Remember your last vacation via automobile when you took the wrong exit off the freeway, and became hopelessly lost? After you found a gas station or convenience store to ask for directions, you may have been told something like "to get there from here you need to turn around, go in this direction, and proceed down the road." Had you been in communication with a traffic helicopter flying overhead, you could have been guided to the correct exit and turns, since they have a much broader perspective on the problem. In other words, having a bird's eye view, instead of being in the middle lane of a five lane freeway, would be a definite navigational plus.
The traditional Public Switched Telephone Network (PSTN) and the Internet (or other Internet Protocol-based network) are both similar to a freeway. There are specific paths that either the traditional or VoIP call takes from the source to the destination. In addition, there are specific orientations that must be respected, just like the configuration of the interstate highway system: The transmit path and the receive path are distinct, and must not be confused. And on occasion, you find a stretch of highway that requires the payment of a tollmuch as long distance calls come with a higher cost than local calls. An in order to bill for those services, you need some type of usage measurement, a way to render a billing statement, and some means to collect.
A bird's eye view of either the PSTN or Internet highways show three basic functions that are required: routing, transmission, and billing. These have been understood in the PSTN for many years, and now must be migrated to Internet technologies if VoIP networks are to be fully operational on an end-to-end basis. For the PSTN, most of the intelligence to accomplish those functions resides in the Central Office (CO) switches that are strategically placed around the countryand the world. CO switches contain two key elements: a 'switching fabric,' which makes the physical connection, and 'switching logic' that provides call routing and control functions, custom calling features, and interfaces to other systems such as billing.
The first "switches," both physical and logical, were telephone operators employed to sit at their stations and swap patch cords to connect calls. If you needed to speak with a friend or relative, they physically connected the call. If you had a billing problem, they corrected it manually. Since that time, however, many generations of CO switches were developed that provided increasing levels of automation. First came relay-based mechanical switches in the central office, and later fully electronic switches were developed and deployed. However these replacements are much easier talked about than implemented, as the upgrade of a large switching system, with possibly tens of thousands of connected lines takes weeks, if not months, to complete. (Perhaps this was the origin of the term forklift upgrade!).
But now, a new generation of switches, called softswitches, is poised to solve many of these network migration challenges, in addition to providing a clear path to the future of integrated voice, video, and data applications. Simply stated, a softswitch replaces much of the hardware switching with computer-resident software functions. Secondly, the physical switching function is separated from the control logic function, thus enabling different devices, perhaps in different locations, to perform these operations. In addition, the softswitch provides mechanisms to bridge the connectivity gap between existing circuit-switched (PSTN) and evolving-packet switched (VoIP) services, providing an easier migration path to the future. When taken together, these architectural advancements also show economic promise for a telecommunications industry that has had its financial struggles in the past few years.
The softswitch architecture decomposes the switching function into two major elements: a call agent, and a media gateway. The majority of the network intelligence resides in the call agent (also called the media gateway controller or call controller), which handles the call routing, network signaling, billing, and other related functions. The media gateway provides the physical connectivity for the end-to-end path, and may therefore incorporate a variety of LAN and WAN interfaces, including Ethernet, IP, T-1/T-3, ATM, and so on. The call agent controls one or more media gateways may be geographically dispersed, depending upon the location of the connecting networks, and the type of transmission formats (analog, LAN, WAN, etc.) that are involved.
Our next tutorial will describe the architecture of a softswitch-based network in more detail, and define the control protocols that are used between the call agent and media gateway.
Copyright Acknowledgment: © 2005 DigiNet ® Corporation, All Rights Reserved
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.