The two-way transmission of voice over a packet-switched IP network, which is part of the TCP/IP protocol suite. The terms "IP telephony" and "voice over IP" (VoIP) are synonymous. However, the term VoIP is widely used for the actual services offered (see VoIP for more details), while IP telephony often refers to the technology behind it. In addition, IP telephony is an umbrella term for all real-time applications over IP, including voice over instant messaging (IM) and videoconferencing.
Starting in the late 1990s, the Internet and its TCP/IP protocol suite began to turn the data communications and telephone industry upside down. IP has become the universal transport for almost all data and video communications worldwide. It is increasingly becoming the infrastructure for voice traffic as well. Today, every communications carrier has built or is using an IP backbone for some or all of its voice services. In addition, large enterprises are either already using IP for some amount of internal voice traffic or have plans to implement it or create test beds.
Data Over Voice Became Voice Over Data
Data was first transmitted over telephone networks, starting in the 1960s, and by the late 1980s, data routinely traveled over digital voice circuits. By the 1990s, the majority of worldwide communications traffic had changed from voice to data, and as IP networks began to flourish, the economics of using IP for voice began to emerge.
Although the backbone of the global telephone network had been converted to digital for some time, the circuit-switched nature of the public switched telephone network (PSTN) is wasteful. Even though one person talks and the other listens, both "to" and "from" channels are always dedicated. In addition, newer voice codecs cut the digital requirement from the traditional 64 Kbps (PCM) down to 8 Kbps with respectable quality. Thus, the bandwidth requirement for voice on an IP network is 1/16th that of the PSTN's dedicated, digital circuits.
Varying Quality
Starting in the mid-1990s, advertiser-supported, free telephone service from PC to PC or between phones and PCs using the public Internet became popular, especially for international calls. Call quality over the Internet can be erratic because the Internet provides no guarantee of quality of service (QoS). However, when an organization has control over its network, quality can be excellent. Private enterprises with their own IP networks, as well as major telcos and IP telephony carriers that have developed IP backbones, can provide voice quality that competes with the traditional PSTN.
Transport and Signaling
IP telephony uses two protocols: one for transport and another for signaling. Transport is provided by UDP over IP for voice packets and either UDP or TCP over IP for signals. Signaling commands that establish and terminate the call as well as provide special features such as call forwarding, call waiting and conference calling are defined in a signaling protocol such as H.323, SIP, MGCP or MEGACO (see IP telephony signaling protocol).
The integration of packet-switched IP with the traditional SS7-based telephone system is a complex undertaking with numerous protocols competing for attention. See ITXC and IP on Everything.
Integrating IP Telephony with the PSTN
This illustration shows the interaction between the traditional telco system and IP carriers, which are often one and the same. In order to understand the flow between these devices, it is important to note the difference between voice packets (blue lines) and signaling (red lines). (Illustration assistance courtesy of GNP Computers and Pulver.com.)
Not Quite IP Telephony
We can be certain that they didn't have the IP protocol in mind when they set up this telephone switchboard in 1882. It was used to switch phone calls between all the lawyers in Richmond, Virginia. (Image courtesy of AT&T.)
Starting in the late 1990s, the Internet and its TCP/IP protocol suite began to turn the data communications and telephone industry upside down. IP has become the universal transport for almost all data and video communications worldwide. It is increasingly becoming the infrastructure for voice traffic as well. Today, every communications carrier has built or is using an IP backbone for some or all of its voice services. In addition, large enterprises are either already using IP for some amount of internal voice traffic or have plans to implement it or create test beds.
Data Over Voice Became Voice Over Data
Data was first transmitted over telephone networks, starting in the 1960s, and by the late 1980s, data routinely traveled over digital voice circuits. By the 1990s, the majority of worldwide communications traffic had changed from voice to data, and as IP networks began to flourish, the economics of using IP for voice began to emerge.
Although the backbone of the global telephone network had been converted to digital for some time, the circuit-switched nature of the public switched telephone network (PSTN) is wasteful. Even though one person talks and the other listens, both "to" and "from" channels are always dedicated. In addition, newer voice codecs cut the digital requirement from the traditional 64 Kbps (PCM) down to 8 Kbps with respectable quality. Thus, the bandwidth requirement for voice on an IP network is 1/16th that of the PSTN's dedicated, digital circuits.
