VoIP

Protocols which are used to carry voice signals over the IP network are commonly referred to as Voice over IP or VoIP protocols. They may be viewed as commercial realizations of the experimental Network Voice Protocol (1973) invented for the ARPANET.ce providers. Some cost savings are due to utilizing a single network to carry voice and data, especially where users have existing underutilized network capacity they can use for VoIP at no additional cost. VoIP to VoIP phone calls on any provider are typically free, while VoIP to PSTN calls generally costs the VoIP user.

There are two types of PSTN to VoIP services: DID (Direct Inward Dialing) and access numbers. DID will connect the caller directly to the VoIP user while access numbers require the caller to input the extension number of the VoIP user. Access numbers are usually charged as a local call to the caller and free to the VoIP user(citation/example needed) while DID usually has a monthly fee. There are also DID that are free to the VoIP user but is chargeable to the caller .

Functionality
VoIP can facilitate tasks that may be more difficult to achieve using traditional networks:

Incoming phone calls can be automatically routed to your VoIP phone, regardless of where you are connected to the network. Take your VoIP phone with you on a trip, and wherever you connect to the Internet, you can receive incoming calls.
Free phone numbers for use with VoIP are available in the USA, UK and other countries from organizations such as VoIP User.
Call center agents using VoIP phones can work from anywhere with a sufficiently fast and stable Internet connection.
Many VoIP packages include PSTN features that most telcos normally charge extra for, or may be unavailable from your local telco, such as 3-way calling, call forwarding, automatic redial, and caller ID.

Mobility
VoIP allows users to travel anywhere in the world and still make and receive phone calls:

Subscribers of phone-line replacement services can make and receive local phone calls regardless of their location. For example, if a user has a New York City phone number and is traveling in Europe and someone calls the phone number, it will ring in Europe. Conversely, if a call is made from Europe to New York City, it will be treated as a local call. Of course, there must be a connection to the Internet e.g. WiFi to make all of this possible.
Users of Instant Messenger based VoIP services can also travel anywhere in the world and make and receive phone calls.
VoIP phones can integrate with other services available over the Internet, including video conversation, message or data file exchange in parallel with the conversation, audio conferencing, managing address books and passing information about whether others (e.g. friends or colleagues) are available online to interested parties.

Drawbacks
VoIP technology still has a few shortcomings that have led some to believe that it is not ready for widespread deployment. However, many industry analysts predicted that 2005 was the "Year of Inflection," where more IP PBX ports shipped than conventional digital PBX ports. This date has been moved on an annual basis and only now (mid 2006) is it beginning to happen. However, many purchasers of VoIP ports just want a phone, so the statistics can be misleading when interpreted by marketeers.

Faxes
One drawback is the difficulty in sending faxes due to software and networking restraints in most home systems. However, an effort is underway to define an alternate IP-based solution for delivering Fax-over-IP, namely the T.38 protocol. Another possible solution to overcome the drawback is to treat the fax system as a message switching system which does not need real time data transmission - such as sending a fax as a email attachment (see iFax) or remote printout (see Internet Printing Protocol). The end system can completely buffer the incoming fax data before displaying or printing the fax image.

Internet Connection
Another drawback of VoIP service is its frequent reliance upon another separate service - an Internet connection. The quality and overall reliability of the phone connection is entirely reliant upon the quality, reliability, and speed of the internet connection which it is using. Shortcomings with internet connections and Internet Service Providers (ISPs) can cause a lot of grief with VoIP calls. Higher overall network latencies can lead to significantly reduced call quality and cause certain problems such as echoing.

VoIP isn't entirely reliant upon internet connections, however. VoIP systems can also utilize regular telephone lines and business-grade connections like T1's for voice service. A few business VoIP Providers offer dedicated point-to-point T1 connections, thereby not relying on an internet connection for service. Although residential VoIP service typically uses only an internet connection, business-grade VoIP service can use a variety of connection methods to provide ongoing phone service.

Many VoIP users still maintain a traditional analog voice line (business line) which allows them to dial emergency numbers and utilize a traditional fax machine.

Power Outages
Another drawback of VoIP is the inability to make phone calls during a power outage, but this problem also exists with many phones used with conventional land lines and can be remedied with a battery backup. During a power outage you also have the choice to forward your phone to your cell phone or another phone number so you would still be able to receive calls. Although you can't call out on your home phone system during a power outage, at least you can still receive calls.

If VoIP is used in solitary LAN (with no internet connection), it would consume more resources compared to a PABX.

Modems are now available with lithium ion battery backup so that you can use the service with no power.

Implementation challenges
Because UDP does not provide a mechanism to ensure that data packets are delivered in sequential order, or provide Quality of Service guarantees, VoIP implementations face problems dealing with latency and jitter. This is especially true when satellite circuits are involved, due to long round trip propagation delay (400 milliseconds to 600 milliseconds for geostationary satellite). The receiving node must restructure IP packets that may be out of order, delayed or missing, while ensuring that the audio stream maintains a proper time consistency. This functionality is usually accomplished by means of a jitter buffer.

Another challenge is routing VoIP traffic through firewalls and address translators. Private Session Border Controllers are used along with firewalls to enable VoIP calls to and from a protected enterprise network. Skype uses a proprietary protocol to route calls through other Skype peers on the network, allowing it to traverse symmetric NATs and firewalls. Other methods to traverse firewalls involve using protocols such as STUN or ICE.

VoIP challenges:

Delay/Network Latency
Packet loss
Jitter
Echo
Security
Fixed delays cannot be controlled but some delays can be minimized by marking voice packets as being delay-sensitive (see, for example, Diffserv).

The principal cause of packet loss is congestion, which can be controlled by congestion management and avoidance. Carrier VoIP networks avoid congestion by means of traffic engineering.

