Softswitches�Part IV: Media Gateway architecture
Media gateways physically connect dissimilar networks by translating the protocols and data transmission parameters of each to the other. They do a few other things as well.
So far in our current tutorial series regarding softswitches, we have discussed the concepts of telephone switching, and the architecture of a softswitch, with its four functional planes: Transport, Call Control and Signaling, Service and Application and Management, which are used to describe that architecture. Our last tutorial examined the specifics of the media gateway controller, also known as call agent, that provides the call routing, network signaling, billing, and other logical functions within the network. All of these concepts are detailed by the International Packet Communications Consortium (IPCC) in their IPCC Reference Architecture Document.
From our previous discussions, recall that one of the advantages of the softswitch architecture is its ability to separate the logical switching functions from the physical switching functions, and allow each of these to reside in separate devices, which may be geographically separated from each other. While the call agent handles the vast majority of the logical functions, a second key element, the media gateway, provides the physical connection to the LAN or WAN. This media gateway operates within the Transport Plane, which is responsible for the physical transport of the VoIP messages, and with that, the required access to external networks and their associated terminals.
The term gateway is perhaps one of the most widely used networking terms in recent years, and has been made more popular as both LAN and WAN legacy networks have been migrated to newer technologies. If we were to adhere to the Open Systems Interconnection (ISO) networking reference model, as defined in 1978 by the International Organization for Standardization (commonly designated ISO), we would say that a gateway is a device that converts protocols from one system to the protocols on another system, and operates at all seven layers of the OSI model. These two networks are likely to be based upon differing technologies, such as circuit switching on one side and packet switching on the other, or a cellular telephone network on one side, and a packet switching network on the other. Thus, one of the key attributes of the gateway will be two interfaces: one that communicates with each network. For example, lets assume that the gateway connects the Public Switched Telephone Network (PSTN) with Internet Protocol (IP)-based network. We might expect to see a T-1 interface, operating at 1.544 Mbps on the PSTN side, and an Ethernet interface, operating at 10 or 100 Mbps on the IP network side. Many other options and combinations are possible, however, including cellular wireless, high speed optical, cable television, Asynchronous Transfer Mode (ATM), and others.
But making the physical connection, and with that making the necessary interface and speed conversions, is just one part of the gateway responsibilities. Depending upon the networks involved, it may also:
- Convert the information from time slots that are used with Time Division Multiplexing (TDM) systems into packets, used with IP networks.
- Perform media processing functions, such as canceling line echos, managing the jitter buffer, and compensating for packet loss, to thus assure the overall Quality of Service (QoS) of the connection.
- Insert appropriate signals into the media, such as dialing and call progress tones, comfort (background) noise, and so on.
- Include the ability to detect specific call events, such as on/off hook states and voice activity.
Thus, the media gateway consists of three key elements: a physical interface to the first network, another physical interface to the second network, and signal processing functions that are required to make all of the protocol conversions between these two networks. And depending upon the network type, media gateways are frequently located close to the community of end users, to further reduce connection costs, and provide even more efficiencies.
However, some communication between the media gateway and the media gateway controller is required. These functions, plus the associated protocols, will be the subject of our next tutorial.
Copyright Acknowledgement: © 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.