4 reasons of latency :
Processing Delay : time the packet is correctly received at the head node of the link and the time the packet is assigned to an outgoing link queue for transmission. The processing delay is independent of the amount of traffic handled by the corresponding node.
Queueing Delay : time between the moment the packet is assigned to a queue for transmission and the moment it starts being transmitted. Packet waits while other packets in the transmission queue are transmitted.
Transmission Delay
Propagation Delay