Gbit Copper Tapping
Issues if you want to tap Gbit copper links
Full passive taping of Gbit Ethernet Copper links ! This is not possible !
Why ? In 10 and 100 Mbit it is possible!
The explanation is simple, the RJ45 cable has 8 wires paired to 4 groups, in 10/100 mode two of this groups are for each direction. So it is easy to connect with a device on each of the two directions and tap the traffic.
But in Gbit the situation is fully different!
to transport 1 Gbit traffic full duplex (1 Gbit in each direction) a very complex signal is used to reach the performance and the quality. In case of Gbit Ethernet the signal is called 8/10 modulation meaning that at the same time on each cable pair 5 bits are transported. But this happens at the same time in both directions. The PHY chips at the end of each cable has a very complex work to do because it must separate the two signals from each other. This is possible because it knows its on signal, so it deduct its on send signal from the mixed signal on the line, an than it understand, the information sent by its link partner. These few sentences describe in a simplified way a very complex function, there are some more vital functions necessary (autoneg, clock ..) to establish a Gbit full duplex connection over a copper cable.
Now to tap a copper link shown in the picture bellow it is not possible to look in the middle on the wire because all what you will see is a complex modulation of two signals. The only way to terminate the signal (see picture) is to use a PHY chip to separate the signal and then send the signal out again to the link partner. This solution is working but there are some problems coming up with this solution.
1:) It is not passive any longer 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 close in a power down situation (see picture) to bring back the link. Also this solution will not prevent that the link is down for min 3 sec. This 3 sec result in the autoneg behaviour. This can not be changed because it is a vital functionality of IEEE 802.3 standard described in clause 28 & 40. Even this short interruption time could cause big problems in a network,
a:) because in some cases this links can not be re-established without shutting down the service
b:) re routing functions in the network will take place
c:) streaming applications will collapse and cause many issue more,
2:) Some layer 1 informations are not transported over a copper tap: for example pause frames
3:) The clock synchronisation is affected. Sync-E over a standard Gbit copper Tap is not possible, and
IEEE 1588 is affected because of the additional delay what a copper Tap produces.
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