Download : The speed in which data is received through your internet connection. This can have some affect on your voice quality if there is not enough bandwidth for all of the traffic. Typical download requirement would be no less than 10Mbit but can be used with QOS ( Quality of Service ) configured on your router.
Upload : The speed in which data is sent through your internet connection. Just like download , this can affect voice quality if there is not enough bandwidth to send all of the voice traffic over your connection. Typical upload requirement would be no less than 5Mbit but like download can be used with QOS ( Quality of Service ) configured on your router.
Latency : Latency is the amount of time a message takes to traverse a system. In a computer network, it is an expression of how much time it takes for a packet of data to get from one designated point to another. It is sometimes measured as the time required for a packet to be returned to its sender. Latency is best if it is under 50ms, If it is above 50ms it can cause voice quality issues.
Jitter : Jitter can also be known as interference on the line. This can greatly have an effect on the stability and consistency of your phone calls. With high jitter , this can bring about issues such as choppiness and conversations cutting in and out. Jitter can start to cause issues when above 25ms.