Voice over IP Basics (2): Gateway's

Voice over IP Basics (2): Gateway's

January 11, 2001

Bart Teunis


As we all know the Internet spans the whole world and you can find a place to connect to the Internet at the most strange places in the world (i.e. easyinternet from EasyEverything). Today many websites are sound enabled. Voice over IP uses the IP protocol to send audio between one or more computers in real time, so that users can converse. When Vocaltec introduced their first IP telephony software in 1995, they caught the world's attention. VoIP has become one of the hottest items at the moment, but VoIP wouldn't be that hot if there was no connection between the Internet and the traditional PSTN networks. If there where no connection you have two separate networks and that is not what we were waiting for. To make a connection between the two networks you use a gateway.

What does a gateway:

By bridging the traditional circuit-switched telephony world to IP related networks, gateways offer the advantages of VoIP to the most common, cheapest, most mobile, and easiest-to-use terminal in the world : the standard telephone.
Gateways overcome another significant VoIP problem: addressing. To address a remote user on a multimedia PC or IP-phone, you must know the user's IP address. To address a remote user with a gateway product you only need to know the user's phone number.

How does it work:

Picture 1 represents a simple layout of a gateway.

Picture 1: Simple Gateway scheme

On the Phone to network connection the gateway connects to the PSTN. You can connect any phone you like at this side. At the other side the gateway connects to an IP network, this side can communicate with every well equipped PC or IP phone in the big world of communication.
The gateway takes the standard telephone signal ( if it is analog, the gateway digitizes it) and compresses it significantly. After that the gateway makes the packets, which are needed to make use of the IP protocol and routes it to the destination on the IP network. The gateway also reverses the operation for the signal, which comes from the IP network and want to go to the PSTN network. It is important that the both operations take place in the same time to allow a full-duplex (two way) conversation.

Two very important functions of an gateway are:

  1. Reduce latency, humans can tolerate 250 msec of latency before it has a noticeable effect, so the gateway must can reduce this latency otherwise the connections will sound like a traditional call routed over a satellite circuit.
  2. Echo cancellation


In the article above I described the most basic features of the gateway and what they mean in the VoIP world.