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IP Telephony also called Internet telephony, is the technology that makes it possible to
have a telephone conversation where the signal is carried over the Internet or a dedicated network in Internet Protocol (IP) packets, instead of over dedicated voice
transmission lines. This allows the 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.
Its advantages over traditional telephony include:
- lower costs per call, especially for long-distance calls
- lower infrastructure costs: once IP infrastructure is installed, no or little additional telephony infrastructure is
needed.
Note that voice over IP traffic does not necessarily have to travel over the public Internet: it may also be deployed on private IP networks.
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 VoIP
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 between their switching stations, where they convert the
dedicated voice signal 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 support centers which provide support over telephone often use IP telephony exclusively to take advantage
of the data abstraction that comes with it.
The benefit of using this technology is the need for only one class of circuit connection and better use of the available
bandwidth. IP telephony is commonly used to route traffic that may be originated from and terminated at conventional PSTN telephones.
VoIP is now widely deployed by carriers, especially for international telephone calls. Most commonly, users are completely
unaware that their telephone call is being routed over IP infrastructure for most of its distance, instead of entirely over the
circuit switched PSTN.
VoIP is also used by large companies to eliminate call charges between their offices, by using their data network to carry
inter-office calls. They may also use VoIP to reduce the costs of calls outside the company, by carrying them to the nearest
point on their network before handing them off to the PSTN.
There are companies which offer a gateway to the PSTN from any VoIP phone. You can simply dial a conventional telephone number
and the telephone call will be routed over your internet connection to the company that operates the gateway, and they will bill
you, not the local phone company. Electronic Numbering
(ENUM) makes it possible to dial traditional E.164 phone numbers, but be connected
entirely over the internet if the other party uses Enum, so you do not incur any expenses other than the internet connection
fees.
VoIP implementation challenges
Because IP does not by default 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 possible data integrity problems.
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. Another important
challenge is keeping packet latency down to acceptable levels, so that users do not experience significant lag time between when
they speak and the signal is decoded on the other end of the connection.
Solutions to these problems:
- Certain hardware solutions can distinguish VoIP packets and provide priority queuing for this class of service.
- Alternatively packets can be buffered but this can lead to an overall delay similar to that encountered on satellite
circuits.
- The network operator can also ensure that there is enough bandwidth end-to-end to guarantee low-latency low-loss traffic:
this is easy to do in private networks, but much harder to do in the public Internet.
- Jitter (delay variance) problems are mainly generated in lowband access (less than 256 Kbps) because of serialization of big
(1500 bytes) data packets. At these rates, fragmentation mechanisms for these big packets are needed (interleaving small voice
packets) to reduce the delay. Over networks slower than 256 Kbps it is almost impossible to ensure quality voice without a
fragmentation mechanism.y
VoIP protocols
In the overwhelming majority of implementations, the RTP protocol is used to transmit VoIP
traffic ("media").
For signaling, there are several alternative protocols:
- SIP, the IETF Session Initiation Protocol, a
newcomer gaining popularity
- H.323, an older protocol still used by many legacy applications
- Skinny Client Control Protocol,
proprietary protocol from Cisco
- MeGaCo (a.k.a. H.248) and MGCP, both Media Gateway control protocols
- MiNET, proprietary protocol from Mitel
- IAX, the Inter-Asterisk eXchange protocol used by the Asterisk open source PBX server and associated client software.
Mass-market telephony over broadband Internet access
A 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 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 you 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 you on a number with a US area code, but you
could be in London, and if you were to call another number with that area code, it
would be treated as a local call, regardless of where you are 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 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 can no longer be assumed that a conventional voice-grade telephone line will be available in
nearly every home in the United States and Canada.
There is also a free service called Free World Dialup (FWD),
that permits users to make free telephone calls to other FWD users, and that has only limited connections to and from the
public switched telephone
network.
See also
External links
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