quality would be better using a com- phones.) pression codec (G.729A) compared to a full bandwidth VolP Technology CHAPTER 3.3 G.711 codec. Several factors make packet loss re- transcoding (including the one in and out of the voice quirements somewhat variable, such as the following: mailbox). Each transcoding episode results in some Packet loss requirements are tighter for tones (other degradation of voice quality. than DTMF) than for voice. The ear is less able to These problems may be minimized by the use of some detect packet loss during speech (variable pitch), form of network rerouting (e.g., Q-SIG Path Re- than during a tone (consistent pitch). placement). This feature detects that the call coming Packet loss requirements are tighter for short, con- through the main call server has been routed from one tinuous packet loss than for random packet loss tandem node, through the main, and back out to a third over time. Losing ten contiguous packets is worse switch. In these cases, the system then reroutes the call than losing ten packets evenly spaced over an hour directly, thus replacing the path through the main system with a more direct connection. timespan. Packet loss may be more noticeable for larger voice payloads than for smaller ones, because more voice is lost in a larger payload. Packet loss may be more tolerable for one codec over The two main types of echo are acoustic and impedance, another. although the sources of echo can be many. Echo will result Even small amounts of packet loss can greatly affect when a VoIP call leaves the LAN through a poorly TTY (TDD) devices' ability to work properly. administered analog trunk into the PSTN. Another major Packet loss for TCP signaling traffic increases sub- cause is from an impedance mismatch between four-wire stantially over 3 percent loss due to retransmissions. and two-wire systems. Echo also results when an imped- ance mismatch exists in the conversion between TDM Again, acceptable rates for packet loss vary with the needs of the end users. (Time Divi