USB specifies a maximum cable length of 5m for a full-speed device (see also http://www.usb.org/about/faq/ans5/). The primary reason for this limit is the maximum allowed round-trip delay of about 1500ns. If a USB device does not answer to host commands within the allowed time, the host considers the command to be lost. When adding up the USB device response time, delays from using the maximum number of hubs, and delays from the connecting cables, the maximum acceptable delay per cable turns out to be 26 ns. The USB 2.0 specification requires cable delay to be less than 5.2 ns per meter (which is close to the maximum achievable speed for standard copper cable). This allows for a 5 meter cable.