IP-PBX systems and softswitches are somewhat apples and oranges. While in most cases softswitches are IP-PBX systems, the reverse is not always true.
When a system is referred to as an IP-PBX, that usually indicates only that the system supports VOIP communication with the phone and/or the PSTN (via something like SIP trunks). Avaya and Nortel have IP-PBX systems, along with a host of other new and established vendors in the telecom space.
A “softswitch” in the truest sense is a PBX that derives its function primarily from software. Voicemail, call handling, call center functionality, etc. are implemented through software and use hardware only for basic connectivity to the PSTN or phone. Asterisk is an example of a softswitch.
Softswitches are almost always IP-PBX systems, as this is the easiest way to avoid specific hardware dependency. Asterisk, for example, can operate as a pure IP-PBX softswitch on standard PC server hardware. Communication with both the PSTN and phones is accomplished through a software-based VOIP stack.
IP-PBX systems, on the other hand, are not always softswitches. Almost all IP-PBX systems offered by hardware giants like Nortel and Avaya achieve their functions through hardware: expansion boards that fit into the main chassis. In my opinion, while these systems may derive some capabilities from the firmware, it is not correct to call such a system a softswitch.
There are also some hybrid systems like those from Vertical Communications and AltiGen. These systems are software-based in the sense that most of their functionality comes from software running on a Windows Server PC, but with the exception of Vertical’s HMP system, they require specific hardware to function.
There are no capacity or capacity limitations intrinsic to IP-PBX systems or softswitches since we are talking about architecture, but real-world implementations of softswitches can grow simply because they are sometimes used for carrier-grade switching. Any capacity difference you see in the market is likely not the result of the technology, but rather the manufacturer’s marketing approach (the SMB market is very attractive, but don’t assume that just because IP-PBX systems they generally target the 25 -250 seat size which you can’t get any bigger).
Connectivity to a third-party application server is not something that is necessarily affected by the IP-PBX/softswitch distinction, as even most hardware-based systems today support gateways that allow for integration. with external application servers. That being said, a softswitch will generally make the job a lot easier. Most softswitches include programming APIs that allow the gateways to be programmed in a common programming language such as VB or a .NET language. Asterisk is programmable if you don’t mind getting your hands dirty, and both Vertical’s AltiGen and TeleVantage include COM object-based SDKs that allow extensive control of the switches for IVR creation and custom PC-based call handling.
Compare this to some of the proprietary script-based gateways of some hardware-based systems (including some hardware-based IP-PBX systems) where there is no standard programming interface and the end user cannot modify the gateway without help ( read: cost) from the provider.
The main difference between Softswitch and IP-PBX is that they are analogous to Switch/MSC and PBX respectively in TDM networks.
Only Softswitch can act as Switch/MSC and the access technology can be based on IP/GSM/CDMA/CDMA2000/UMTS/WCDMA.
Where the purpose of IP-PBX is….
has. Convert IP phone calls to TDM calls to interface with PSTN switches or other switches.
b. Switch calls within IP-PBX phones
Below is a difference based on a broad definition.
Soft switching …..
A programmable network switch that can process signaling for all types of packet protocols. Also known as a “media gateway controller,” “call agent,” or “call server,” these devices are used by carriers that support converged communications services by integrating SS7 phone signaling with packet networks. Using network processors at their core, softswitches can support IP, DSL, ATM, and frame relay in the same unit.
According to the International Softswitch Consortium, a softswitch should be able to…
(1) control connection services for a media gateway and/or native IP endpoints,
(2) select processes that can be applied to a call,
(3) provide routing for a call within the network based on signaling and customer database information,
(4) transfer control of the call to another element of the network, and
(5) interconnect and support management functions such as provisioning, failure, billing, etc.
The software makes it flexible…
The switching technology in a softswitch is in the software (hence the name) rather than in the hardware as is the case with traditional switching center technology. This software programmability allows it to support existing and future IP telephony protocols (H.323, SIP, MEGACO, etc.).
IP switchboard….
(Internet Protocol Private Branch Exchange) A telephone switch that supports voice over IP (VoIP). IP PBXs convert IP phone calls into traditional circuit-switched TDM connections to the PSTN. They also support traditional analog and digital phones, allowing businesses to slowly migrate to an all-IP telephony environment.
That’s it. It seems complicated and confusing. It can be… but it doesn’t have to be.