mically to changing network conditions even during a call or session. The second area for improvement is related to the encapsulation of higher-level- protocol data units (PDUs) into ATM cells. If a way can be found to create networks that send IP packets with low latency and jitter without encapsulating them, it would have two advantages. First, the processing time for encapsulation of the IP PDU would be eliminated. Second, effective capacity would increase because ATM has a relatively large header size for its payload. 3 ATM was a well-suited technology at the time it was developed. But now it may be possible to implement technologies that use variable-sized packets and also are able to reduce latency and jitter to the point where the network can support real-time voice and video when operating at or near capacity. So far we have talked about alternative physical media, physical-layer implementa- tion, and data-link-layer implementation. However, the voice channels have still been DS-0 circuits, and the data channels have used IP packets. In the next section we will change this scenario by looking at voice services over the IP at the network layer of the OSI model. 19.4 Voice over IP (VoIP) VoIP in packet pipes improves resource utilization because, for each call, only the ca- pacity needed is actually used. In circuit-switched technology, a full-duplex channel is always provided for each call. Switching to VoIP increases the total capacity of the pipe significantly for several reasons: In most phone calls, most of the time, only one person is talking at a time. Voice activity detection (VAD) is a natural way for VoIP to send fewer bits and packets when the one party is not speaking. Depending on a variety of conditions, low-bit-rate codecs reduce the resources needed to carry voice-quality audio. The ATM header-to-payload ratio is 9 to 44. Additional bytes are lost due to padding when the last cell containing a PDU has fewer than 44 bytes. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digital