y and jitter, but Quality is not one discrete value where the low side is cannot tolerate loss. The data philosophy has centered on good and the high side is bad. There is a tradeoff between providing reliable data transmission over unreliable real-world limits and acceptable voice quality. Lower media, almost regardless of delay. Bandwidth in the data delay, jitter, and packet loss values can produce the best world is largely shared, SO congestion and delay are often voice quality, but also may come with a cost to upgrade present and can cause problems for multimedia applica- the network infrastructure to achieve the low network tions such as voice. values. For example, in a Wide Area Network (WAN) The factors that affect the quality of data trans- link between Los Angeles and Manila, the link could add mission are different from those affecting the quality of a large, fixed delay into the overall delay budget. This voice transmission. Data is generally not greatly af- delay in the conversation may be noticeable to the end fected by delay. Voice transmissions are degraded by users as either an actual delay, or as echo, SO the users relatively small amounts of delay and cannot be may not get "toll quality" voice, but the cost savings can retransmitted. Additionally, a small amount of packet far override the slight loss of quality and the voice quality (data) loss does not affect voice quality at the receiver's can still be acceptable for the user's purposes. ear, but even a small loss of data can corrupt an entire The concept of voice quality is also greatly influenced file or application. So in some cases, introducing VoIP by the user's business needs. IP telephony quality can be to a high-performing data network can yield very poor engineered to several different levels to accommodate voice quality. Communications Engineering Desk Reference; ISBN: 9780123746481 Copyright © 2003 Elsevier Inc. All rights reserved CHAPTER 3.3 VolP Technology Implementing VoIP requires attention to many fac- The ITU-T has recommend