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April 25, 2011

What is a VoIP Gateway?

By Mae Kowalke, TMCnet Contributor


Despite the power and usefulness of the technology involved, people are often confused by VoIP gateways. So much so, in fact, that of all types of VoIP equipment, gateways are returned the most simply because it is not clear how, when, where and why to use them.

In a recently published guide, VoIP Supply takes the mystery out of VoIP gateways by explaining the technology and offering straightforward tips on how to best use it. This article summarizes the guide’s main points about what a VoIP gateway is and how it works.

A VoIP gateway converts telephony traffic from analog (Time Division Multiplexing, or TDM) to digital (Internet Protocol, or IP) and vice-versa.

Traffic coming in from the public switched telephone network (PSTN) is fed through a gateway and converted to digital packets so it can be transported over a local area network (LAN) or other IP-based network.

Conversely, digital IP traffic is fed back through a VoIP gateway for conversion back to analog so it can be transported out over the PSTN.

In other words, a VoIP gateway acts as a bridge between an IP network and the PSTN.

Things get a bit more complex when considering two other factors: VoIP protocols and voice codecs. A protocol is a method of transporting voice packets across a network. A voice codec is a method of encoding and/or decoding digital data streams or signals.

For a VoIP gateway to work properly, the protocol and codec employed must be compatible with the VoIP phone system and/or VoIP service being used. Incompatible protocols and codecs can cause the system to not work at all, or can result in decreased call quality.

Most VoIP gateways support a single protocol. The most common are Session Initiation Protocol (SIP), Cisco (News - Alert) Skinny Client Control Protocol (CSCCP, proprietary for Cisco equipment), MGCP (mostly outdated), and H.323 (mostly outdated).

Most VoIP gateways support multiple codecs. The codec used determines how much the voice stream or signal is compressed, and this affects both call quality and bandwidth needed. More compression means lower voice quality but also less bandwidth demanded.

The most common voice codecs used by VoIP gateways are GSM (13 Kbps), iLBC (15 Kbps), G.711 (64 Kbps), G.722 (48/56/64 Kbps), G726 (16/24/32/40 Kbps), G.728 (16 Kbps) and G.729 (8 Kbps).

VoIP gateways come in two types: analog and digital. Analog gateways connect traditional phones to a VoIP phone system, or a VoIP phone system to the PSTN. Digital gateways connect a VoIP phone system to digital voice lines (e.g. a T1) or to connect a traditional PBX (News - Alert) system to an IP network.

Read the full guide for more about how VoIP gateways work, an overview of common features and standard configurations, and a five-step method for selecting a gateway.


Mae Kowalke is a TMCnet contributor. She is Manager of Stories at Neundorfer, Inc., a cleantech company in Northeast Ohio. She has more than 10 years experience in journalism, marketing and communications, and has a passion for new tech gadgets. To read more of her articles, please visit her columnist page.

Edited by Patrick Barnard




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