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Voice over IP(Redirected from VoIP)
IP Telephony, also called 'Internet telephony', is technology that enables routing of voice conversations over the Internet or a dedicated Internet Protocol (IP) network instead of dedicated voice transmission lines. This allows elimination of circuit switching and the associated waste of bandwidth. Instead, packet switching is used, where IP packets with voice data are sent over the network only when data needs to be sent, i.e., when a caller is talking. Advantages over traditional telephony:
Voice over IP traffic may be deployed on any IP network, without an internet connection, for instance on a LAN inside a building. The protocols used to carry the signal over the IP network are commonly referred to as Voice over IP or VoIP protocols.
Corporate and telco use of VoIPAlthough 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. IP telephony is commonly used to route traffic starting and ending at conventional PSTN (Public Switched Telephone Network) telephones. VoIP is widely deployed by carriers, especially for international telephone calls. 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. Companies can acquire their own gateways, eliminating third-party costs — worthwhile in some situations. VoIP implementation challengesBecause IP does not provide any mechanism to ensure that data packets are delivered in sequential order, or provide any Quality of Service guarantees, implementations of VoIP face problems dealing with latency and possibly data integrity. One of the central challenges for VoIP implementers is restructuring streams of received IP packets, which can come in any order and have packets missing, to ensure that the ensuing audio stream maintains a proper time consistency. To help with this, the network provider can ensure that there is enough bandwidth end-to-end to guarantee low-latency, high quality voice. This is easy to do in private networks, but much more difficult to do with less than 256 kbit/s without a fragmentation mechanism. The other main challenge is getting VoIP traffic to traverse firewalls and NAT. Intermediary devices called Session Border Controllers are often used to achieve this, though some proprietary systems such as Skype do firewall and NAT traversal without the need for an SBC. Keeping packet latency down to acceptable levels on satellite circuits can also be a problem, simply due to the distance of the transmissions. VoIP protocolsIn the overwhelming majority of implementations, RTP is used to transmit VoIP traffic ("media"). Notable exception is IAX which carries both signaling and voice data over a single UDP stream, which results in fewer problems while traversing firewall and NAT devices. For signaling, there are several alternative protocols:
Several different speech codecs can be used for stream audio compression. Commonly used codecs for VoIP traffic include G.711 and G.729, both ITU-T-specified codecs. Mass-market telephony over broadband Internet accessA new development 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. This requires an analog telephone adapter (ATA) to connect a telephone to the broadband Internet connection. Companies in the US, such as Broadvoice, Verizon, Vonage, VoicePulse, and Packet8, use IP to offer unlimited calling to the US, and sometimes to Canada or to selected countries in Europe or Asia, for a flat monthly fee. One advantage of this is the ability to make and receive calls as one would at home, anywhere in the world, at no extra cost. As calls go via IP, this does not incur charges as call diversion does via the PSTN, and the called party does not have to pay for the call. For example, somebody may call someone on a number with a US area code, but one could be in London, and if someone were to call another number with that area code, it would be treated as a local call, regardless of where that person is in the world. However, the broadband phone is likely to complement, rather than replace a PSTN line, as it still needs a power supply, while calling the US emergency services number 911, may not automatically be routed to the nearest local emergency dispatch center, or be of any use for subscribers outside the US. 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 a hitch, but in other cases they won't 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. There is also a free service called Free World Dialup (FWD), that permits users to make free telephone calls to other FWD users, although has only limited connections to and from the public switched telephone network. See also
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