tation is that the industry will standardize around the SIP protocol with fewer requirements to support multiple protocols. However, SIP is a complex protocol that allows for many dif- ferent implementations of similar functionality, and, as such, a great deal of interworking issues need to be resolved within this protocol. When attempting to determine which voice gateways to use for a deployment, you need to understand that the primary goal of a voice gateway is to accurately reproduce voice with as little negative effects as possible while performing as many interworking func- tions as possible. A well-designed VoIP gateway should add no more than 5 milliseconds of delay to a solution and should also enhance the overall voice experience by providing the following: A high-quality conversion from PCM to IP voice with a fast and accurate sampling rate. The capability to detect and cancel echo that exists on the TDM connection (known as Echo Cancellation). A high-quality voice gateway has the capability to cancel echo that occurs as much as 128 ms after the initial voice sample. The capability to detect and suppress silence, which is an important capability that enables fewer packets and thus uses less bandwidth on the IP connection when si- lence is detected. The downside of any Voice Activity Detection (VAD) system is that the time between the DSP detecting a voice packet and the voice being sent can result in words being clipped. A high-quality voice gateway has a minimal amount of clip- ping, but all voice gateways employing VAD will have some amount of clipping. Chapter 1: Overview of IP Telephony 9 Finally, voice gateways need to have a great deal of flexibility as to the TDM connections they connect to. TDM connections vary greatly by region and are something that is dependent on location. In North America, the primary analog connection is a Foreign Exchange Office (FXO) connection, and the primary digital connection is a T1, which multiplexes 23 or 24 audio changes into a single connection. A T1 can either 