There are several causes for Jitter. Some of the most common causes are:
Network Congestion
Jitter is generally caused by congestion in the IP network. The congestion can occur at the router interfaces or in a provider or carrier network if the circuit has not been provisioned correctly. Too many devices hooked up to the same system, all being used simultaneously, will run out of bandwidth, slowing your connection to a crawl.
Wireless Networks
While a wireless network allows mobility and eliminates the need for wires running through the office, there is a chance that you will experience a degraded network link. WiFi is not necessarily secure or stable enough for our mobile devices to depend on for our phone calls.
Other notable causes
- Misbehaving hardware
- Hardware capacity on source/destination
- Network capacity
- Bad routing through network/internet (traffic routed in a suboptimal fashion introducing delays, etc)