Varying Quality
Starting in the mid-1990s, advertiser-supported, free telephone service from PC to PC or between phones and PCs using the public Internet became popular, especially for international calls. Call quality over the Internet can be erratic because the Internet provides no guarantee of quality of service (QoS). However, when an organization has control over its network, quality can be excellent. Private enterprises with their own IP networks, as well as major telcos and IP telephony carriers that have developed IP backbones, can provide voice quality that competes with the traditional PSTN.
Transport and Signaling
IP telephony uses two protocols: one for transport and another for signaling. Transport is provided by UDP over IP for voice packets and either UDP or TCP over IP for signals. Signaling commands that establish and terminate the call as well as provide special features such as call forwarding, call waiting and conference calling are defined in a signaling protocol such as H.323, SIP, MGCP or MEGACO (see IP telephony signaling protocol).
The integration of packet-switched IP with the traditional SS7-based telephone system is a complex undertaking with numerous protocols competing for attention. See ITXC and IP on Everything.
Integrating IP Telephony with the PSTN
This illustration shows the interaction between the traditional telco system and IP carriers, which are often one and the same. In order to understand the flow between these devices, it is important to note the difference between voice packets (blue lines) and signaling (red lines). (Illustration assistance courtesy of GNP Computers and Pulver.com.)
Not Quite IP Telephony
We can be certain that they didn't have the IP protocol in mind when they set up this telephone switchboard in 1882. It was used to switch phone calls between all the lawyers in Richmond, Virginia. (Image courtesy of AT&T.)
 | Reproduced with permission from Computer Desktop Encyclopedia. Copyright (c) 1981-2009 The Computer Language Company Inc. All rights reserved. |
Additional Resources
- PhonerLite 1.68 (Windows)
- PhonerLite is a clearly arranged and easy to used application for Windows. PhonerLite enables your PC to use it for Internet telephony (VoIP, Voice over IP). Pre-conditions are a full-duplex sound card, a microphone and speakers alternatively a headset, an Internet connection and a registration at a provider supporting the...
- Software downloads 2009-11-11
- Is your network ready for IP Telephony?
- Successfully deploying IP telephony to your enterprise also means understanding the requirements for delivering toll-quality voice over your company's network infrastructure and then appropriately planning for, choosing and deploying the right IP telephony solution. Make sure your network is ready for IP telephony, download this paper now.
- White papers 2009-11-10
- Building Reliable IP Telephony Systems
- Reliability is the most critical aspect of a business phone system. IP telephony systems will deliver differing service levels because their architecture is fundamentally different. Ironically, optimal architecture and design does not only deliver high reliability, but also produces a system that is easy to use and manage. ...
- White papers 2009-11-10
- Strategic Guide to Business Phone Systems:
- This 32 page eBook was created to help you understand the key issues involved in choosing and deploying the best UC system for your organization. This guide takes you through the steps in choosing a business phone system and will provide a firm understanding of IP Telephony and Unified Communications.
- White papers 2009-11-10
- Skype plays footsie with open source
- Where to for SkypeIt's always tough to predict where something will end up in the future. Those unexpected little surprises tend to throw things off a bit. I think what Skype is trying to do is expand into a growing market, namely the Linux market. To do...
- Discussion threads 2009-11-04
- InterGate Policy Manager 9.3.1 (Mac)
- InterGate Policy Manager - A comprehensive Internet access management solution, providing fine grained control of web content access, usage of Peer to Peer, Instant Messaging and Skype networks, logging and reporting of user activity, as well a core IP router and firewall to provide advanced protection to your network. Advanced...
- Software downloads 2009-10-09
- Brekeke PBX 2.3.8.2 (Windows)
- An award-winning product, Brekeke PBX is a SIP-based IP-PBX system for call centers, enterprises and service providers. Brekeke PBX solutions are cost-effective and provide flexibility to meet each telephony system's requirements. Brekeke PBX comes in two editions: Basic Edition with traditional telephony features and Pro Edition which includes a full...
- Software downloads 2009-10-04
- Telephony Solution Provider Creates Affordable IP-Based Fax Software
- Commetrex, which specializes in telecommunications technologies, saw an opportunity when Microsoft announced that it would have partners develop fax functions for Microsoft Exchange Server 2010. Commetrex developed BladeWare Fax-to-Email Server (Fax2Email), a cost-effective, easy-to-manage product that provides customers - including small and midsize businesses - with inbound fax functions that...
- Case studies 2009-10-01
- FCC's Net neutrality push: Is wireless access different?
