ±11 MHz from the center frequency, and attenuated by at least 50 dB from its peak energy at ±22 MHz from the center frequency. Since the spectral mask only defines power output restrictions up to ±22 MHz from the center frequency, it is often assumed that the energy of the channel extends no further than these limits. In reality, if the transmitter is sufficiently powerful, the signal can be quite strong even beyond the ±22 MHz point. Therefore, it is incorrect to say that channels 1, 6, and 11 do not overlap. It is more correct to say that, given the separation between channels 1, 6, and 11, the signal on any channel should be sufficiently attenuated to minimally interfere with a transmitter on any other channel. However, this is not universally true; for example, a powerful transmitter on channel 1 can easily overwhelm a weaker transmitter on channel 6. In one lab test, throughput on a file transfer on channel 11 decreased slightly when a similar transfer began on channel 1, indicating that even channels 1 and 11 can interfere with each other to some extent. Although the statement that channels 1, 6, and 11 are "non-overlapping" is incomplete, the 1, 6, 11 guideline has merit. If transmitters are closer together than channels 1, 6, and 11 (e.g. 1, 4, 7, and 10), overlap between the channels will probably cause unacceptable degradation of signal quality and throughput. The channels that are available for use in a particular country differ according to the regulations of that country. In the United States, for example, FCC regulations only allow channels 1 through 11 to be used. Channels 10 and 11 are the only channels which work in all parts of the world, because Spain hasn't licensed channels 1 to 9 for 802.11b operation.

802.11a

The 802.11a amendment to the original standard was ratified in 1999. The 802.11a standard uses the same core protocol as the original standard, operates in 5 GHz band, and uses a 52-subcarrier orthogonal frequency-division multiplexing (OFDM) with a maximum raw data rate of 54 Mbit/s, which yields realistic net achievable throughput in the mid-20 Mbit/s. The data rate is reduced to 48, 36, 24, 18, 12, 9 then 6 Mbit/s if required. 802.11a has 12 non-overlapping channels, 8 dedicated to indoor and 4 to point to point. It is not interoperable with 802.11b, except if using equipment that implements both standards. Since the 2.4 GHz band is heavily used, using the 5 GHz band gives 802.11a the advantage of less interference. However, this high carrier frequency also brings disadvantages. It restricts the use of 802.11a to almost line of sight, necessitating the use of more access points; it also means that 802.11a cannot penetrate as far as 802.11b since it is absorbed more readily, other things (such as power) being equal. Different countries have different regulatory support, although a 2003 World Radiotelecommunications Conference made it easier for use worldwide. 802.11a is now approved by regulations in the United States and Japan, but in other areas, such as the European Union, it had to wait longer for approval. European regulators were considering the use of the European HIPERLAN standard, but in mid-2002 cleared 802.11a for use in Europe. In the US, a mid-2003 FCC decision may open more spectrum to 802.11a channels. Of the 52 OFDM subcarriers, 48 are for data and 4 are pilot subcarriers with a carrier separation of 0.3125 MHz (20 MHz/64). Each of these subcarriers can be a BPSK, QPSK, 16-QAM or 64-QAM. The total bandwidth is 20 MHz with an occupied bandwidth of 16.6 MHz. Symbol duration is 4 micro seconds with a guard interval of 0.8 microseconds. The actual generation and decoding of orthogonal components is done in baseband using DSP which is then upconverted to 5 GHz at the transmitter. Each of the subcarriers could be represented as a complex number. The time domain signal is generated by taking an Inverse Fast Fourier transform (IFFT). Correspondingly the receiver downconverts, samples at 20 MHz and does an FFT to retrieve the original coefficients. The advantages of using OFDM include reduced multipath effects in reception and increased spectral efficiency. 802.11a products started shipping in 2001, lagging 802.11b products due to the slow availability of the 5 GHz components needed to implement products. 802.11a was not widely adopted overall because 802.11b was already widely adopted, because of 802.11a's disadvantages, because of poor initial product implementations, making its range even shorter, and because of regulations. Manufacturers of 802.11a equipment responded to the lack of market success by improving the implementations (current-generation 802.11a technology has range characteristics much closer to those of 802.11b), and by making technology that can use more than one 802.11 standard. There are dual-band, or dual-mode or tri-mode cards that can automatically handle 802.11a and b, or a, b and g, as available. Similarly, there are mobile adapters and access points which can support all these standards simultaneously.

