Hi,
1- What is the main difference between Base-T and Base-TX in twisted pair cables. Most importantly, how many pairs are used and which is better.
Here is link that describe the difference and the type of cables you can use with each.
The Telecommunications Industry Association (TIA) created and promoted a standard similar to 1000BASE-T that was simpler to implement, calling it 1000BASE-TX (TIA/EIA-854).[20] The simplified design would have, in theory, reduced the cost of the required electronics by only using two unidirectional pairs (one pair TX, one RX) instead of four bidirectional pairs. However, this solution has been a commercial failure, likely due to the required Category 6 cabling and the rapidly falling cost of 1000BASE-T products.
link:
https://en.wikipedia.org/wiki/Gigabit_Ethernet#1000BASE-TX
2- In full Duplex, no collision exists because the use of two different pairs for sending and receiving. what about the the half duplex? Do we use 1 or 2 pairs?
Full and half duplex is the function of the physical NIC and switch port. So, if a device connect to a switch port with cat 5a, 6 or 6a, you can still configure the switch port and the NIC as half duplex.
3- Since Ethernet is baseband and telephone line broadband. Does that mean that the throughput of the telephone line is always larger than the Ethernet? If so, are cable internet are slower than the DSL?
Not today, this was the case maybe 7, 8 or 10 years ago where DSL with speed of 1 or 2Mb was popular. Today, most providers can hand off a 10Mb, 100Mb, Gig or even 10Gig Ethernet for your long distance communications as well as Internet. Even for home use, the cable providers are now providing much faster connections than DSL.
HTH