- Stop coddling wireless companiesThey are no longer fledgling upstarts but the new normal. It is simply mind-boggling why they get away with a much looser regulatory framework than landlines. Why is it, for instance, that you do not have a choice of carrier for international calls the way you do...
- Discussion threads 2009-09-22
- The best VoIP solution is ...
- What about hosted?Wondering if you guys would be willing to discuss the most obvious omission... hosted. It's like choosing which car to buy when its cheaper to take a limo.RE: The best VoIP solution is ...Your article was only comprehensive if you consider just the IP-PBX and desktop telephone;...
- Discussion threads 2009-09-22
- Next Softphone 1.0 (Windows)
- NX is a softphone with a Computer Telephony Integration - CTI tool for Asterisk and third party application. API is based on .INI files, through which one can integrate IP telephony to any CRM / ERP software. FEATURES * SIP Proxy authentication. * Dial/Receive phone calls directly from your contact...
- Software downloads 2009-08-25
- VaxTele SIP Server SDK 2.0 (Windows)
- VaxTele SIP Server SDK Development Kit is a new and flexible approach to develop SIP based VoIP servers, IP-PBX, SIP gateways, SIP PBX. It is really easy to develop IP-Telephony, PC to PC and PC to Phone services by using VaxTele SIP Server SDK. VaxTele SIP Server SDK includes COM...
- Software downloads 2009-08-20
- IVM Telephone Answering Attendant 5.02 (Windows)
- IVM is a telephone answering machine, voice mail, call attendant, and interactive voice response IVR program for Windows. It connects to the phone using a professional telephony board or directly to a VoIP service. This software is an effective voice mail, call attendant, info-line, audio text or autodial solution for...
- Software downloads 2009-08-19
- Overcoming IP Telephony Performance Issues in a Converged Network
- When implementing Voice over IP VoIP onto a data network, you must ensure that the equipment capable of handling the additional load, and, more importantly, that the traffic will be properly classified, marked, and prioritized for optimal performance. Voice traffic is highly intolerant to delay, jitter, and packet loss, so...
- White papers 2009-08-07
- OfficeSIP Server 2.4 (Windows)
- Enable your employees call each other for free with OfficeSIP Server. Multi-office and trans-national corporations pay thousands of dollars for long-distance calls every month even after considerate savings made possible with IP telephony. Voice over IP is great for saving money on long-distance calls, so why not to make the...
- Software downloads 2009-07-22
- Alaskan Firm Taps Microsoft Partners to Save $1 Million With Unified Communications
- Alutiiq serves customers worldwide with construction, IT, and other services. By 2008, it needed to upgrade its IP-based telephony system, but its existing vendor was unresponsive and upgrade costs were high. Instead, Alutiiq chose Gold Systems, a Microsoft Gold Certified Partner, to deploy Microsoft Office Communications Server 2007 and Microsoft...
- Case studies 2009-07-01
- Unified Communications: IP Networks, Connecting People Any Time, Any Where
- Business communications in today's work culture is having a significant impact on how companies and people operate. People want information right now, want to share complex applications to brainstorm ideas, make decisions, etc. This is driven by global workforce, rapid travel and faster business cycles. Applications like instant messaging, IP...
- White papers 2009-06-26
- Video Telephony: Completing the Picture of Unified Communications Video Telephony: Completing the Picture of Unified Communications Effectiveness
- Unified communications UC is a term that has been heralded in the telephony and network markets as the IT architecture that will streamline business processes and break down the distinct silos of communications within the enterprise. IDC defines unified communications as a solution platform that adds advanced IP telephony calling...
- White papers 2009-06-17
- IP Solutions Firm Adds Collaboration Features to All-in-One Communications Software Suite
- Interactive Intelligence is a global provider of unified communications solutions for contact center automation, enterprise IP telephony, and enterprise messaging. Founded in 1994, the company has more than 3,000 customers worldwide and is headquartered in Indianapolis, Indiana. Interactive Intelligence has long recognized the benefits of building its contact center, enterprise...
- Case studies 2009-06-01
- Automobile Company Cuts Communications Costs by 50 Percent With Unified Solution
- Automobilgesellschaft Weilbacher, an authorized Mercedes-Benz sales and service company in Germany, has five locations throughout the country and nearly 200 employees. Each day, the company receives more than 300 customer calls, which are routed through a central call center. The company had a mix of IP telephony systems supporting its...
- Case studies 2009-06-01
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