Windows Update uses the Background Intelligent Transfer Service (BITS) to download the updates, using idle network bandwidth. However BITS will use the speed as reported by the network interface (NIC) to calculate bandwidth. This can lead to bandwidth calculation errors, for example when a fast network adapter (e.g. 10 Mbit/s) is connected to the network via a slow link (e.g. 56 kbit/s) according to Microsoft "BITS will compete for the full bandwidth the NIC ... BITS has no visibility of the network traffic beyond the client."