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Delay, also known as latency, is the time it takes voice data to arrive at its destination. It's natural but becomes a nuisance in excess. For example, a person may speak and wait several seconds for the other party to hear what he or she said.
Variants in delay make up jitter. Jitter can disrupt a conversation by passing audio inconsistently, making it choppy and lagged. The greater the range in delay, the worse the quality. Often, jitter results in two users talking over one another.
Whenever voice data enters a queue, it creates gaps in the packet sequence. IP devices detect and correct these gaps with a buffer. It imposes a small delay to allow packets to arrive and shuffle into the proper order. A buffer too small does not allow enough time to do so. One too large causes greater delays than necessary.
Besides adjusting the jitter buffer, setting QoS on the networking components can correct delay. Essentially, the QoS allocates data to specific devices before others. In this case, the IP phones get an expressway for packets to travel, jumping the line in any queue along the way.
The distance a packet travels may also produce delay. That said, latency between two points that is consistently high is better than latency that fluctuates per call. Like distance, security can halt a packet's journey out of or into a network. Should other troubleshooting methods fail, security settings should be re-evaluated.