802.11g

In June 2003, a third modulation standard was ratified: 802.11g. This flavor works in the 2.4 GHz band (like 802.11b) but operates at a maximum raw data rate of 54 Mbit/s, or about 24.7 Mbit/s net throughput like 802.11a. 802.11g hardware will work with 802.11b hardware. Details of making b and g work well together occupied much of the lingering technical process. In older networks, however, the presence of an 802.11b participant significantly reduces the speed of an 802.11g network. The modulation scheme used in 802.11g is orthogonal frequency-division multiplexing (OFDM) for the data rates of 6, 9, 12, 18, 24, 36, 48, and 54 Mbps, and reverts to (like the 802.11b standard) CCK for 5.5 and 11 Mbps and DBPSK/DQPSK+DSSS for 1 and 2 Mbps. Even though 802.11g operates in the same frequency band as 802.11b, it can achieve higher data rates because of its similarities to 802.11a. The 802.11g standard swept the consumer world of early adopters starting in January 2003, well before ratification. The corporate users held back and Cisco and other big equipment makers waited until ratification. By summer 2003, announcements were flourishing. Most of the dual-band 802.11a/b products became dual-band/tri-mode, supporting a, b, and g in a single mobile adaptor card or access point. Despite its major acceptance, 802.11g suffers from the same interference as 802.11b in the already crowded 2.4GHz range. Devices operating in this range include microwave ovens and cordless telephones. While 802.11g held the promise of higher throughput, actual results were mitigated by a number of factors: conflict with 802.11b-only devices (see above), exposure to the same interference sources as 802.11b, limited channelization (only 3 fully non-overlapping channels like 802.11b) and the fact that the higher data rates of 802.11g are often more susceptible to interference than 802.11b, causing the 802.11g device to reduce the data rate to effectively the same rates used by 802.11b. The move to dual-mode/tri-mode products also carries with it economies of scale (e.g. single chip manufacturing). For the consumer, dual-band/tri-mode products ensure the best possible throughput in any given environment.

Non-Standard Channel Bonding

Chipmaker Atheros sells a proprietary channel bonding feature called Super G for manufacturers of access points and client cards.[http://www.techworld.com/mobility/features/index.cfm?FeatureID=442] This feature can boost network speeds up to 108 Mbit/s by using Channel bonding. Also range is increased to 4x the range of 802.11g and 20x the range of 802.11b. This feature may interfere with other networks and may not support all b and g client cards. In addition, packet bursting techniques are also available in some chipsets and products which will also considerably increase speeds. This feature may not be compatible with other equipment. Broadcom, another chipmaker, has a similar proprietary feature called AfterBurner.

802.11n

In January 2004 IEEE announced that it had formed a new 802.11 Task Group (TGn) to develop a new amendment to the 802.11 standard for local-area wireless networks. The real data throughput is estimated to reach a theoretical 540 Mbit/s (which may require an even higher raw data rate at the physical layer), and should be up to 40 times faster than 802.11b, and near 10 times faster than 802.11a or 802.11g. It is projected that 802.11n will also offer a better operating distance than current networks. There are two competing proposals of the 802.11n standard, expected to be ratified: WWiSE (World-Wide Spectrum Efficiency), backed by companies including Broadcom, and TGn Sync backed by Intel and Philips. Previous competitors TGnSync, WWiSE, and a third group, MITMOT, said in late July 2005 that they would merge their respective proposals as a draft which would be sent to the IEEE in September; a final version will be submitted in November. The standardization process is expected to be completed by the second half of 2006. 802.11n builds upon previous 802.11 standards by adding MIMO (multiple-input multiple-output) and orthogonal frequency-division multiplexing (OFDM). MIMO uses multiple transmitter and receiver antennas to allow for increased data throughput through spatial multiplexing and increased range by exploiting the spatial diversity, perhaps through coding schemes like Alamouti coding. The [http://www.enhancedwirelessconsortium.org/ Enhanced Wireless Consortium (EWC)] was formed to help accelerate the IEEE 802.11n development process and promote a technology specification for interoperability of next-generation wireless local area networking (WLAN) products.

Certification

Because the IEEE only sets specifications but does not test equipment for compliance with them, a trade group called the Wi-Fi Alliance runs a certification program that members pay to participate in. Virtually all companies selling 802.11 equipment are members. The Wi-Fi trademark, owned by the group and usable only on compliant equipment, is intended to guarantee interoperability. Currently, "Wi-Fi" can mean any of 802.11a, b, or g. As of fall 2003, Wi-Fi also includes the security standard Wi-Fi Protected Access or WPA. Eventually "Wi-Fi" will also mean equipment which implements the IEEE 802.11i security standard (aka WPA2). Products that say they are Wi-Fi are supposed to also indicate the frequency band in which they operate (2.4 or 5 GHz).

