Found what I was lookin for:
"10BASE-T and 100BASE-TX Ethernet use only two pairs of wire in 4-pair CAT5/CAT5e/CAT6 cable, leaving the other two pairs free to transmit power for Power over Ethernet (PoE) applications. However, Gigabit Ethernet or 1000BASE-T uses all four pairs of wires, leaving no pairs free for power. So how can PoE work over Gigabit Ethernet?
The answer is through the use of phantom power—power sent over the same wire pairs used for data. When the same pair is used for both power and data, the power and data transmissions don’t interfere with each other. Because electricity and data function at opposite ends of the frequency spectrum, they can travel over the same cable. Electricity has a low frequency of 60 Hz or less, and data transmissions have frequencies that can range from 10 million to 100 million Hz.
10- and 100-Mbps PoE may also use phantom power. The 802.3af PoE standard for use with 10BASE-T and 100BASE-TX defines two methods of power transmission. In one method, called Alternative A, power and data are sent over the same pair. In the other method, called Alternative B, two wire pairs are used to transmit data, and the remaining two pairs are used for power. That there are two different PoE power-transmission schemes isn’t obvious to the casual user because PoE Powered Devices (PDs) are made to accept power in either format."
That information is from Blackbox.com; http://www.blackbox.com/Store/Results.aspx/Networking/PoE/n-4294953056
PoE is a DC signal/current. It can share the medium with no problems (ignoring any "noise" that the DC source may inject on the line. The same method is used for satellite TV. The LNB that exists on the antenna requires power, which it receives in the form of a DC current from either the STB or a separate power injector. This DC power shares the same medium as the L-band signal coming from the antenna to the STB.
In general, however, multiple signals can share a medium. It all depends on how the system is engineered, and requires good coordination practices as well. The best example is the "airwaves". Right now, mobile phones, wireless devices, trunked radio, etc. are all sharing the air around you. The frequency separation, coding and modulation schemes, spectral mask restrictions, etc. prevent them from interfering with each other to the point of failure.
But this can also be found it other "wired" media as well. Think of DWDM in fiber optics - separate frequencies sharing the same fiber. The endpoints play a key role here - the sender must be very specific in its transmission, and the receiver must be able to cleanly discriminate between the different frequencies. DSL is another example - you can use the phone and your internet connection at the same time. Even cable TV - many different channel frequencies sharing the same medium.
From history prospective. In Line power is something Cisco came up with initially. As usual later on Industry followed it and came up with 802.3af. But technically both are different and while buying POE enabled devices, watch out from data sheet if it supports both standards or either one.
Kent, I have read exactly the same article but I have a big doubt: Why they say that Current is operating at 60Hz if VDC is like the name says (Direct Current)? The current does not change in time, VDC is 0Hz. isn't it??
If the current is operating at 60Hz so it must be VAC not VDC... BIG Doubt here. That article doesn't explain very well I think.
This is the Youtube Video of Black Box, they explain the same thing there:
From past prospective. In Line power is something Cisco came up with initially. Usually later on Business followed it and came up with 802.3af. However technically are both various and while buying POE enabled gadgets, be aware from data sheet if it supports both standards or either one.