This blog post will explain what VoIP gateways are. VoIP gateways connect legacy phone systems to VoIP resources. VoIP gateways connect VoIP phone systems with legacy phone lines. Someone could add VoIP resources to a legacy phone system to save money and add features. Gateways connect to legacy phone systems through digital or analog trunk ports. There are analog and digital gateways. Analog gateways deal with the PSTN. FXS and FXO ports are used to make a connection to the PSTN. Digital gateways deal with T1, E1, PRI, and BRI services. An FXS gateway connects an analog phone to a VoIP system. One to a few ports are included with an FXS gateway. An analog phone is connected to a port on an FXS gateway. FXS gateways use RJ11 ports. In case a business deploys an SIP phone system, there is a chance they may not want to replace their analog phones. There is an image of the back of a FXS gateway below.
An FXO gateway connects a VoIP system with PSTN lines. When an SIP client is connected to a SIP phone system, an FXO gateway can use a PSTN line. FXO gateways come with one to a few ports to support multiple PSTN lines. In case of the network crashing or if the Internet is down, an SIP phone system can be configured to use the PSTN lines provided by the FXO gateway. Protocols such as SIP, ISDN and PSTN are supported. If a business has multiple offices, employees can connect to a centralized SIP phone system to utilize their FXO gateway. Long distance charges are eliminated that way. There is an image of the back of a FXS gateway below.
In order how to understand how FXO and FXS gateways work, you will need to know how FXO and FXS ports work. FXO and FXS are ports that are used by the PSTN to make and receive calls on an analog phone. FXO and FXS ports are paired together to make phone calls. FXS stands for foreign exchange subscriber. An FXS port is a wall jack that sends the power, dial tone, and ringing voltage to FXO devices. FXO stands for foreign exchange office. An FXO port connects analog phones and fax machines, and modems to an outside phone line. The FXO port is on the devices themselves. FXO devices receive service from a PSTN center. FXO and FXS ports have to be connected to make and receive calls. When someone makes an outbound call on an FXO device, they dial a number and connect to the other end using the PSTN. Dialed numbers are encoded as DTMF digits and they are sent through an FXS port. If someone is to receive a call on an FXO device, the ring voltage will be sent to that persons FXO device and it will ring. If someone answers the FXO device, the call is established.