Softswitches�Part II: Functional planes of the softswitch architecture

In our previous tutorial, we briefly discussed the concepts of telephone switching, and three key functions, routing, transmission, and billing, that are involved in connecting a call from the source to the destination. Another key concept was a distinction between the switching fabric, which makes the physical connection, and the switching logic, which handles routing, billing, and associated functions.

Legacy Central Office (C.O.) switches in the existing Public Switched Telephone Network (PSTN) incorporate both the physical and logical functions into one (very large) system. One of the key concepts behind the softswitch (definition) is that it physically separates these functions, allowing the physical switching functions to reside in one device, and the logical functions to reside in another. In addition, there is no requirement for these two functions to be co-resident; instead they may be separated geographically if for any reason that arrangement would make more architectural sense. Two key functional elements were also introduced, the media gateway, which provides the physical connection to the LAN or WAN, and the call agent, which provides the call routing, network signaling, billing, and other logical functions. In this tutorial, we will dig deeper into the softswitch architecture, and examine the core functions that comprise the network.

As the upgrade and/or replacement of large C.O. switches with any new technology can be challenging, an industry group, originally called the International Softswitch Consortium, was founded in 1998 to address these issues and promote the deployment of next generation switching and VoIP technologies. That group has now morphed into the International Packet Communications Consortium (IPCC), which has broadened their scope of interest to include video over IP, plus packet technologies and services transmitted over wireline, wireless, and cable-based infrastructures. The IPCC is comprised of service providers, applications developers, and system integrators, with technical working groups that define reference architectures, interconnectivity schemes, and management systems, all intended to facilitate VoIP and softswitch deployment.

One of their key contributions to this industry is the IPCC Reference Architecture, available at This document defines the architecture of a softswitch, in order to promote greater interoperability between service provider and vendors products. In order to describe that architecture from the top down, four major functional areas, called functional planes, are defined, which specify the major functions of a VoIP network and the associated softswitches. These functional planes are named Transport, Call Control and Signaling, Service and Application, and Management. In addition, these four planes may be further sub-divided into domains. These elements provide the following functions:

  • Transport Plane: transports media, call setup and call signaling messages across the VoIP network. The underlying physical technology may vary, based upon the requirements for the media being carried. Three domains are defined. The IP Transport Domain transports packets across the VoIP network, and includes devices such as routers and switches. The Interworking Domain, which interacts with networks that are external to the VoIP network, includes signaling gateways, media gateways, and interworking gateways. The Non-IP Access Domain allows non-IP terminals, such as ISDN telephones or mobile telephones, to connect to the VoIP network through access gateways or residential gateways.
  • Call Control and Signaling Plane: controls the major devices in the VoIP network, and also handles the establishment and teardown of media connections. The call agent (also called the media gateway controller) operates in this plane.
  • Service and Application Plane: provides the control and logic functions for services and applications that are available on the VoIP network, and would include devices such as application server and feature servers.
  • Management Plane: provides the services required for subscriber and service provisioning, support and billing functions. This plane may interact with the other three planes using the industry-standard Simple Network Management Protocol (SNMP), or proprietary protocols.

Our next tutorial will describe the functional components that make up this softswitch reference architecture.

Copyright Acknowledgement: © 2005 DigiNet ® Corporation, All Rights Reserved

Author’s Biography
Mark A. Miller, P.E. is President of DigiNet ® Corporation, a Denver-based consulting engineering firm. He is the author of many books on networking technologies, including Voice over IP Technologies, and Internet Technologies Handbook, both published by John Wiley & Sons.

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