Digital to analog (D/A) conversion issues also currently plague toll networks. Although almost all the telephony backbone networks in first-world countries today are digital, sometimes multiple D/A conversions occur.
Each time a conversion occurs from digital to analog and back, the speech or waveform becomes less "true." Although today's toll networks can handle at least seven D/A conversions before voice quality is affected, compressed speech is less robust in the face of these conversions.
It is important to note that D/A conversion must be tightly managed in a compressed speech environment. When using G.729, just two conversions from D/A cause the MOS score to decrease rapidly. The only way to manage D/A conversion is to have the network designer design VoIP environments with as few D/A conversions as possible.
Although D/A conversions affect all voice networks, VoIP networks using a PCM codec (G.711) are just as resilient to problems caused by D/A conversions as today's telephony networks are.