Variation in delay is called jitter. The effects of jitter can be mitigated by storing voice packets in a buffer (called a play-out buffer) upon arrival, before playing them out. This avoids a condition known as buffer underrun, in which the playout process runs out of voice data to play because the next voice packet has not yet arrived, but increases delay by the length of the buffer.

Common causes of echo include impedance mismatches in analog circuitry, and acoustic coupling of the transmit and receive signal at the receiving end.

Reliability
Conventional telephones are connected directly to telephone company phone lines, which in the event of a power failure are kept functioning by back-up generators or batteries located at the telephone exchange. However, household VoIP hardware uses broadband modems and other equipment powered by household electricity, which may be subject to outages dictating the use of an uninterruptible power supply or generator to ensure availability during power outages. Early adopters of VoIP may also be users of other phone equipment, such as PBX and cordless phone bases, that rely on power not provided by the telephone company. Even with local power still available, the broadband carrier itself may experience outages as well. While the PSTN has been matured over decades and is typically extremely reliable, most broadband networks are less than 10 years old, and even the best are still subject to intermittent outages. Furthermore, consumer network technologies such as cable and DSL often are not subject to the same restoration service levels as the PSTN or business technologies such as T-1 connection.

Quality of Service
Some broadband connections may have less than desirable quality. Where IP packets are lost or delayed at any point in the network between VoIP users, there will be a momentary drop-out of voice. This is more noticeable in highly congested networks and/or where there is long distances and/or interworking between end points. Technology has improved the reliability and voice quality over time and will continue to improve VoIP performance as time goes on.

Emergency calls
The nature of IP makes it difficult to geographically locate network users. Emergency calls, therefore, cannot easily be routed to a nearby call center, and are impossible on some VoIP systems. Moreover, in the event that the caller is unable to give an address, emergency services may be unable to locate them in any other way. Following the lead of mobile phone operators, several VoIP carriers are already implementing a technical work-around. For instance, one large VoIP carrier requires the registration of the physical address the VoIP line will be used at. When you dial the emergency number for your country, they will route it to the appropriate local system. They also maintain their own emergency call center that will take non-routable emergency calls (made, for example, from a software based service that is not tied to any particular physical location) and then will manually route your call once learning your physical location.

The United States government had set a deadline, requiring VoIP carriers to implement E911; however, the deadline is being appealed by several of the leading VoIP companies.

This is a different situation with IPBX systems, where these corporate systems often have full E911 capabilities built into the system.

Integration into global telephone number system
While the traditional Plain Old Telephone System (POTS) and mobile phone networks share a common global standard (E.164) which allocates and identifies any specific telephone line, there is no widely adopted similar standard for VoIP networks. Some allocate an E.164 number which can be used for VoIP as well as incoming/external calls. However, there are often different, incompatible schemes when calling between VoIP providers which use provider specific short codes.

Single point of calling
With hardware VoIP solution it is possible to connect the VoIP router into the existing central phone box in the house and have VoIP at every phone already connected. Software based VoIP services require the use of a computer, so they are limited to single point of calling, though handsets are now available, allowing them to be used without a PC. Some services provide the ability to connect WiFi SIP phones so that service can be extended throughout the premises, and off-site to any location with an open hotspot[1]. However, note that many hotspots require browser-based authentication, which most SIP phones do not support[2].

Mobile phones
Telcos and consumers have invested billions of dollars in mobile phone equipment. In developed countries, mobile phones have achieved nearly complete market penetration, and many people are giving up landlines and using mobiles exclusively. Given this situation, it is not entirely clear whether there would be a significant higher demand for VoIP among consumers until either a) public or community wireless networks have similar geographical coverage to cellular networks (thereby enabling mobile VoIP phones, so called WiFi phones) or b) VoIP is implemented over legacy 3G networks. However, "dual mode" handsets, which allow for the seamless handover between a cellular network and a WiFi network, are expected to help VoIP become more popular [3]. The first company launching mobile VoIP in the world was ((truphone)). Phones like the Nokia E60, E61 have been the first "dual mode" handsets capable of delivering mobile VoIP with long battery lifetimes.

Security
The majority of consumer VoIP solutions do not support encryption yet. As a result, it is relatively easy to eavesdrop on VoIP calls and even change their content. There are several open source solutions that facilitate sniffing of VoIP conversations. A modicum of security is afforded due to patented audio codecs that are not easily available for open source applications, however such security through obscurity has not proven effective in the long run in other fields. Some vendors also use compression to make eavesdropping more difficult. However, real security requires encryption and cryptographic authentication which are not widely available at a consumer level[4]. The existing secure standard SRTP is available on Analog Telephone Adapters(ATAs) as well as various softphones.

The Voice VPN solution provides secure voice for enterprise VoIP networks by applying IPSec encryption to the digitized voice stream.

Pre-Paid Phone Cards
VoIP has become a major provider of phone services to travellers, migrant workers and ex-pats, who either, due to not having a fixed or mobile phone or high overseas roaming charges, choose instead to use VoIP services to make their phone calls. Pre-Paid phone cards can be used either from a normal phone or from Internet Cafes that have phone services. The undeveloped markets are usually markets where Pre-Paid cards are used; however in cities with high tourist or immigrant communities they are also common.

Caller ID
Caller ID support among VoIP providers varies, although the majority of VoIP providers now offer full Caller ID w/ Name on Outgoing calls. When calling a traditional PSTN number from some VoIP providers, Caller ID isn't supported, and therefore the target person will not know who is calling. The number shows up as 'Unknown' or '000-012-3456'.

In a few cases, VoIP providers may allow a caller to spoof the Caller ID information, making it appear as though they are calling from a different number. Business grade VoIP equipment and software often makes it easy to modify caller ID information. Although this can provide many businesses great flexibility, it is also open to abuse.

