4D PAM 5
1000 BASE TX utilises Forward Error Correction in the form of a 4 Dimension 8 State encoder to compensate for the complexity of the coding techniques employed. The signal to line or to our cable utilises a 5 level or 5 voltage signal which may be seen as voltage levels of +1V, +0.5V, 0V, -0.5V, -1V. In fact we only use 4 levels for the data (+1,+0.5,-0.5,-1) and the other level (0) is used for other data transmission requirements.

Lets start with the diagram above. All 4 pairs are used. So any cable installed supports only one port from a Switch. The cable is no longer expandable. 1000 BASE TX is designed to operate on a properly installed Cat 5 cable. Using higher quality cables such as Cat 5 enhanced or Cat 6 cables does improve the signal to noise of the circuit but should not be needed if the cable and cabling products are installed correctly with good quality termination products.

Note that in the diagram above there are devices called a "HYBRID". This is an electronic technique for coupling the transmitted data and received data. These are quite complex devices but essentially may be thought of as very sophisticated solid state transformers. (but there is no transformer) Each pair of the circuit carries 250MBits/sec in both directions which is why we say the circuit is Full Duplex. By adding the data on all 4 pairs we derive the total data transmission rate of 1000 MBits per second. That's a lot of data to try and imagine running over that cable you install. Each transmitted symbol or piece of data is only 8 nano seconds long which is not long at all, it's very short indeed. Yet 10,000 million bit per second data circuits over Cat 6 has already been developed. I can't stress enough that your techniques for installing the cable is quite critical in making this stuff work properly. Why do you think I bother telling you of the complexity of the signalling techniques. You don't absolutely need to understand it in detail but I hope my simplified explanation give you an emotional feeling that the signals are small, very complex high frequency signals that need a damn good cable installation to operate on correctly.

In the diagram above you can see a comparison between the MLT3 encoded signal and a PAM 5 signal. Note that in both cases the symbol period is 8 nano seconds. This is because each transmitter is timed with the same clock rate of 125MHz. The subtle difference is that although each PAM 5 symbol is sourced and transmitted at 125 Million times per second, each symbol represents two data bits so in fact 125 times 2 equals 250 Million bits per second or 250 MBits/sec. Behind the scenes the raw data from our computer undergoes some pretty sophisticated encoding before it is delivered to the final PAM 5 encoder. The final stage is PAM 5 that transcribes every two bits of the incoming data stream into a voltage symbol. Thus if we only look at every 2 bits of data we only need 4 levels or 4 voltages to transmit the data at 2 bits for each symbol. The reason for choosing 5 levels gives us a huge redundancy in unused symbols in the coding system which is used in enhanced 1000 BASE TX/T2 versions of this Ethernet software.

The technique used to encode our data works roughly like this. The data is being delivered to the transmitter which will represent every level or voltage with 2 bits of data, either 00, 01,10 or 11. This means at our clock rate of 125 MHz we are receiving 256 data codes or 2⁸ data codes (2 bits) every 8 nanoseconds. Remember we pick 2 because each level is representing 2 bits of data. (2x125=250MBits) Because we have 4 pairs of cable with 4 possible levels (2 bits each) then we could achieve the required 256 symbols required by utilising all four pairs or 4⁴ being 256 symbols. But if this is all we use, 4 levels across 4 pairs then we have no more symbols left over for controlling our data circuit such as Idle or Start of Frame or End of Frame or other control signals. By choosing a 5 level, 5 voltage system the amount of symbol combinations across 4 pairs becomes 5⁴ or 625 symbol combinations of which we only need 256 for our data so the remaining symbols may be used for 100% redundancy (256 symbols), and 113 symbols still remaining for controlling the circuit.