Standards

The following IEEE standards[http://standards.ieee.org/] and task groups exist within the IEEE 802.11 working group:
- IEEE 802.11 - The original 1 Mbit/s and 2 Mbit/s, 2.4 GHz RF and IR standard (1999)
- IEEE 802.11a - 54 Mbit/s, 5 GHz standard (1999, shipping products in 2001)
- IEEE 802.11b - Enhancements to 802.11 to support 5.5 and 11 Mbit/s (1999)
- IEEE 802.11c - Bridge operation procedures; included in the IEEE 802.1D standard (2001)
- IEEE 802.11d - International (country-to-country) roaming extensions (2001)
- IEEE 802.11e - Enhancements: QoS, including packet bursting (2005)
- IEEE 802.11F - Inter-Access Point Protocol (2003)
- IEEE 802.11g - 54 Mbit/s, 2.4 GHz standard (backwards compatible with b) (2003)
- IEEE 802.11h - Spectrum Managed 802.11a (5 GHz) for European compatibility (2004)
- IEEE 802.11i - Enhanced security (2004)
- IEEE 802.11j - Extensions for Japan (2004)
- IEEE 802.11k - Radio resource measurement enhancements
- IEEE 802.11l - (reserved, typologically unsound)
- IEEE 802.11m - Maintenance of the standard; odds and ends.
- IEEE 802.11n - Higher throughput improvements
- IEEE 802.11o - (reserved, typologically unsound)
- IEEE 802.11p - WAVE - Wireless Access for the Vehicular Environment (such as ambulances and passenger cars)
- IEEE 802.11q - (reserved, typologically unsound, can be confused with 802.1q VLAN trunking)
- IEEE 802.11r - Fast roaming
- IEEE 802.11s - ESS Mesh Networking
- IEEE 802.11T - Wireless Performance Prediction (WPP) - test methods and metrics
- IEEE 802.11u - Interworking with non-802 networks (e.g., cellular)
- IEEE 802.11v - Wireless network management
- IEEE 802.11w - Protected Management Frames Note - there is no standard or task group named "802.11x". Rather, this term is used informally to denote any current or future 802.11 standard, in cases where further precision is not necessary. (The IEEE 802.1X standard for port-based network access control, is often mistakenly called "802.11x" when used in the context of wireless networks.) Note - 802.11F and 802.11T are recommendations, not standards and are capitalized as such.

Community networks

With the proliferation of cable modems and DSL, there is an ever-increasing market of people who wish to establish small networks in their homes to share their high speed Internet connection. Wireless office networks are often unsecured or secured with WEP, which is said to be easily broken, although a substantial amount of data has to be collected before it can be cracked successfully. These networks frequently allow "people on the street" to connect to the Internet. There are also efforts by volunteer groups to establish wireless community networks to provide free wireless connectivity to the public.

Security

In 2001, a group from the University of California, Berkeley presented a paper describing weaknesses in the 802.11 WEP (wired equivalent privacy) security mechanism defined in the original standard; they were followed by Fluhrer, Mantin, and Shamir's paper entitled "Weaknesses in the Key Scheduling Algorithm of RC4". Not long after, Adam Stubblefield and AT&T publicly announced the first verification of the attack. In the attack they were able to intercept transmissions and gain unauthorized access to wireless networks. The IEEE set up a dedicated task group to create a replacement security solution, 802.11i (previously this work was handled as part of a broader 802.11e effort to enhance the MAC layer). The Wi-Fi Alliance announced an interim specification called Wi-Fi Protected Access (WPA) based on a subset of the then current IEEE 802.11i draft. These started to appear in products in mid-2003. 802.11i (aka WPA2) itself was ratified in June 2004, and uses the Advanced Encryption Standard, instead of RC4, which was used in WEP and WPA. In January 2005, IEEE setup yet another task group TGw to protect management and broadcast frames, which previously were sent unsecured. See IEEE 802.11w

