Comparing VoFR and VoIP Packet Overhead
The main handicap is the greater packet overhead required by IP. Both FR and IP packets are constructed with packet header information, voice/fax header information, and the digitized, compressed voice and fax information (the payload).
Each payload represents a fragment of speech called a ‘talk spurt.’ Representing a long talk spurt requires the VDID (Voice/Data Integration Device) to accumulate the speech information over an extended period of time, and possibly introducing delay into the conversation. Conversations with added delay sound unnatural and cause the corporate telephony user to bypass the VDN (Voice/Data Network), defeating any hoped-for cost savings. This delay-limiting requirement dictates a maximum talk spurt size for voice (and consequently, fax) of 50 bytes at a compressed data rate of 8Kbps (1000 bytes per second, or one byte per millisecond). Fifty bytes represent 50 milliseconds of speech activity, the longest practical period without contributing too much to the overall delay.
Tables A and B show the bandwidth consumed in Kbps for FR and IP telephony packets. The speech compression CODEC is assumed to need 8Kbps. The sum of the CODEC bandwidth and the packet overhead is the peak bandwidth consumed by an active conversation. The peak only occurs for a few seconds and is replaced by 0Kbps during the natural periods of silence in a typical conversation. While one person talks, the other listens, yielding silence about 50% of the time over a 20-30 second period. Pauses and interruptions in the conversation contribute another 10%. The net bandwidth consumption averaged over the 20-30 second period is about 40% of the peak bandwidth.
The method in which silence suppression is implemented can affect both the perception of voice quality and bandwidth consumed. Absence of sound is often perceived as a broken connection because the listener is accustomed to hearing a certain minimal level of background noise or ‘comfort noise’. There are two methods of solving this problem. The first is to inject white noise generated by the VDID at either end of the connection to let the listener know that the connection is alive. The advantage of this method is that it does not consume any bandwidth over the WAN link. However, it results in a reduced level of perceived voice quality. The other method involves actually sampling the background noise. This method produces a significant improvement in voice quality, but consumes more bandwidth. The way that the comfort noise feature is implemented differs with each vendor and can often be configured by the network manager. Bandwidth consumption can vary from 0 to 2Kbps, plus overhead.
Each payload represents a fragment of speech called a ‘talk spurt.’ Representing a long talk spurt requires the VDID (Voice/Data Integration Device) to accumulate the speech information over an extended period of time, and possibly introducing delay into the conversation. Conversations with added delay sound unnatural and cause the corporate telephony user to bypass the VDN (Voice/Data Network), defeating any hoped-for cost savings. This delay-limiting requirement dictates a maximum talk spurt size for voice (and consequently, fax) of 50 bytes at a compressed data rate of 8Kbps (1000 bytes per second, or one byte per millisecond). Fifty bytes represent 50 milliseconds of speech activity, the longest practical period without contributing too much to the overall delay.
Tables A and B show the bandwidth consumed in Kbps for FR and IP telephony packets. The speech compression CODEC is assumed to need 8Kbps. The sum of the CODEC bandwidth and the packet overhead is the peak bandwidth consumed by an active conversation. The peak only occurs for a few seconds and is replaced by 0Kbps during the natural periods of silence in a typical conversation. While one person talks, the other listens, yielding silence about 50% of the time over a 20-30 second period. Pauses and interruptions in the conversation contribute another 10%. The net bandwidth consumption averaged over the 20-30 second period is about 40% of the peak bandwidth.
The method in which silence suppression is implemented can affect both the perception of voice quality and bandwidth consumed. Absence of sound is often perceived as a broken connection because the listener is accustomed to hearing a certain minimal level of background noise or ‘comfort noise’. There are two methods of solving this problem. The first is to inject white noise generated by the VDID at either end of the connection to let the listener know that the connection is alive. The advantage of this method is that it does not consume any bandwidth over the WAN link. However, it results in a reduced level of perceived voice quality. The other method involves actually sampling the background noise. This method produces a significant improvement in voice quality, but consumes more bandwidth. The way that the comfort noise feature is implemented differs with each vendor and can often be configured by the network manager. Bandwidth consumption can vary from 0 to 2Kbps, plus overhead.
