Twisted pair Ethernet (10 and 100 Mbit/s) uses two pairs of wires. 10BASE-T Ethernet was designed to be be used on CAT3 quality wiring. 100BASE-TX was designed to be used with CAT5 wiring or better. Since the early 2000s most new structured cable installations are built with Category 5e or Category 6 cable. There are cases where some older wiring needs to be used to Ethernet applications. There are cases where old cabling can be used for Ethernet with just changing the connectors on the wall to RJ-45 connectors (or building special adapter cables). On some other cases there is needs to use adapters like VDSL modems.
Ethernet (10 Mbit/s works well, 100 Mbit/s sometimes) can be run on many cases over the old telephone wiring. Around 2002-2006 there was quite a bit of interest in reusing old telephone wiring for Ethernet connections in Finland and some other countries. And still there could be applications where this could be relevant even today.
The reason for the interest to reuse old cabling at that time was rising interest for fast Internet connections and the high price of installing new cable: it costs easily 150 euros per connection on easy cases and easily over 300 euros per connection on harder cases. There was also changes on the telephone regulations in Finland so that the cabling owner could more freely than earlier use free pairs on the existing cabling for other than traditional telephone purposes. Many cables installed in Finland for in-house telephone use had good enough performance for many other uses than just analogue telephone (planned many years ago to be good enough for ISDN services).
Taloverkon kunnostaminen document tells in Finnish how to use those old telephone wires for Ethernet. There was also an article titled “Puhelinjohto muuntuu ethernet-käyttöön” in Tietokone magazine 06/2004 about using old telephone wire for Ethernet.
Finnish telecommunications administration official recommendation in 1989 required at least 3-pair telephone cables. Recommendation on the basis of blocks of flats have been widely used in the MHS model, the inner cable 3 x 2 x 0.5. On the phone pairs is normally only one. Row houses other hand, is already used in homes in the past pulled up to 5 pairs of vmohbu, underground cables
In Finland, the idea of a Helsinki-driven Petri Krohn says that in practice the 3-pair cable to meet the 10BaseT’s or 10 mega-bit Ethernet required by the cat-3 classification. Up to 100-mega-traffic has been a work-mhs-vmohbu and cables.
Krohn believes that the 1970-1980′s, used a four-threaded MMS cable might be a candidate for Ethernet communications. In this case, however, is not free pairs for Ethernet when one pair is used for wired phones.
Functionality also depends on age and manufacturer of cable. Mhs-cable manufacturer in Finland Draka NK Cables Product Development Manager, Esko Aro is no doubt that the MHS in to work at least 10 Mbps data communications.
Here is some information picked from 10Base-T Ethernet puhelimen ylätaajuudelle discussion and translated to English:
Part of the house web sites we have pulled the phone cable alongside the new Cat5e cable to the old pipe. The house of stairs between the backhaul is however, carried out by running 100Base-TX Ethernet VMOHBU Underground. In the same way has been implemented as well as parents of the 1980s terraced 10 items and 100Mbit / s speeds and MOHBU VMOHBU-cables.
Later in the 1995 block of flats is made after the telephone network used for 3-pairs of the MHS 3 x 2 x 0.5 cable for indoor installation. This is successfully used in-house Ethernet network cabling. (Cable Cat3 is a classification, but also to 100Mbit / s is a short distance without any problems.)
So it is possible in many cases use existing telephone wiring for Ethernet connections when there is free wire pairs on the cabling and changing the connectors is not too hard. Before planning to do any bigger changes to this direction, it is a good idea to make sure what kind of cabling there is (check cable type, measure with suitable network cable tester) and weight the pros/cons against other alternatives (rewiring or using VDSL/xDSL technologies etc..).
There has been some cases where I have successfully used old telephone wiring and similar wiring to run Ethernet connections. The old cabling that has worse performance than modern UTP cable can still work well with Ethernet when you don’t need the support full 100 meter distance, if you just need the signal to go 10-30 meters, the cable can be considerably worse in performance than what is specified in standard for reliable operation up 100 meters! I have found out that 10Mbit/s 10BASE-T Ethernet is very robust to run in many kinds of wirings without too much problems (it was designed for CAT3 cabling). The 100Mbit/s 100BASE-TX is more picky on cable quality (need better cabling). And you can forget even truing 1Gbit/s Ethernet, because it needs four pairs and very good quality cable (CAT5 or better).
So sometimes re-using old wiring for Ethernet can make sense. And sometimes it is just better do the re-wiring with new cable or fiber.