IP telephony

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IP telephony (IPT) is the uniting of voice over IP (VoIP) and telephony capabilities, specifically the whole world of signaling.

The Holy Grail: Uniting Voice and Data

If there is one application that has been most closely associated with the word convergence it is IP telephony (IPT). Voice data integration in the same network has been a goal for many in the industry since the wall came down between voice and data in the late 1960s. It was in the late 1960s that packet switching emerged as a concept in the original ARPAnet. Packet-based data networks were designed with data in mind. Packets were moved through the network first-come-first-served. Switches could store packets before forwarding them, and would do so if the target output link was currently being used to transmit a packet. Packet networks emerged as queuing networks where bandwidth, delay, and jitter could all vary widely.

Circuit networks, the model on which traditional telephone networks were built, put a circuit in place through the network for a particular communication and fix the bandwidth, jitter, and delay of that path for the duration of the communication.

If these worlds were ever combined, circuits were borrowed from the circuit network to carry packets. Getting better-than-necessary behavior may be inefficient, but it doesn’t break applications. Packets were never taken from data networks to carry interactive voice.

As time marched on, however, two trends came together. First, we created ways to add class of service (CoS) and quality of service (QoS) to the packet network. Second, the world of wireless telephony exploded, effectively lowering the expectations of the users concerning voice quality in the network. The IPT era began around 1996 and moved aggressively into the mainstream around 2005.

Defining VoIP and IPT

Defining VoIP and IPT

The terms Voice over IP (VoIP) and IP Telephony (IPT) are often used interchangeably. There is an important difference however. VoIP is the art and science of placing voice in IP packets. IPT is VoIP plus signaling and features. Note this means that all IPT solutions are VoIP, but not all VoIP solutions are IPT.

The classic example of a VoIP application is replacing the tie lines between traditional PBXs with an IP backbone. The backbone network is simply being used to carry voice between the two PBXs. It is a form of trunking. Embedding voice in an application is often simply VoIP. For example, several e-learning platforms (e.g., CentraOne), make it possible for an instructor to speak in real time to the students in the virtual classroom. Students can also be given the chance to respond or ask questions, also within the application. This is a VoIP enabled application.

IPT is about doing what people do with telephones: it’s about making calls. The telephones involved are IP-based phones. The signaling intelligence to determine who is calling whom is supported using IP-based protocols like the Session Initiation Protocol (SIP).

Cisco has further muddied the waters by introducing the more vacuous term IP Communications (IPC). In the Cisco world, this reflects a completely converged network that includes data, voice, and video.

IPT Network Components

An IP Telephony (IPT) solution can comprise several components. The central component of the solution, or any IP network for that matter, is a routed backbone. There are many makes and models of routers, with each manufacturer claiming that its approach to router design has some fundamental advantage over its competitors. Typically, routers with very low latencies (e.g., can move packets through the router quickly) are better. The routers in the core of the network may need to support additional capabilities required by the applications in the network. That might entail such things as the integration of routing with lower layer switching, the support of special routing or reservation protocols, or the ability to communicate and use quality of service (QoS) information of various types.

At the edges of the IP network are the devices that connect the packet world to the traditional circuit-based telephony world: the media gateways. The fundamental purpose of the gateway is to digitally encode and packetize the voice signals so they can be sent across an IP network. The gateway can connect traditional analog phones to the IP backbone, can bridge between the IP world and a traditional PBX world, or can connect the entire PSTN to the IP backbone. Gateways can be stand-alone devices, or they can be integrated into routers or PBXs.

The media gateway controller, sometimes referred to as a call manager, voice server, or softswitch, is responsible for finding destination stations on an IP telephony network and dealing with the authentication, authorization, and accounting (AAA) functions. The controller is responsible for establishing calls between calling parties. The media gateway controller can be a standalone device, running on a conventional server, or integrated into a PBX.

Premises-Based IPT Solution

Finally, there are the user devices. If the corporation has traditional phone systems and PBXs, media gateways can connect them to the IPT infrastructure. Within the IP network, native IPT equipment can exist, including IP telephones, with either an Ethernet or Wi-Fi interface, or softphones. A softphone is IPT software running on a laptop or desktop system.

Premises-Based IPT Solution

The visual depicts a premises-based deployment of an IPT solution as it might be deployed by a company. The media gateway controller has a link to the PSTN to integrate SS7 information into the solution. The visual depicts a single IP PBX with an integrated media gateway. Additional PBXs, IP telephones, traditional phones, and softphones could exist at other offices as well. The carrier network backbone is a QoS-capable data service.

Carrier-Based IPT Solution

Carrier-Based IPT Solution

The visual depicts a carrier-based deployment of an IPT solution. The media gateway controller and media gateway have migrated into the carrier infrastructure, leaving only the various kinds of IP telephones on the premises. Additional IP telephones and softphones could exist at other offices as well. Note that this model makes it slightly more difficult to integrate traditional phones on the premises. Depending on the carrier service offered, this can be achieved using traditional Centrex services, or by placing an IP PBX (or the equivalent thereof) on the premises.


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