1. In a system for testing a communication network component, said test system comprising a simulator for simulating operational responses of elements of the network with which said network component is intended to interact and a message processor, said network component communicating with said test system by messages via said message processor, a method for testing said network component comprising: responsive to receipt of a first message from said network component by said message processor, said message processor determining whether said first message is a predefined type, and if said first message is said predefined type, manipulating said first message responsive to a user-changeable preprogrammed pattern; following said message processor sending said first message to said simulator, said simulator sending to said message processor a second message according to the operational responses of said network elements responsive to said first message; responsive to receipt of said second message, said message processor determining whether said second message is a predefined type, and if said second message is a predefined type, manipulating said second message responsive to a user changeable preprogrammed pattern; sending said manipulated second message to said network component; and said message processor monitoring further messages from said network component to determine whether said network component responds appropriately to said manipulated second message.