Adoption

Mass-market telephony
A major development starting in 2004 has been the introduction of mass-market VoIP services over broadband Internet access services, in which subscribers make and receive calls as they would over the PSTN. Full phone service VoIP phone companies provide inbound and outbound calling with Direct Inbound Dialing. Many offer unlimited calling to the U.S., and some to Canada or selected countries in Europe or Asia as well, for a flat monthly fee.

These services take a wide variety of forms which can be more or less similar to traditional POTS. At one extreme, an analog telephone adapter (ATA) may be connected to the broadband Internet connection and an existing telephone jack in order to provide service nearly indistinguishable from POTS on all the other jacks in the residence. This type of service, which is fixed to one location, is generally offered by broadband Internet providers such as cable companies and telephone companies as a cheaper flat-rate traditional phone service. Often the phrase "VoIP" is not used in selling these services, but instead the industry has marketed the phrase "Internet Phone" or "Digital Phone" which is aimed at typical phone users who are not necessarily tech-savvy. Typically, the provider touts the advantage of being able to keep one's existing phone number. According to a study by Telephia, the top ten providers in the United States include Vonage, Verizon VoiceWing, AT&T CallVantage, SunRocket, Lingo, NetZero, BroadVoice, America Online, Packet8, and Earthlink. Verizon VoiceWing and AT&T CallVantage are both listed in second place with 5.5% market share.[5]

At the other extreme are services like Gizmo Project and Skype which rely on a software client on the computer in order to place a call over the network, where one user ID can be used on many different computers or in different locations on a laptop. In the middle lie services which also provide a telephone adapter for connecting to the broadband connection similar to the services offered by broadband providers (and in some cases also allow direct connections of SIP phones) but which are aimed at a more tech-savvy user and allow portability from location to location. One advantage of these two types of services is the ability to make and receive calls as one would at home, anywhere in the world, at no extra cost. No additional charges are incurred, as call diversion via the PSTN would, and the called party does not have to pay for the call. For example, if a subscriber with a home phone number in a U.S. area code calls someone else in his home area code, it will be treated as a local call regardless of where that person is in the world. Often the user may also select a phone number with any desired area code; this is generally done to minimize the phone tariffs of those who frequently call.

For some users, the broadband phone complements, rather than replaces, a PSTN line, due to a number of inconveniences compared to traditional services. VoIP requires a broadband Internet connection and, if a telephone adapter is used, a power adapter is usually needed. In the case of a power failure, VoIP services will generally not function. Additionally, a call to the U.S. emergency services number 9-1-1 may not automatically be routed to the nearest local emergency dispatch center, and would be of no use for subscribers outside the U.S. This is potentially true for users who select a number with an area code outside their area. Some VoIP providers offer users the ability to register their address so that 9-1-1 services work as expected.

Another challenge for these services is the proper handling of outgoing calls from fax machines, TiVo/ReplayTV boxes, satellite television receivers, alarm systems, conventional modems or FAXmodems, and other similar devices that depend on access to a voice-grade telephone line for some or all of their functionality. At present, these types of calls sometimes go through without any problems, but in other cases they will not go through at all. And in some cases, this equipment can be made to work over a VoIP connection if the sending speed can be changed to a lower bits per second rate. If VoIP and cellular substitution becomes very popular, some ancillary equipment makers may be forced to redesign equipment, because it would no longer be possible to assume a conventional voice-grade telephone line would be available in almost all homes in North America and Western-Europe. The TestYourVoIP website offers a free service to test the quality of or diagnose an Internet connection by placing simulated VoIP calls from any Java-enabled Web browser, or from any phone or VoIP device capable of calling the PSTN network.

Corporate and telco use
Although few office environments and even fewer homes use a pure VoIP infrastructure, telecommunications providers routinely use IP telephony, often over a dedicated IP network, to connect switching stations, converting voice signals to IP packets and back. The result is a data-abstracted digital network which the provider can easily upgrade and use for multiple purposes.

Corporate customer telephone support often use IP telephony exclusively to take advantage of the data abstraction. The benefit of using this technology is the need for only one class of circuit connection and better bandwidth use. Companies can acquire their own gateways to eliminate third-party costs, which is worthwhile in some situations.

VoIP is widely employed by carriers, especially for international telephone calls. It is commonly used to route traffic starting and ending at conventional PSTN telephones.

Many telecommunications companies are looking at the IP Multimedia Subsystem (IMS) which will merge Internet technologies with the mobile world, using a pure VoIP infrastructure. It will enable them to upgrade their existing systems while embracing Internet technologies such as the Web, email, instant messaging, presence, and video conferencing. It will also allow existing VoIP systems to interface with the conventional PSTN and mobile phones.

Electronic Numbering (Enum) uses standard phone numbers (E.164), but allows connections entirely over the Internet. If the other party uses Enum, the only expense is the Internet connection.

Use in Amateur Radio
Amateur radio has adopted VoIP by linking repeaters and users with Echolink, IRLP, Dstar and EQSO. Echolink and IRLP are programs/systems based upon the Speak Freely VoIP open source software. In fact, Echolink allows users to connect to repeaters via their computer (over the internet) rather than by using a radio. By using VoIP Amateur Radio operators are able to create large repeater networks with repeaters all over the world where operators can access the system with actual ham radios.

Ham Radio operators using radios are able to tune to repeaters with VoIP capabilities and use DTMF buttons to command the repeater to connect to various other repeaters, thus allowing them to talk to people all around the world, however powerful their radio. Dingotel offers a similar feature for non ham radio users by providing a P2P network to link FRS radios.

Click to call
Click-to-call is a service which lets users click a button and immediately speak with a customer service representative. The call can either be carried over VoIP, or the customer may request an immediate call back by entering their phone number. One significant benefit to click-to-call providers is that it allows companies to monitor when online visitors change from the website to a phone sales channel.

