McDewey

Figure 3-1 Voice Compression

The analog voice stream is digitized into PCM samples and delivered to the compression algorithm in 10 ms increments. The look ahead is discussed in Algorithmic Delay.
Standards for Delay Limits
The International Telecommunication Union (ITU) considers network delay for voice applications in Recommendation G.114. This recommendation defines three bands of one-way delay as shown in Table 4.1.
Table 4.1 Delay Specifications
Note: These recommendations are for connections with echo adequately controlled. This implies that echo cancellers are used. Echo cancellers are required when one-way delay exceeds 25 ms (G.131).
These recommendations are oriented for national telecom administrations. Therefore, these are more stringent than when normally applied in private voice networks. When the location and business needs of end users are well-known to the network designer, more delay can prove acceptable. For private networks 200 ms of delay is a reasonable goal and 250 ms a limit. All networks need to be engineered such that the maximum expected voice connection delay is known and minimized.
Sources of Delay
Range in Milliseconds Description 0-150 Acceptable for most user applications. 150-400 Acceptable provided that administrators are aware of the transmission time and the impact it has on the transmission quality of user applications. Above 400 Unacceptable for general network planning purposes. However, it is recognized that in some exceptional cases this limit is exceeded. Page 3 of 18 Cisco - Understanding Delay in Packet Voice Networks 7/31/2008 http://kbase.cisco.com/paws/servlet/ViewFile/5125/delay-details.xml?convertPaths=1