The first telephones and switchboards date back to the late 1800s. Today, the switching - connecting two lines together - is done entirely by software. The softswitch is a device within a telephone network, and has many different components. The softswitch is controlled by the Call Agent, which sends information to gateways, such as a Media Gateway, Access Gateway Control Function, or a Signaling Gateway. The gateways interact with the PSTN and with IP networks. Softswitches make modern telephony technology, including VoIP, possible.
The call agent (not to be confused with any kind of tech support or customer service agent) controls call signaling and routing, and coordinates different gateways. The media gateways, in turn, interface with a number of different networks: IP servers that carry internet traffic, including VoIP and SIP Trunking; “Asynchronous Transfer Mode” (ATM) networks that transmit data, voice and video in cells, rather than packets; PBX’s; and “Integrated Access Devices” (IAD), which have the capacity to accommodate banks of hundreds of phones; cellular networks, which retrieve voice and data wirelessly; and even satellite phones (satphones).
The end user does not interact with the softswitch at all. All the switching happens in centralized locations, such as a building in the middle of a telephone network, or near a server operated by your VoIP service provider. Softswitches allow different networks to work in tandem. A great example of this is combining VoIP with Satellite phone networks to bring telephone service to remote rural areas.
A VoIP softswitch interacts with other softswitches, both VoIP softswitches and PSTN softswitches. Within the internet, a VoIP softswitch, when directed to make a call, will first look within its own database for the IP address of the recipient. If it can’t find it, it will then pass on the request to other softswitches until it locates the correct IP address/SIP address or hands the phone call off to the PSTN.