The PBX is alive and kicking
PBX stands for "Private Branch Exchange"—a term from the traditional telephony world where customers received telephone lines and needed a device on their premises that connects those lines to handsets.
The Session Initiation Protocol (SIP) takes that to the next level and makes the PBX a software component in the computer network.
SIP is changing the telephony landscape like HTTP changed the publishing world. By using SIP you can combine devices from different vendors to build your telephone system. The SIP-based IP-PBX ties those devices together and makes sure that they can talk to each other.
How does this work?
1. The pbxnsip PBX server is the core of the enterprise communication system.
2. It is connected to the companies local area network (LAN) which provides enough bandwidth to transport data and voice.
3. Typically most of the SIP-phones are registered inside the LAN to the PBX. They can make internal calls and they can call the PBX to check the mailbox.
4. The PC in the network can access the PBX through the LAN. This makes computer telephony-integration simple. The PBX can initiate phone calls from an address book, run soft phones that are registered to the PBX and/or use the web browser to log into the PBX and change settings, initiate conference calls or check call lists.
5. If the LAN contains WiFi access points, cordless devices can be registered to the PBX just like other SIP devices. If the access point includes routing functionality, the WiFi phones can hang off a subnet. Other devices like DECT-enabled devices are registered as regular SIP phones.
6. In order to make phone calls into the PSTN, typically a PSTN gateway is being employed. This makes sure that there is excellent audio quality and that phone calls are possible even if the Internet connection should be down.
7. Calls can be routed to regular PSTN destinations such as old analog phones.
8. Calls to cell phones that are associated to extensions are also run over the PSTN gateway. These calls can run in parallel to the ringing of internal extensions and whoever picks up first will get the call.
9. Many companies also would like to include the public Internet in their communication infrastructure.
10. In order to reach the the Internet, the PBX needs to go through the corporate firewall. This can be done by assigning a second IP address to the PBX which can be routed from the public Internet.
11. Regular SIP phones can be registered through the public Internet. The built-in mini-SBC of the PBX will take care about home DSL routers and make sure that those phones can register to the PBX.
12. If public WiFi access points are available, the PBX can even keep registrations of WiFi-enabled cell phones alive and integrate them as regular extension into the corporate communication infrastructure.
That's the point when you receive a phone call at the airport and your other telephone party as you how is the weather in your corporate headquarter. "I have no idea" might be your answer.