Delay is caused when packets of data (voice) take more time than expected to reach their destination. This causes some disruption is the voice quality. However, if it is dealt with properly, its effects can be minimized.
When packets are sent over a network towards a destination machine/phone, some of them might be delayed. Reliability features in the voice quality mechanism sees to it that a conversation is not deadlocked waiting for a packet that went to have a walk somewhere in the green. In fact, there are many factors affecting the journey of packets from source to destination, and one of them is the underlying network.
The delayed packet may come late or may not come at all, in case it is lost. QoS (Quality of Service) considerations for voice are relatively tolerant towards packet loss, as compared to text. If you lose a word or a zero in your balance, your text might mean something completely different! If you lost a “hu” or a “ha” in a speech, it does not make a really big impact, except some hitch in voice quality. Besides, voice smoothing mechanism regulates it so that you don’t feel the bump.
When a packet is delayed, you will hear the voice later than you should. If the delay is not big and is constant, your conversation can be acceptable. Unfortunately, the delay is not always constant, and varies depending on some technical factors. This variation in delay is called jitter, which causes damage to voice quality.
Delay causes echo in VoIP calls.