Legal issues

As the popularity of VoIP grows, and PSTN users switch to VoIP in increasing numbers, governments are becoming more interested in regulating VoIP in a manner similar to legacy PSTN services.

In the U.S., the Federal Communications Commission now requires all VoIP operators who do not support Enhanced 911 to attach a sticker warning that traditional 911 services aren't available. The FCC recently required VoIP operators to support CALEA wiretap functionality. The Telecommunications Act of 2005 proposes adding more traditional PSTN regulations, such as local number portability and universal service fees. Other future legal issues are likely to include laws against wiretapping and network neutrality.

Some Latin American and Caribbean countries, fearful for their state owned telephone services, have imposed restrictions on the use of VoIP, including in Panama where VoIP is taxed. In Ethiopia, where a totalitarian government is monopolizing telecommunication service, it is a criminal offence to offer services using VoIP. The country has installed firewalls to prevent international calls being made using VoIP. These measures were taken after a popularity in VoIP reduced the income generated by the state owned telecommunication company.

In the European Union, the treatment of VoIP service providers is a decision for each Member State's national telecoms regulator, which must use competition law theory to define relevant national markets and then determine whether any service provider on those national markets has "significant market power" (and so should be subject to certain obligations). A general distinction is usually made between VoIP services that function over managed networks (via broadband connections) and VoIP services that function over unmanaged networks (essentially, the Internet).

VoIP services that function over managed networks are often considered to be a viable substitute for PSTN telephone services (despite the problems of power outages and lack of geographical information); as a result, major operators that provide these services (in practice, incumbent operators) may find themselves bound by obligations of price control or accounting separation.

VoIP services that function over unmanaged networks are often considered to be too poor in quality to be a viable substitute for PSTN services; as a result, they may be provided without any specific obligations, even if a service provider has "significant market power".

The relevant EU Directive is not clearly drafted concerning obligations which can exist independently of market power (e.g. the obligation to offer access to emergency calls), and it is impossible to say definitively whether VoIP service providers of either type are bound by them. A review of the EU Directive is under way and should be complete by 2007.

In India, it is legal to use VoIP, but it is illegal to have VoIP gateways inside India. This effectively means that people who have PCs can use them to make a VoIP call to any number, but if the remote side is a normal phone, the gateway that converts the VoIP call to a POTS call should not be inside India.

Technical details
The two major competing standards for VoIP are the IETF standard SIP and the ITU standard H.323. Initially H.323 was the most popular protocol, though its popularity has decreased in the "local loop" due to its poor traversal of NAT and firewalls. For this reason as domestic VoIP services have been developed, SIP has been far more widely adopted. However in backbone voice networks where everything is under the control of the network operator or telco, H.323 is the protocol of choice. Many of the largest carriers use H.323 in their core backbones, and the vast majority of callers have little or no idea that their POTS calls are being terminated over VoIP. So really SIP is a useful tool for the "local loop" and H.323 is like the "fiber backbone". With the most recent changes introduced for H.323, however, it is now possible for H.323 devices to easily and consistently traverse NAT and firewall devices, opening up the possibility that H.323 may again be looked upon more favorably in cases where such devices encumbered its use previously.

Where VoIP travels through multiple providers' Soft Switches the concepts of Full Media Proxy and signalling proxy are important. In H.323, the data is made up of 3 streams of data: 1) H.225.0 Call Signaling; 2) H.245; 3) Media. So if you are in London, your provider is in Australia, and you wish to call America, then in full proxy mode all three streams will go half way around the world and the delay (up to 500-600 ms) and packet loss will be high. However in signaling proxy mode where only the signaling flows through the provider the delay will be reduced to a more user friendly 120-150 ms. These proxy concepts could lead the way to true global providers.

One of the key issues with all traditional VoIP protocols is the wasted bandwidth used for packet headers. Typically to send a G.723.1 5.6 kbit/s compressed audio path will require 18 kbit/s of bandwidth based on standard sampling rates. The difference between the 5.6 kbit/s and 18 kbit/s is packet headers. There are a number of bandwidth optimisation techniques used, such as silence suppression and header compression. This can typically save 35% on bandwidth usage. But the really interesting technology comes from VoIP off shoots such as TDMoIP which take advantage of the concept of bundling conversations that are heading to the same destination and wrapping them up inside the same packets. These can offer near toll quality audio in a 6-7 kbit/s data stream.

Computer networking

Such networks involve at least two devices capable of being networked with at least one usually being a computer. The devices can be separated by a few meters (e.g. via Bluetooth) or thousands of kilometers (e.g. via the Internet). Computer networking is sometimes considered a sub-discipline of telecommunications.

History
Carrying instructions between calculation machines and early computers was done by human users. In September 1940 George Stibitz used a teletype machine to send instructions for a problem set from his Model K at Dartmouth College in New Hampshire to his Complex Number Calculator in New York and received results back by the same means. Linking output systems like teletypes to computers was an interest at the Advanced Research Projects Agency (ARPA) when, in 1962, J.C.R. Licklider was hired and developed a working group he called the "Intergalactic Network", a precursor to the ARPANet. In 1964, researchers at Dartmouth developed the Dartmouth Time Sharing System for distributed users of large computer systems. The same year, at MIT, a research group supported by General Electric and Bell Labs used a computer (DEC's PDP-8) to route and manage telephone connections. Throughout 1960s Leonard Kleinrock, Paul Baran and Donald Davies had independently conceptualized and developed network systems consisting of datagrams or packets that could be used in a packet switching network between computer systems. In 1969 the University of California at Los Angeles, SRI (in Stanford), University of California at Santa Barbara, and the University of Utah were connected as the beginning of the ARPANet network using 50 kbit/s circuits.

