they must separate the two signals from each other. This is only possible because they know their own signal, so they can deduct their own send signals from the mixed signals on the line and then interpret the information sent by their link partners. To tap a copper link as shown in the picture above it is not possible to just tap the middle of the wire because all you will see is a complex modulation of two signals. The only way to terminate the signal (as shown in the picture) is to use a PHY chip to separate the signal and then send the signal on to the link partner. This solution works but causes some other problems. It is not passive any longer, so in the case of a failure the link can go down and the services on the link are interrupted. To minimize this problem each copper tap has a bypass switch (relays), which closes in a power down situation (as shown in the picture), to bring the link back up. Also this solution will not detect that the link is down for a minimum of three seconds. These three seconds are a result of the autonegotiation behavior. This can not be changed because it is a vital function of the IEEE 802.3 standard as described in clause 28 and 40. Even this short interruption time could cause big problems in a network. In some cases these links cannot be re-established without shutting down the services. Rerouting functions in the network may take place Streaming applications can collapse and cause more issues. Some layer 1 information is not transported over a copper tap (e.g. pause frames) The clock synchronization is affected. Sync-E over a standard Gbit copper tap is impossible and IEEE 1588 is affected, because of the additional delay a copper tap produces. See also Virtual TAP device DShield ShieldsUP References External links Make-a-Passive-Network-Tap tap Intrusion detection systems