External links

- [http://standards.ieee.org/getieee802/download/802.11-1999.pdf The 802.11-1999 Standard]
- [http://standards.ieee.org/getieee802/download/802.11a-1999.pdf The 802.11a-1999 Standard]
- [http://standards.ieee.org/getieee802/download/802.11b-1999.pdf The 802.11b-1999 Standard]
- [http://standards.ieee.org/getieee802/download/802.11g-2003.pdf The 802.11g-2003 Standard]
- [http://www.war-driving.co.uk Learn the insecurities of Wifi, how to crack them & other general wireless security exploits]
- [http://www.ieee802.org/11/ The working group's website]
- [http://www.tutorial-reports.com/wireless/wlanwifi/ IEEE 802.11 Tutorial] Includes information on Architecture, Standards (802.11 b/a/g), Security and Comparisons
- [http://standards.ieee.org/getieee802/802.11.html Download the 802.11 protocol reference] (much of the document is missing in the on-line version)
- [http://www.oreillynet.com/pub/a/wireless/2005/05/20/80211map.html 802.11 Protocol Map] O'Reilly Network article by Gast, Matthew that includes a map outlining the relationship between the various 802.11 protocols, standards and regulatory bodies.
- [http://www.networkworld.com/news/tech/0214tech.html 802.11b backgrounder]
- [http://ftp.die.net/mirror/papers/802.11/ "Attack verification"], paper by Stubblefield (mirror; original gone)
- [http://802.11-security.com/security/links Wireless LAN Security whitepapers & howtos]
- [http://grouper.ieee.org/groups/802/11/Reports/tgn_update.htm Status of the 802.11n standard]
- [http://www.enhancedwirelessconsortium.org/ Enhanced Wireless Consortium]
- 802.11 Wireless Networks: The Definitive Guide, Second Edition [http://www.oreilly.com/catalog/802dot112/chapter/ch15.pdf Chapter 15: A Peek Ahead at 802.11n: MIMO-OFDM] (PDF) Category:IEEE 802 Category:Wireless networking Category:Wi-Fi ko:IEEE 802.11 ja:IEEE802.11

802.11r

802.11 is a set of IEEE standards that govern wireless networking transmission methods. They are commonly used today in their 802.11a, 802.11b, and 802.11g versions to provide wireless connectivity in the home, office and some commercial establishments.

Fast BSS Transitions

802.11r is the unapproved IEEE 802.11 standard that specifies fast BSS ("Basic Service Set") transitions. This will permit connectivity aboard vehicles in motion, with fast handoffs from one base station to another managed in a seamless manner. Handoffs are supported under the "a", "b" and "g" implementations, but only for data. The handover delay is too long to support applications like voice and video. The primary application currently envisioned for the 802.11r standard is VOIP ("voice over IP", or Internet-based telephony) via mobile phones designed to work with wireless Internet networks, instead of (or in addition to) standard cellular networks.

Protocol Operation

Access Points and Stations are both given the optional ability Fast Transition The BSS – BSS transition goes through five stages:
- Scanning – active or passive for other APs in the area
- Authentication with a (one or more) target AP
- Re-association to establish connection at target AP At this point, the AP and Station have a connection, but still can't do anything.
- Key (PTK) derivation – 4-way handshake of session keys based on 802.1x authentication
- QoS admission control to re-establish QoS streams

Status

Currently, the standard body TGr is combining the two remaining proposals, TAP (Transition Acceleration Protocol) and Just-In-Time 2 Phase Association, into refining the accepted proposal into a draft which will go to a letter ballot in November 2005.

External References

- [http://grouper.ieee.org/groups/802/11/Reports/tgr_update.htm IEEE P802 Status of Project IEEE 802.11r]
- [http://66.102.7.104/search?q=cache:xxpIIlf9q5sJ:www.ieee802.org/21/sept05_meeting_docs/21-05-0381-00-0000-802-11-liaison-September05.ppt+802.11w&hl=en&client=firefox-a IEEE 802.21 liason report that contains protocol summary of 802.11r]
- [http://www.networkworld.com/news/tech/2005/082205techupdate.html 802.11r strengthens wireless voice] Pat Calhoun and Bob O'Hara, Network World, August 22, 2005 Category:IEEE 802 Category:Wireless networking

Category:Telephony

Articles about telephony. Category:Telecommunications Category:Electronic engineering ja:Category:電話

Hieroi logoi

Hieroi logoi, (görögül: Szent elbeszélések), a Kr. e. VI. században keletkezett görög költemény, az orphikus vallási irányzat világkeletkezési elképzelését tartalmazó, ránk csak töredékesen maradt mű, amely mítikus alakokká formálta az elvont fogalmakat, s ezet ok-okozati viszonyát az istenségek generációinak egymásutánjában ábrázolja. Kezdetben volt Kronosz, az Idő, aki nemzette Aithért (Levegőég), az Aithérben egy ezüst tojást teremtett, ebből kelt ki Phanész (Ragyogás), aki leányával, Nüxszel nemzette Uranoszt (Égbolt) és Gaiát (Föld). Ezután a Hésziodosz kozmogóniájából ismert istennemzedékek következnek. Zeusz újrateremti a világot úgy, hogy Phanészt elnyeli, vagyis minden őbenne van. Zeusz fiát, Dionüszoszt a titánok széttépték és megették, Zeusz ezért villámokkal sújtja őket, tagjaikból lettek az emberek, akikben így a vad, törvénytipró titáni és a helyes, dionüszoszi elem egyaránt jelen van. Ha az ember törekszik magában a titánit megfékezni, Dionüszosz megszabadítja a gonoszságtól, és lelke megtisztul.

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