 pins. X.21 connectors are almost never provided in the U.S. ISDN connections rely on an RJ-11C plug on the network side of the NT-1 and an RJ-45 connection on the CPE side (the side that connects to the codec). It is also important to specify whether the customer or the supplier will supply the CSU/DSUs, NT-1s and cables. Packet-Switched Interfaces Group systems have historically relied upon circuit switching, but this is rapidly changing. Today, ITU-T standards address connections over IP. Although people associate them with personal conferencing systems, these standards apply to group systems as well. Group systems with Ethernet LAN interfaces facilitate multimedia exchanges (data collaboration). AUDIO-RELATED FEATURES Because audio quality is critical to the success of a videoconference, it is imperative to consider components such as microphones, speakers and echo cancellers that comprise the audio subsystem. Advances in audio conferencing technology have greatly improved sound quality; digital systems are clearer. Echo cancellation eliminates clipping (which causes sound to drop out when someone from one end interrupts the conversation at the other end). Modern systems are more tolerant of poor room acoustics, ambient noise, and degradation caused by the transmission line. Today's room-system video communication products deliver high-quality audio with digital signal processing (DSP), echo control, noise reduction, and full-duplex transmission. Differences between various codec manufacturers' audio quality levels have substantially evened out. Nevertheless, there can be significant differences between high- and low-end rollabouts. Desktop systems present even more variation. Audio Codecs Digital audio is usually described by sampling rate, the number of bits per sample, the data rate (measured in bps), and the number of audio channels. The fundamentals of audio sampling, quantizing and encoding are covered in Chapter 6. The higher the audio bit rate, the less room will be left over for the video portion of 