A computer 1 is provided with an emulation processor to generate a high volume of simulated data traffic, addressed to a predetermined network address. The emulation programme causes the computer to modify its address routing table so as to open a port 3 to a dummy network address. This address allows the test data to be forwarded to the exchange 7. As the IP address is a dummy IP address, it does not appear in Internet routing tables and so the DSLAM 7 cannot forward the traffic beyond the local exchange 5. Instead, an interception gateway 8 recognises this address as requiring to be diverted to a performance testing platform 9, which measures characteristics such as packet loss rate, to allow analysis to determine the performance of the local loop 4 in isolation, without passing through the rest of the network 6, and without needing to take account of any effects from the rest of the network. Very high volumes of test data can be transmitted, up to the maximum capacity of the local loop 4 and the router 1,
