Every network has a physical layer, and when that physical layer is degraded or disorganized, the network problems it creates are indistinguishable at the software level from configuration errors, switch failures, and application problems. Network engineers spend hours investigating software and configuration when the actual cause is a cable run that has developed a marginal termination after years of vibration, a patch panel port where someone forced a cable at an angle and damaged the contacts, or a telecommunications room where years of adds and changes have created a tangled mess where individual cables can no longer be traced.