This might seem a stupid question but why do Ethernet cables have 8 wires? Cat5 cables were just using 4 of the 8 wires, so only 4 are actualy 'needed'. Why not 12 or 16 wires?
This is an interesting question since I've never seen anything that authoritatively states the design decisions behind that choice. Everything that I've come across, whether on the Interwebs or from conversation with people smarter than me in this area, seem to indicate two possibilities:
- Future proofing
- Extra shielding
By the time of the Cat5 spec we had seen the explosion of data cable runs. Telephone had been using Cat3, or something similar for some time, serial connections had been run throughout University campuses, ThickNet had spidered its way around, ThinNet had started to see significant use in microcomputer labs and in some cases offices. It was obvious that networking computing equipment was the wave of the future. We had also learned the terrible costs of changing out cabling to meet the demands of longer segments or higher speeds. Let's face it, replacing cabling is a nightmarish chore and expensive.
The notion of limiting this cost by developing a cable that could be run, and left in place for some length of time, was definitely an appealing one. So forward thinking engineers, who were probably tired of replacing wiring, could easily have found it worthwhile to design extra pairs into the spec. After all, especially at a time when the price of bulk copper was relatively low. Which is more expensive - adding 4 extra wires or having a team of people remove old wiring and add new?
Since typical Cat5 is UTP (unshielded twisted pair) it does not contain the extra grounded foil to slough off the extraneous electro-magnetic interference. It has been described to me that, when properly grounded, the unused wires will help buffer the in-use pairs in a similar, albeit less effective way, than actual shielding. This could have been an important feature in the long runs and (electrically) noisy environments we were accustomed to running cabling at the time.
To me the future proofing argument is the most compelling.
For the same reason why the first and second pair are connected to pins 4, 5 and 3, 6: compatibility with telephone systems. In telephony main pair is the middle pair and second pair is the next one from middle (pins 2, 5 in RJ11 and 3, 6 in RJ45).
If you're using Fast Ethernet or Ethernet, you can route telephone signal in regular cable and it will work without a splitter (you can connect telephone or computer directly to socket).
Why 8 and not 6? I don't know. It's possible that people responsible for Ethernet were thinking that 100Mbps speeds will require two sending and two receiving pairs (those were the "parallel interfaces age" ;-) ) or that having the capacity for 2 phone lines would be beneficial.
More wires could lead to too expensive cables so 8 was chosen as a compromise.
4-pair UTP cable with RJ-45 connectors was invented for audio telephone use. Its adoption and evolution as a medium for high-speed digital data communications has been a matter of convenience: adapting pre-existing mass-produced products for new uses rather than devising a completely new technical standard specific to one new application.
The way this works has been demonstrated in the development of speed standards. 100BASE-TX, -T2, and -T4 were developed in parallel, targeting adoption on different sorts of pre-existing wiring plants. T4 provides 100Mbps over all 4 pairs of Cat3, which was in place in many miles of conduits in businesses already for phone and prior networking technologies. T2 can work on 2 pairs of Cat3 at the cost of a more complex signaling model and interference sensitivity, which explains its de facto non-existence. TX needs 2 pairs of Cat5, which leaves 2 pairs usable for other applications: a distinct physical network or telephone service, split out with a very simple adapter. That capability is why TX survived while T4 was essentially only used transitionally. Swapping out old 4-pair phone or Cat3 UTP for 4-pair Cat5 UTP in the same runs wasn't free but it was a fairly simple path to a big improvement in communication capabilities.
Ultimately, no one uses 2-pair Cat5 UTP because 2-pair UTP of any grade has never been an economically reasonable option in money or space terms for anything longer than a patch cable. It isn't much thinner or cheaper than 4-pair because 2 pairs need the same physical and electrical protection as 4 if you're going to run it in walls and conduits.
Regarding your additional question about number of cores...
Each of the four pairs in a Cat 5 cable has differing precise number of twists per metre to minimize crosstalk between the pairs. Although cable assemblies containing 4 pairs are common, Category 5 is not limited to 4 pairs. Backbone applications involve using up to 100 pairs. This use of balanced lines helps preserve a high signal-to-noise ratio despite interference from both external sources and crosstalk from other pairs. Category 5 cabling is most commonly used for faster Ethernet networks, such as 100BASE-TX and 1000BASE-T.
As the first notice: In this type of cabling system there is using for transmit one signal not one conductor, but pair. In one line wire is signal in the + phase, in the second one in the - phase (logical "1" is in one line for example +10V, but in the second line of pair is -10V). This is, because on the RX end of cable signal is evaluated as difference between the first and the second line in the pair. It's good method to eliminate interferences on the line - on the line can be interfered other signal to the line, but it will be added on the both line in the same direction - for example, if you have add on the both line +10V and you will have logical "1", you will have on the first line +20V, on the second line 0V (-10V + 10V), but difference is stil the same, 20V. Similar for logical "0" will be difference on the pair 0V. Be honest: I don't know precisely, how many volts are on the wire, it is only for imagination.
The second notice: If the first cabling system used 4 pairs, it was, because 2 pairs was planned for computer network and other 2 pairs was planned for voice communication. This was good up to 100Mbps networks. From 1Gbps networks, computers are using 4pairs and communication on this speed is only half-duplex. If you want have full-duplex communication on the 1Gbps speed, you have to use optical cabling.