Software Makes It Flexible. The switching technology in a softswitch is in software (hence its name) rather than in the hardware as with traditional switching center technology. This software programmability allows it to support existing and future IP telephony protocols (H.323, SIP, MEGACO, etc.).
IP PBX is a telephone switch that supports voice over IP (VoIP). IP PBXs convert IP phone calls into traditional circuit-switched TDM connections for the PSTN. They also support traditional analog and digital telephones, allowing enterprises to migrate slowly to an all-IP telephony environment.
When a system is referred to as an IP-PBX, that typically indicates only that the system supports VOIP communication to the handset and/or the PSTN (via something like SIP trunks).
A "softswitch" in the truest sense is a PBX that derives its feature functionality primarily from software. Voicemail, call handling, call center functionality, etc., are all implemented via software and use hardware only for basic connectivity to the PSTN or the handset.
Softswitches are almost always IP-PBX solutions, as this is the easiest way to avoid a reliance on specific hardware. Asterisk, for example, can operate as a pure IP-PBX softswitch on standard PC server hardware. Communication with both the PSTN and the handsets is accomplished via a software-based VOIP stack.
IP-PBX systems, on the other hand, are not always softswitches. Almost all IP-PBX systems achieve their features via hardware -- expansion boards that fit into the main chassis. In my opinion, while these systems can derive some capabilities from firmware, it isn't proper to call such a system a softswitch.
There aren't any capacity or capability limitations intrinsic to IP-PBX systems or softswitches given that we're talking about architecture, but the real-world implementations of softswitches can grow larger simply because they are sometimes used for carrier-level switching. Any capacity differences you see in the market are likely not the result of the technology but rather the marketing focus of the manufacturer .
We provide custom IP PBX software, softswitch and other VoIP related software development. To know more visit https://www.freeswitchservice.com