It is a device capturing and controling all the signals, which are being created by the whole system, especially the made and received phone call signals. The device is mostly used for connecting regular phone system with the Internet and backwards.
Gatekeeper is an administrational tool for H.232 multimedia networks. A single unit can control interactions for each zone, which comprises the terminals, MCU's and gateway within the frames of a particular domain. Gatekeeper is an optional element, but when it available, it becomes the central administrative entity.