Networks, and the technologies needed to connect and communicate through and between them, continue to drive computer hardware, software, and peripherals industries. This expansion is mirrored by growth in the numbers and types of users of networks from researcher

Categorizing

By network layer
See the seven layer OSI reference model and the four or five layer TCP/IP model

Application layer
Presentation layer (Only in the OSI model)
Session layer (Only in the OSI model)
Transport layer
Network layer
Data Link layer
Media access control sublayer
Logical link control sublayer
Physical layer

By scale
Personal area network (PAN)
Local area network (LAN)
Wireless local area network(WLAN)
Campus area network (CAN)
Metropolitan area network (MAN)
Wide area network (WAN)

By connection method
HomePNA
Power line communication
Ethernet
WiFi

By functional relationship
Active Networking (Low-level code movement versus static data)
Client-server
Peer-to-peer (Workgroup)

By network topology
Bus network
Star network
Ring network
Mesh network
Star-bus network
Tree topology network

By Services provided
Storage area networks
Server farms
Process control networks
Value-added network
SOHO network
Wireless community network
XML appliance
Jungle Networks

Protocol stacks
Computer networks may be implemented using a variety of protocol stack architectures, computer buses or combinations of media and protocol layers, incorporating one or more of:

ARCNET
AppleTalk
ATM
Bluetooth
DECnet
Ethernet
FDDI
Frame relay
HIPPI
IEEE 1394 aka FireWire, iLink
IEEE 802.11 aka Wireless LAN (Wi-Fi certification)
IEEE-488
TCP/IP protocol suite
IPX
Myrinet
QsNet
RS-232
SPX
System Network Architecture
Token ring
TCP
TCP Tuning for discussion of improving performance of same
USB
UDP
X.25 protocol suite
For a list of more see Network protocols.

For standards see IEEE 802.

Suggested topics
Further reading for acquiring an in-depth understanding of computer networks include:

Communication theory

Data transmission

Wired transmission
Public switched telephone network
Modems and dialup
Dedicated lines – leased lines
ISDN
DSL
Time-division multiplexing(TDM)
Packet switching
Frame relay
PDH
Ethernet
RS-232
RS-485
Optical fiber transmission
Synchronous optical networking(SONET)
Fiber distributed data interface

Wireless transmission
Extreme Short range
ZigBee
Short range
Bluetooth
InfraRed(IrDA)
Medium range
WiFi(IEEE 802.11)
WiMax(IEEE 802.16)
Long range
Satellite
MMDS
SMDS
Mobile phone data transmission (channel access methods)
CDMA
CDPD
GSM
TDMA
Paging networks
DataTAC
Mobitex
Motient

WiFi

It was developed to be used for mobile computing devices, such as laptops, in LANs, but is now increasingly used for more services, including Internet and VoIP phone access, gaming, and basic connectivity of consumer electronics such as televisions and DVD players, or digital cameras. More standards are in development that will allow Wi-Fi to be used by cars in highways in support of an Intelligent Transportation System to increase safety, gather statistics, and enable mobile commerce (see IEEE 802.11p). Wi-Fi® and the Wi-Fi CERTIFIED™ logo are registered trademarks of the Wi-Fi Alliance® - the trade organization that tests and certifies equipment compliance with the 802.11x standards.

User
A person with a Wi-Fi enabled device such as a computer, cell phone or PDA can connect to the Internet when in proximity of an access point. The region covered by one or several access points is called a hotspot. Hotspots can range from a single room to many square miles of overlapping hotspots. Wi-Fi can also be used to create a mesh network. Both architectures are used in community networks, municipal wireless networks like Wireless Philadelphia, and metro-scale networks like M-Taipei.

Wi-Fi also allows connectivity in peer-to-peer mode, which enables devices to connect directly with each other. This connectivity mode is useful in consumer electronics and gaming applications.

When the technology was first commercialized there were many problems because consumers could not be sure that products from different vendors would work together. The Wi-Fi Alliance began as a community to solve this issue so as to address the needs of the end user and allow the technology to mature. The Alliance created the branding Wi-Fi CERTIFIED to show consumers that products are interoperable with other products displaying the same branding.

Wi-Fi in gaming
Some gaming consoles and handhelds make use of Wi-Fi technology to enhance the gaming experience:

The Nintendo DS handheld is Wi-Fi compatible, although there is no built in encryption, most games do not support WPA encryption, only the weaker WEP.
The Sony PSP includes WLAN to connect to Wi-Fi hotspots or make wireless connections.
The Xbox 360 features 1 Wi-Fi accessory: A wireless network adapter.
The PlayStation 3 premium model ($599) features built-in Wi-Fi.
The Wii features Wi-Fi.

Technical information

Wi-Fi: How it works
A typical Wi-Fi setup contains one or more Access Points (APs) and one or more clients. An AP broadcasts its SSID (Service Set Identifier, "Network name") via packets that are called beacons, which are usually broadcast every 100 ms. The beacons are transmitted at 1 Mbit/s, and are of relatively short duration and therefore do not have a significant effect on performance. Since 1 Mbit/s is the lowest rate of Wi-Fi it assures that the client who receives the beacon can communicate at least 1 Mbit/s. Based on the settings (e.g. the SSID), the client may decide whether to connect to an AP. If two APs of the same SSID are in range of the client, the client firmware might use signal strength to decide which of the two APs to make a connection to. The Wi-Fi standard leaves connection criteria and roaming totally open to the client. This is a strength of Wi-Fi, but also means that one wireless adapter may perform substantially better than the other. Since Wi-Fi transmits in the air, it has the same properties as a non-switched ethernet network. Even collisions can therefore appear as in non-switched ethernet LAN's. Unlike a wired Ethernet, and like most packet radios, Wi-Fi cannot do collision detection, and instead uses a packet exchange (RTS/CTS used for Collision Avoidance or CA) to try to avoid collisions.

