dors have built stand alone network appliances. And one vendor has taken this a step further by creating an Ethernet IP telephone. Vendors designing IP telephony products face two fundamental issues. One is latency, or in laymen's terms the time required for data to travel from point A to point B. Latency does not in of itself destroy data, but it makes for awkward conversation when it climbs above a few tenths of a second. The other fundamental issue is packet loss. TCP/IP, in its present incarnation, provides no guarantees to users that they can transmit a given amount of data within a given period of time. Network performance can fluctuate from moment to moment. Sometimes data will be transmitted immediately, sometimes it will be delayed or not sent at all. In many respects, the problems facing designers are similar to digital cellular network design in that the underlying transport medium provides no guarantee that data will arrive intact. Therefore, vendors must assume the underlying network is unreliable and compensate for this by using error correction and interpolation techniques to minimize and compensate for lost or severely delayed packets. These problems are not insurmountable by any means, but they do require serious attention. Common IP Telephony Gateway ArchitecturesVendors have taken several approaches to IP telephony. They are: PC server-based gateways built using industry standard (i.e. Dialogic, NMS) voice cards PC server-based gateways built using specialized DSP cards designed for IP telephony Stand-alone network appliances which connect directly to 10BaseT networks Multi-purpose NIC cards with telephony capability Stand-alone IP telephones Rather than focus on specific products, this article will explore the relative benefits and disadvantages of each of these architectures. While product lines vary on a weekly basis, the fundamentals outlined in this article will still be relevant ...
