Windows 2000's TCP/IP network stack contains a little-known feature that can sometimes cause more problems than it solves. This feature, called "media sensing," allows Windows to detect whether or not a network cable is plugged into a NIC.
If a cable is physically plugged into the adapter (and attached at the other end to a network device as well), Windows refers to this as the network media being in a "link state." Almost all network devices have an LED that indicates when the network media is in a link state (usually with a label that says, conveniently enough, "Link"). If the network cable is unplugged, media sensing kicks in and removes the bound protocols from the adapter until the media is re-inserted. This is done so that the rest of the system will know that this particular network interface is not to be used.
However, there are circumstances where this sort of thing can be counterproductive. For instance, some NICs do not correctly report the link state back to the PC or to Windows. This is usually due to a problem with the NIC's driver or the firmware on the NIC itself. Sometimes this can be fixed with an upgraded driver or firmware, but this is often slow to come. Another place where this can be problematic is with wireless adapters, which may report a dropped link prematurely when signal strength dips temporarily, and can cause a wireless adapter to appear to be failing randomly.
If you are experiencing link problems with a given adapter, you may be able to work around it by disabling media sensing. To do this, navigate to HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Tcpip\Parameters in the registry and add a DWORD value named DisableDHCPMediaSense. Set it to 1 and reboot.
Note that there are some side effects to disabling media sensing. For one, on any machine that is multi-homed -- i.e., using more than one network interface -- media sensing is used to reroute connectivity through other adapters if one fails or is disconnected. Also, media sensing is used on laptops to detect network settings and automatically refresh the connection without needing to disconnect and reconnect the network stack manually. Also note that after you disable media sensing, a "Network Disconnected" or "Cable Disconnected" icon may show continuously even when you have network access.
Serdar Yegulalp is the editor of the Windows 2000 Power Users Newsletter.
This was first published in February 2003