Channels
Except for 802.11a, which operates at 5 GHz, Wi-Fi uses the spectrum near 2.4 GHz, which is standardized and unlicensed by international agreement, although the exact frequency allocations vary slightly in different parts of the world, as does maximum permitted power. However, channel numbers are standardized by frequency throughout the world, so authorized frequencies can be identified by channel numbers.

The frequencies for 802.11 b/g span 2.400 GHz to 2.487 GHz. Each channel is 22 MHz wide yet there is a 5 MHz step to the next higher channel.

The maximum number of available channels for wi-fi enabled devices are: – 13 for Europe - 11 for North America - 14 for Japan

In North America, only channels 1, 6, and 11 are deployed for 802.11b/g.

Advantages of Wi-Fi

Wireless Internet on the beach, Taba, EgyptAllows LANs to be deployed without cabling, typically reducing the costs of network deployment and expansion. Spaces where cables cannot be run, such as outdoor areas and historical buildings, can host wireless LANs.
Wi-Fi silicon pricing continues to come down, making Wi-Fi a very economical networking option and driving inclusion of Wi-Fi in an ever-widening array of devices.
Wi-Fi products are widely available in the market. Different brands of access points and client network interfaces are interoperable at a basic level of service. Products designated as Wi-Fi CERTIFIED by the Wi-Fi Alliance are interoperable and include WPA2 security.
Wi-Fi networks support roaming, in which a mobile client station such as a laptop computer can move from one access point to another as the user moves around a building or area.
Wi-Fi is a global set of standards. Unlike cellular carriers, the same Wi-Fi client works in different countries around the world.
Widely available in more than 250,000 public hot spots and millions of homes and corporate and university campuses worldwide.
As of 2006, WPA and WPA2 encryption are not easily crackable if strong passwords are used
New protocols for Quality of Service (WMM) and power saving mechanisms (WMM Power Save) make Wi-Fi even more suitable for latency-sensitive applications (such as voice and video) and small form-factor devices.

Disadvantages of Wi-Fi
Wi-Fi can be interrupted by other devices, notably 2.4 GHz cordless phones and microwave ovens.
Spectrum assignments and operational limitations are not consistent worldwide; most of Europe allows for an additional 2 channels beyond those permitted in the US (1-13 vs 1-11); Japan has one more on top of that (1-14) - and some countries, like Spain, prohibit use of the lower-numbered channels. Furthermore some countries, such as Italy, used to require a 'general authorization' for any Wi-Fi used outside an operator's own premises, or require something akin to an operator registration. For Europe; consult http://www.ero.dk for an annual report on the additional restrictions each European country imposes.
EIRP in the EU is limited to 20dbm.
Power consumption is fairly high compared to some other standards, making battery life and heat a concern.
The most common wireless encryption standard, Wired Equivalent Privacy or WEP, has been shown to be breakable even when correctly configured.
Wi-Fi Access Points typically default to an open (encryption-free) mode. Novice users benefit from a zero configuration device that works out of the box but might not intend to provide open wireless access to their LAN. WPA Wi-Fi Protected Access which began shipping in 2003 aims to solve these problems and is now generally available, but adoption rates remain low.
Many 2.4 GHz 802.11b and 802.11g Access points default to the same channel, contributing to congestion on certain channels.
Wi-Fi networks have limited range. A typical Wi-Fi home router using 802.11b or 802.11g with a stock antenna might have a range of 45 m (150 ft) indoors and 90 m (300 ft) outdoors. Range also varies with frequency band, as Wi-Fi is no exception to the physics of radio wave propagation. Wi-Fi in the 2.4 GHz frequency block has better range than Wi-Fi in the 5 GHz frequency block, and less range than the oldest Wi-Fi (and pre-Wi-Fi) 900 MHz block. Outdoor range with improved antennas can be several kilometres or more with line-of-sight.
Wi-Fi pollution, meaning interference of a closed or encrypted access point with other open access points in the area, especially on the same or neighboring channel, can prevent access and interfere with the use of other open access points by others caused by overlapping channels in the 802.11g/b spectrum as well as with decreased signal-to-noise ratio (SNR) between access points. This is a widespread problem in high-density areas such as large apartment complexes or office buildings with many Wi-Fi access points.
It is also an issue when municipalities or other large entities such as universities seek to provide large area coverage. Everyone is considered equal when they use the band (except for amateur radio operators who are the primary licensee); often this causes contention when one user seeks to claim priority in this unlicensed band. This openness is also important to the success and widespread use of Wi-Fi, but makes Part 15 (US) unsuitable for "must have" public service functions.
Interoperability issues between brands or deviations from the standard can disrupt connections or lower throughput speeds on other user's devices within range. Wi-Fi Alliance programs test devices for interoperability and designate devices which pass testing as Wi-Fi CERTIFIED.
Wi-Fi networks can be monitored and used to read and copy data (including personal information) transmitted over the network when no encryption such as VPN is used.

Examples of Standard Wi-Fi Devices

Wireless Access Point (WAP)
A wireless access point (AP) connects a group of wireless stations to an adjacent wired local area network (LAN). An access point is similar to an ethernet hub, but instead of relaying LAN data only to other LAN stations, an access point can relay wireless data to all other compatible wireless devices as well as to a single (usually) connected LAN device, in most cases an ethernet hub or switch, allowing wireless devices to communicate with any other device on the LAN.

Wireless Routers
A wireless router integrates a wireless access point with an IP router and an ethernet switch. The integrated switch connects the integrated access point and the integrated ethernet router internally, and allows for external wired ethernet LAN devices to be connected as well as a (usually) single WAN device such as cable modem or DSL modem. A wireless router advantageously allows all three devices (mainly the access point and router) to be configured through one central configuration utility, usually through an integrated web server.

