OK, I really should call it 1000BASE-T instead of GigE, but everyone calls it GigE.
What does GigE mean: 1 Gbps over 8 wire CAT5.
How do they do it?
Data is flowing, in both directions over all 8 wires at the same time. The 8 wires are grouped into 4 pairs that are electrically balanced.
That means 250 Mbps per pair in each direction.
Each baud, or symbol on the wire conveys more than one bit of information. It is encoded into a multi level phase modulated system called 4D/PAM5. No need to worry about the way they cram 100 pounds of stuff into a 10 pound sack, just know that there are some high frequencies on the wire. Radio frequency type of high frequencies. It is a miracle that CAT5 with GigE do not jam every 2 way radio around more than they do. Keeping the pairs electrically balanced is one reason why they don't radiate much.
GigE Surge Protector Special Considerations
What's this got to do with surge protectors?
Balanced transmission lines:
Normally, when we are talking about transmission lines, we are talking about the connection between a radio transmitter or receiver and an antenna. But like I said, these GigE signals are in the radio frequency range and they must be treated like RF signals. That means you must worry about line impedance and balance. That falls primarily to the person that is designing the printed circuit board. Where most engineers blanch at the thought of RF transmission line theory (a much hated course at engineering colleges) some engineers actually like that kind of stuff. But most of them work at jobs designing radio frequency equipment and antennas.
Just so happens, I earned my chops with microwave antenna design and transmission line design long before I started doing surge suppressors. In any event here is an example of balanced pairs on a printed circuit board.
Balanced Pairs - Controlled Impedance: Note the pairs of blue lines running left to right? Those are data pairs. If you keep them together they will remain balanced and not radiate. It is also important to control their spacing, width and length to keep impedances matched as much as possible. Low quality designs only focus on making the connection, not treating the data pairs as transmission lines. The quality of the design and layout has a direct effect on Return Loss, Near End Cross Talk (NEXT) and Power Sum Near End Cross Talk (PS-NEXT). With GigE, this is very important as you don't want your own signal bounce back at you on a data pair interfering with the desired signal from the far end. This is not a problem with 100 Base T where each pair has data flowing in only one direction. But with 1000 Base T you must keep Return Loss (RL) and NEXT to a minimum or you will have CRC errors and possible packet loss.
Here is a competitor. Note no effort to maintain paired circuits, no effort to use low impedance traces.
They are too skinny, impedance is unbalanced and high. Moreover, skinny pairs cause another problem:
Ampacity - How much current can a trace conduct? Right up until it melts and then explodes. Copper melts at 1984 F (A truly Orwellian number).
Normal PCBs are designed to limit the temperature rise to something reasonable like 100F. But in surge suppressors, the traces form an additional layer of protection - fusible links. They will blow at some point and protect your equipment from damage if the other components have not already done so.
So it is normal to design a surge protector PCB to be able to handle much more current than the data signals. That means big thick fat conductors for the data circuits. My data circuits can take more than 12 amps without exploding for a short amount of time. Not sure the CAT5 will survive, or the RJ45, but the trace will for a few microseconds.
Keep in mind, a surge protector is a sacrificial device. If you open the housing and the PCB is all black with exploded traces, that is a very good thing as long as the equipment it is protecting is still operating. Exploded traces and components means the device did its job.
One of our early products, given a full military funeral. It did it's job. It was a hero. The WISP radio this protected and the customer's computer were fully functional after the lightning strike.
This is not a sign that something went wrong with the surge protector, it is proof something went right!
Other considerations to look for in a quality surge protector:
Shielded RJ45 connectors. They are very import to take shield impulses directly to ground. If you use shielded cables you need to use shielded jacks on the protector. Many competitors use all plastic jacks to save money.
100% Surface Mount Construction. Most of my designs are surface mount. There are still a few components on a few products that are through hole, but with the exception of CAT6 Rated RJ45 jacks all of my GigE protectors are 100% SMT. Why is that important?
Through hole wave soldering machines use a fluxer, normally a foam fluxer to coat the bottom of the PCB with flux prior to entering the pre-heater. The pre-heater dries and activates the flux. Then the molten solder fountain solders the connections and cooks off most of the flux. But there is still a residue. Look at the back of many PCBs that have through hole components. You will see a bit of a film on them. This solder residue is hygroscopic - it will attract moisture and accelerate corrosion formation. The flux vapors also get into the RJ45 creating connection problems and corrosion. Not a problem with surface mount technology.
RF Bypass - RF Immunity
Most manufacturers do not design their products to be used in the presence of strong RF fields like at the top of an FM broadcast towers. I have as much RF protection built into our surge protection products as I can fit in there. We have customers tell us that our products actually make an Ethernet circuit work where it would not work without our product protecting the circuit.
This goes clear back to the Motorola 450 AP and SM where there were CRC errors with their own surge protector but ours would cure the errors. I know why, but I am not going to elaborate, just say it is part of my secret sauce.
POE Timing Glitches
Some manufacturers use POE to send sync pulses. They do it by interrupting the POE current for a few microseconds each second. The problem is that those fast rise times interact with the inductance of the data cables causing large voltage spikes. Spikes high enough to activate the surge suppression components. And when that happens, it interferes with the data, sometimes causing errors, sometimes causing a GigE circuit to downshift to 100 Mbps. Our products can snub the spike without hurting the data. You just don't find all these features in one place with any competing product.