IADs are most thought of when we need to attach a TDM PBX or Key System to a VoIP/SIP Service. They range in price from a few hundred dollars to several thousand depending upon the connectivity required and desired features. The most basic functionality needed is to accept an IP or Ethernet connection and convert it into something that the PBX or Key system understands, T1, PRI, FXS, and FXO. Additional capabilities would include QoS, multi-codec support, SIP, MGCP, and H.323 protocols. (I apologize in advance for not having the time to go through each one of the aforementioned protocols. Check out Wikipedia or any of your telecom resources for more information).
Going beyond the most basic functionality, an IAD can offer FXS/FXO ports to support point-of-sale (POS) devices, alarm systems and video monitoring systems. Due to certain bandwidth requirements, these services today continue to better managed using TDM facilities. Although, the time for all of these to be efficiently handled over a VoIP network is coming soon.
Another interesting use is for business continuity. Certain IADs can support multiple IP addresses pointing to multiple softswitches on either the same or different networks. When a given IP Address fails, it can switch all traffic to the back up. Inherent in SIP is the ability to load balance across multiple IP addresses. The IAD aids in that process.
How an IAD presents an IP connection to the TDM PBX is also important. As stated above it varies from an FXO/FXS connection to a T1. A useful connection type is to present it as a PRI. A PRI is communicates faster with most PBXs than a voice T1. It also removes the need for a router by presenting an interface common to the TDM CPE, thus saving money.
Finally, an IAD can be used with either an IP PBX or TDM CPE to provide firewall protection, NAT traversal techniques and secure VPN access including encryption.
In short, these devices are used just to provide TDM CPE access to an IP network. They are workhorses. Broadvox's primary partners include Adtran, AudioCodes, Dialogic, Ingate and others. Visit their sites for specific product information and to attain costing data.