???? ????? ??????????: Protocols

Voice over IP has been implemented in various ways using both?proprietary?and?open protocols?and standards. Examples of technologies used to implement Voice over IP include:

• H.323
• IP Multimedia Subsystem?(IMS)
• Media Gateway Control Protocol (MGCP)
• Session Initiation Protocol?(SIP)
• Real-time Transport Protocol?(RTP)
• Session Description Protocol?(SDP)

The H.323 protocol was one of the first VoIP protocols that found widespread implementation for long-distance traffic, as well as local area network services. However, since the development of newer, less complex protocols, such as MGCP and SIP, H.323 deployments are increasingly limited to carrying existing long-haul network traffic. In particular, the Session Initiation Protocol (SIP) has gained widespread VoIP market penetration.

A notable proprietary implementation is the?Skype protocol, which is in part based on the principles of?Peer-to-Peer?(P2P) networking.

Consumer market

A major development that started in 2004^[4]?was the introduction of mass-market VoIP services that utilize existing?broadband Internet access, by which subscribers place and receive telephone calls in much the same manner as they would via the?public switched telephone network?(PSTN). Full-service VoIP phone companies provide inbound and outbound service with?Direct Inbound Dialing. Many offer unlimited domestic calling for a flat monthly subscription fee. This sometimes includes international calls to certain countries. Phone calls between subscribers of the same provider are usually free when flat-fee service is not available.^[5]

A?VoIP phone?is necessary to connect to a VoIP service provider. This can be implemented in several ways:

• Dedicated VoIP phones connect directly to the IP network using technologies such as wiredEthernet?or wireless?Wi-Fi. They are typically designed in the style of traditional digital business telephones.
• An?analog telephone adapter?is a device that connects to the network and implements the electronics and firmware to operate a conventional analog telephone attached through a modular phone jack. Some residential Internet gateways and?cablemodems?have this function built in.
• A?softphone?is application software installed on a networked?computer?that is equipped with a microphone and speaker, or headset. The application typically presents a dial pad and display field to the user to operate the application by mouse clicks or keyboard input.

PSTN and mobile network providers

It is becoming increasingly common for telecommunications providers to use VoIP telephony over dedicated and public IP networks to connect switching stations and to interconnect with other telephony network providers; this is often referred to as "IP?backhaul".^[6][7]

Smartphones?and?Wi-Fi?enabled mobile phones may have SIP clients built into the firmware or available as an application download. Such clients operate independently of the mobile telephone phone network and use either the cellular data connection or WiFi to make and receive phone calls.

[edit]Corporate use

Because of the bandwidth efficiency and low costs that VoIP technology can provide, businesses are gradually beginning to migrate from traditional copper-wire telephone systems to VoIP systems to reduce their monthly phone costs.

VoIP solutions aimed at businesses have evolved into "unified communications" services that treat all communications?phone calls, faxes, voice mail, e-mail, Web conferences and more?as discrete units that can all be delivered via any means and to any handset, including cellphones. Two kinds of competitors are competing in this space: one set is focused on VoIP for medium to large enterprises, while another is targeting the small-to-medium business (SMB) market.^[8]

VoIP allows both voice and data communications to be run over a single network, which can significantly reduce infrastructure costs.^[9]

The prices of extensions on VoIP are lower than for PBX and key systems. VoIP switches may run on commodity hardware, such as PCs or Linux systems. Rather than closed architectures, these devices rely on standard interfaces.^[9]

VoIP devices have simple, intuitive user interfaces, so users can often make simple system configuration changes. Dual-mode cellphones enable users to continue their conversations as they move between an outside cellular service and an internal?Wi-Fi?network, so that it is no longer necessary to carry both a desktop phone and a cellphone. Maintenance becomes simpler as there are fewer devices to oversee.^[9]

Skype, which originally marketed itself as a service among friends, has begun to cater to businesses, providing free-of-charge connections between any users on the Skype network and connecting to and from ordinary?PSTN?telephones for a charge.^[10]

In the United States the Social Security Administration (SSA) is converting its field offices of 63,000 workers from traditional phone installations to a VoIP infrastructure carried over its existing data network.^[11][12]

Operational cost

VoIP can be a benefit for reducing communication and infrastructure costs. Examples include:

• Routing phone calls over existing data networks to avoid the need for separate voice and data networks.^[13]
• Conference calling, IVR, call forwarding, automatic redial, and caller ID features that traditional?telecommunication companies (telcos)normally charge extra for, are available free of charge from?open source?VoIP implementations

[edit]Flexibility

VoIP can facilitate tasks and provide services that may be more difficult to implement using the PSTN. Examples include:

• The ability to transmit more than one telephone call over a single broadband connection.
• Secure calls using standardized protocols (such as?Secure Real-time Transport Protocol). Most of the difficulties of creating a?secure telephone?connection over traditional phone lines, such as digitizing and digital transmission, are already in place with VoIP. It is only necessary to?encrypt?and?authenticate?the existing data stream.
• Location independence. Only a sufficiently fast and stable Internet connection is needed to get a connection from anywhere to a VoIP provider.
• Integration with other services available over the Internet, including video conversation, message or data file exchange during the conversation, audio conferencing, managing address books, and passing information about whether other people are available to interested parties.
• Unified Communications, the integration of VoIP with other business systems including E-mail, Customer Relationship Management (CRM), and Web systems.

Source: http://english-eschool.blogspot.com/2011/05/protocols.html

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