The switching fabric does this same function under the
control of a processor (in other words, we automated the operator function).
Detach this function from the processor, and you have a relatively inexpensive device
that can be placed throughout the network. For example, an operator providing service
to a remote rural area cannot afford to deploy a full-blown circuit switch into this market,
but it can deploy an inexpensive media gateway in the market and place the controller
someplace more central, supporting media gateways in many different rural areas.
So now you have the switching matrix being provided by the media gateway. The
media gateway does not have any processing power, and therefore it must rely on a
Figure 2.4 Splitting the switch functions
Applications
Call control
Switching matrix
Applications
Call control
Switching matrix
Traditional circuit switch VoIP media gateway
VoIP media gateway controller
VoIP application server
50 Chapter 2
controller to tell it how connections are to be made, and to control those connections.
The media gateways do not have any signaling between themselves per se; they communicate
with their controller. A number of protocols are used between the media
gateway and the controllers. This is another issue with many VoIP implementations,
because it is difficult to interwork many different brands of devices if they do not support
the right protocols.
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