VoIP Gateways Demystified

By Lori Shepherd
Updated: February 01, 2011

For IT Managers and business owners who are planning to implement a Voice over Internet Protocol (VoIP) solution, will also need to consider choosing the perfect VoIP gateway for their business environment. VoIP gateways aren’t rocket science, but deciphering through the options may seem daunting. Here’s a straightforward approach to consider which helps demystify this technology.

Gateways: What are they?

First and foremost, VoIP gateways are nothing more than hardware that converts TDM telephony traffic from the publicly switched telephone network (PSTN) into digital IP packets, so that it can be transported over an IP network, such as a LAN. Think of VoIP gateways as a bridge between an IP network and the PSTN. Where voice traffic originates from a VoIP gateway, it will be converted into a form to receive by the destination IP or PSTN network. Therefore if voice traffic originates from the PSTN, the VoIP gateway converts the analog voice signal into a digital signal.

This digital signal gets compressed using a codec, and arranges it into packets which are transferred across the IP network using a signaling protocol. When voice traffic originates from an IP network, the VoIP gateway decompresses the digital packets into a digital signal which is converted into an analog signal to be transferred across the PSTN.

VoIP Protocols and Codecs

With VoIP gateways now defined, you’ll also want to consider VoIP protocols and codecs of which gateways are dependent. A VoIP compatible protocol is technology which determines how a voice packet is transported across a network, and codecs, the voice streaming compression digital packet, are crucial. Without this compatibility, gateways cannot work properly with your VoIP phone system or hosted service, not to mention determining overall call quality.

Analog and Digital

Lastly, understanding that there are two main gateways—analog and digital – is important:

Analog

An analog VoIP gateway is used to connect traditional analog telephones to a VoIP phone system or to connect VoIP phone system to the PSTN. Due to this dual purpose an analog VoIP gateway comes in two different forms – FXS and FXO.

  • FXS gateway – An FXS gateway is used to connect traditional telephones and fax machines to a VoIP phone system.
  • FXO gateway – An FXO gateway is used to connect VoIP phone systems to PSTN lines.

Digital 

A digital VoIP gateway is used to connect VoIP Phone systems to digital voice lines such as T1/E1/BRI. A digital VoIP gateway can also be used to connect traditional PBX systems to IP networks.

Essentially analog and digital VoIP gateways accomplish the same thing.

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