Wireless Ethernet Bridge
A wireless Ethernet bridge connects a wired network to a wireless network. This is different from an access point in the sense that an access point connects wireless devices to a wired network at the data-link layer. Two wireless bridges may be used to connect two wired networks over a wireless link, useful in situations where a wired connection may be unavailable, such as between two separate homes.

Range Extender
A wireless range extender (or wireless repeater) can increase the range of an existing wireless network by being strategically placed in locations where a wireless signal is sufficiently strong and nearby locations that have poor to no signal strength. An example location would be at the corner of an L-shaped corridor, where the access point is at the end of one leg and a strong signal is desired at the end of the other leg. Another example would be 75% of the way between the access point and the edge of its useable signal. This would effectively increase the range by 75%.

DIY Range Optimizations
USB-wifi adapters, food container can-antennas, parabole-reflectors, and many other types of self-built antennae are increasingly made by do-it-yourselvers. For minimal budgets, as low as a few dollars, signal strength and range can be improved dramatically.

Wi-Fi and its support by operating systems
There are two sides to Wi-Fi support under an operating system. Driver level support and configuration and management support.

Driver support is usually provided by the manufacturer of the hardware or, in the case of Unix clones such as Linux and FreeBSD, sometimes through open source projects.

Configuration and management support consists of software to enumerate, join, and check the status of available Wi-Fi networks. This also includes support for various encryption methods. These systems are often provided by the operating system backed by a standard driver model. In most cases, drivers emulate an ethernet device and use the configuration and management utilities built into the operating system. In cases where built in configuration and management support is non-existent or inadequate, hardware manufacturers may include their own software to handle the respective tasks.

Microsoft Windows
Microsoft Windows has comprehensive driver-level support for Wi-Fi, the quality of which depends on the hardware manufacturer. Hardware manufactures almost always ship Windows drivers with their products. Windows ships with very few Wi-Fi drivers and depends on the OEMs and device manufactures to make sure users get drivers. Configuration and management depend on the version of Windows.

Earlier versions of Windows, such as 98, ME and 2000 do not have built-in configuration and management support and must depend on software provided by the manufacturer
Microsoft Windows XP has built-in configuration and management support. The original shipping version of Windows XP included rudimentary support which was dramatically improved in Service Pack 2. Support for WPA2 and some other security protocols require updates from Microsoft. To make up for Windows’ inconsistent and sometimes inadequate configuration and management support, many hardware manufacturers include their own software and require the user to disable Windows’ built-in Wi-Fi support
Microsoft Windows Vista is expected to have improved Wi-Fi support over Windows XP. The original betas automatically connected to unsecured networks without the user’s approval. This is a large security issue for the owner of the respective unsecured access point and for the owner of the Windows Vista based computer because shared folders may be open to public access. The release candidate (RC1 or RC2) does not continue to display this behavior, requiring user permissions to connect to an unsecured network, as long as the user account is in the default configuaration with regards to User Account Control.

Apple Mac OS X & Mac OS
Apple was an early adopter of Wi-Fi, introducing its AirPort product line, based on the 802.11b standard, in July 1999. Apple makes the Mac OS operating system, the computer hardware, and the accompanying drivers and configuration and management software, simplifying Wi-Fi integration. All Intel based Apple computers either come with or have the option to included AirPort Extreme cards. These cards are compatible with 802.11g. Many of Apple’s earlier PowerPC models came with Airport Extreme as well, and all Macs starting with the original iBook at least included AirPort slots.

Mac OS X has Wi-Fi support, including WPA2, and ships with drivers for Apple’s AirPort cards. Many third-party manufacturers make compatible hardware along with the appropriate drivers which work with Mac OS X’s built-in configuration and management software. Other manufacturers distribute their own software.
Apple's older Mac OS 9 does not have built in support for Wi-Fi configuration and management nor does it ship with Wi-Fi drivers, but Apple provides free drivers and configuration and management software for their AirPort cards for OS 9, as do a few other manufacturers. Versions of Mac OS before OS 9 predate Wi-Fi and do not have any Wi-Fi support.

Unix-like systems
Linux, FreeBSD and similar Unix-like clones have much coarser support for Wi-Fi. Due to the open source nature of these operating systems, many different standards have been developed for configuring and managing Wi-Fi devices. The open source nature also fosters open source drivers which have enabled many third party and proprietary devices to work under these operating systems. See Comparison of Open Source Wireless Drivers for more information on those drivers.

Linux has patchy Wi-Fi support[1]. Native drivers for many Wi-Fi chipsets are available either commercially or at no cost[2], although some manufacturers don't produce a Linux driver, only a Windows one. Consequently, many popular chipsets either don't have a native Linux driver at all, or only have a half-finished one. For these, the freely available NdisWrapper and its commercial competitor DriverLoader[3] allow Windows x86 NDIS drivers to be used on x86-based Linux systems but not on other architectures. The FSF has some recommended cards[1] and more information can be found through the searchable Linux wireless site[2] As well as the lack of native drivers, some Linux distributions do not offer a convenient user interface and configuring Wi-Fi on them can be a clumsy and complicated operation compared to configuring wired Ethernet drivers[4].
FreeBSD has similar Wi-Fi support relative to Linux. Wi-Fi support under FreeBSD is best in the 6.x versions, which introduced full support for WPA and WPA2, although in some cases this is driver dependent. FreeBSD comes with drivers for many wireless cards and chipsets, including those made by Atheros, Ralink, Cisco, D-link, Netgear, and many Centrino chipsets, and provides support for others through the ports collection. FreeBSD also has "Project Evil", which provides the ability to use Windows x86 NDIS drivers on x86-based FreeBSD systems as NdisWrapper does on Linux, and Windows amd64 NDIS drivers on amd64-based systems[3].
NetBSD, OpenBSD, and DragonFly BSD have similar Wi-Fi support to FreeBSD. Code for some of the drivers, as well as the kernel framework to support them, is mostly shared among the 4 BSDs.

