As the world moves inexorably towards all-IP, there are still businesses today that use a TDM (time division multiplexing) PBX-based phone system. Eventually, they will end up switching to an IP PBX when their old equipment becomes unserviceable or irreplaceable, but for whatever reason they have chosen not to migrate yet. Even so, there are ways to help these companies reap some advantages from VoIP technology while hanging onto their legacy system.

The solution involves adding one or more gateways. Known as a media gateway, VoIP gateway, IAD (Internet access device) and a few other names, a gateway is a piece of hardware that bridges different types of networks. Its function is to convert audio from the format provided by the PBX (e.g., Q.931 call control, µ-law speech) to packet-based data for the IP network (e.g., SIP call control, G.729A speech) and vice versa.

An FXS gateway will allow you to connect the legacy phone system to an ITSP (Internet telephony service provider) or SIP trunks. This allows least-cost routing of phone calls over both the PSTN (public switched telephone network) and the Internet. Because long-distance calls placed over the Internet are free, this can generate substantial cost savings for the company.

When an FXS gateway is used in conjunction with an FXO gateway, you can pull extensions off a PBX and across the Internet to have analog extensions at another location (branch office) that are part of the same system. So not only do gateways allow for importing SIP trunks into an analog phone system, they allow you to have office extensions at different locations from a single PBX (see diagram).

Wondering how to know when to use an FXO versus an FXS gateway? Just think about the ports the gateway needs. If the gateway is connected to the FXS port on the PBX, then it needs FXO ports (an FXO gateway). If it is connected to the FXO ports on analog endpoints, then it needs FXS ports (FXS gateway). Keep in mind that an FXS port is always on one end of a single connection and an FXO port is on the other end; they work in pairs. For more information about FXS and FXO gateways, download our Ultimate Guide to Media Gateways.

Installing gateways is also a great first step towards familiarizing the company with VoIP. As we mention in our blog post about whether a business should go all-IP, migrating to an IP PBX will eventually be inevitable. It’s better to help them take a gentle step forward now so the eventual migration will be easier.

CONCLUSION

Adding gateways is a great way to get more value out of a legacy PBX system and reduce costs for your customer, all while helping them to get ready for an eventual migration to IP.