Social concerns

Unintended and intended use by outsiders
Florida man charged with stealing WiFiThe wireless access point provides no technological protection from unauthorized use of the network. Many business and residential users do not intend to close (secure) their access points but to leave them open for other users in the area. Some argue that it is proper etiquette to leave access points open for others to use just as one can expect to find open access points while on the road.

Measures to deter unauthorized users include suppressing the AP's service set identifier (SSID) broadcast, allowing only computers with known MAC addresses to join the network, and various encryption standards. Access points and computers using no encryption, or the older (pre-2003) Wired Equivalent Privacy (WEP) encryption are vulnerable to eavesdropping by an attacker armed with packet sniffer software. If the eavesdropper has the ability to change his MAC address then he can potentially join the network by spoofing an authorised address.

WEP encryption can protect against casual snooping but may also produce a misguided sense of security since freely available tools such as AirSnort can quickly recover WEP encryption keys. Once it has seen 5-10 million encrypted packets, AirSnort will determine the encryption password in under a second.[5] The newer Wi-Fi Protected Access (WPA) and IEEE 802.11i (WPA2) encyption standards do not have the serious weaknesses of WEP encryption.

Recreational exploration of other people's access points has become known as wardriving, and the leaving of graffiti describing available services as warchalking. These activities may be illegal in certain jurisdictions, but existing legislation and case-law is often unclear.

However, it is also common for people to unintentionally use others' Wi-Fi networks without explicit authorization. Operating systems such as Windows XP and Mac OS X automatically connect to an available wireless network, depending on the network configuration. A user who happens to start up a laptop in the vicinity of an access point may find the computer has joined the network without any visible indication. Moreover, a user intending to join one network may instead end up on another one if the latter's signal is stronger. In combination with automatic discovery of other network resources (see DHCP and Zeroconf) this could possibly lead wireless users to send sensitive data to the wrong destination, as described by Chris Meadows in the February 2004 RISKS Digest.

In Singapore, using another person's Wi-Fi network is illegal under the Computer Misuse Act; A 17 year old has been arrested for simply tapping into his neighour's wireless Internet connection and faces up to 3 years' imprisonment and a fine.

Wi-Fi vs. amateur radio
In the US and Australia, a portion of the 2.4 GHz Wi-Fi radio spectrum is also allocated to amateur radio users. In the US, FCC Part 15 rules govern non-licenced operators (i.e. most Wi-Fi equipment users). Under Part 15 rules, non-licensed users must "accept" (e.g. endure) interference from licensed users and not cause harmful interference to licensed users. Amateur radio operators are licensed users, and retain what the FCC terms "primary status" on the band, under a distinct set of rules (Part 97). Under Part 97, licensed amateur operators may construct their own equipment, use very high-gain antennas, and boost output power to 100 watts on frequencies covered by Wi-Fi channels 2-6. However, Part 97 rules mandate using only the minimum power necessary for communications, forbid obscuring the data, and require station identification every 10 minutes. Therefore, expensive automatic power-limiting circuitry is required to meet regulations, and the transmission of any encrypted data (for example https) is questionable.

In practice, microwave power amplifiers are expensive and decrease receive-sensitivity of link radios. On the other hand, the short wavelength at 2.4 GHz allows for simple construction of very high gain directional antennas. Although Part 15 rules forbid any modification of commercially constructed systems, amateur radio operators may modify commercial systems for optimized construction of long links, for example. Using only 200 mW link radios and high gain directional antennas, a very narrow beam may be used to construct reliable links with minimal radio frequency interference to other users.

History

Official Wi-Fi logoWi-Fi uses both single carrier direct-sequence spread spectrum radio technology (part of the larger family of spread spectrum systems) and multi-carrier OFDM (Orthogonal Frequency Division Multiplexing) radio technology. Unlicensed spread spectrum was first authorized by the Federal Communications Commission in 1985 and these FCC regulations were later copied with some changes in many other countries enabling use of this technology in all major countries. These regulations then enabled the development of Wi-Fi, its onetime competitor HomeRF, and Bluetooth.

The precursor to Wi-Fi was invented in 1991 by NCR Corporation/AT&T (later Lucent & Agere Systems) in Nieuwegein, the Netherlands. It was initially intended for cashier systems; the first wireless products were brought on the market under the name WaveLAN with speeds of 1 Mbit/s to 2 Mbit/s. Vic Hayes, who was the primary inventor of Wi-Fi and has been named the 'father of Wi-Fi,' was involved in designing standards such as IEEE 802.11b, 802.11a and 802.11g.

Origin and meaning of the term 'Wi-Fi'
Despite the similarity between the terms 'Wi-Fi' and 'Hi-Fi', statements reportedly, made by Phil Belanger of the Wi-Fi Alliance contradict the popular conclusion that 'Wi-Fi' stands for 'Wireless Fidelity.' According to Mr. Belanger, the Interbrand Corporation developed the brand 'Wi-Fi' for the Wi-Fi Alliance to use to describe WLAN products that are based on the IEEE 802.11 standards. In Mr. Belanger's words, "Wi-Fi and the yin yang style logo were invented by Interbrand. We (the founding members of the Wireless Ethernet Compatibility Alliance, now called the Wi-Fi Alliance) hired Interbrand to come up with the name and logo that we could use for our interoperability seal and marketing efforts. We needed something that was a little catchier than 'IEEE 802.11b Direct Sequence'."

The Wi-Fi Alliance themselves invoked the term 'Wireless Fidelity' with the marketing of a tag line, "The Standard for Wireless Fidelity," but later removed the tag from their marketing. The Wi-Fi Alliance now seems to discourage propagation of the notion that 'Wi-Fi' stands for 'Wireless Fidelity', but it has been referred to as such by the Wi-Fi Alliance in White Papers currently held in their knowledge base: