I'm curious about why 10Base-T and then 100Base-T Ethernet networks used cables which had four pairs if they only needed two? Were there some cables that only had two pairs?

If we wanted to increase the speed of Ethernet further in the future, could adding more pairs of cables help?

  • 8
    Because they are not ethernet cables, but UTP cables that have various applications, ethernet being only one possible application, and the cabling standard is four pairs for UTP cabling.
    – Ron Maupin
    Sep 29, 2021 at 12:41
  • With 10base-T and 100base-TX, some vendors used the unused 4-5 and 7-8 pairs for PoE. There were both IEEE-standard and vendor-proprietary forms of this. Oct 1, 2021 at 16:57

3 Answers 3


10BASE-T saw first light as StarLAN that made use of the already existing twisted-pair category 3 telephone cabling (instead of the dedicated coax that 10BASE5/2 required). That cabling standard carries four pairs to each wall jack. StarLAN (10) and subsequently 10BASE-T had no use for more than two pairs, so they ignored the other two. 100BASE-TX borrowed FDDI's copper design ("CDDI") with also just two pairs, so it's the same situation.

The already existing cabling is also the reason for the use of straight-through cables as standard (with the crossover, MDI vs MDI-X logic on the device port side), and crossover cables only in special cases. Fiber uses crossovers throughout.

Cables with only 1-2 & 3-6 pairs are sub-standard, yet functional with 10BASE-T and 100BASE-TX. Of course, there are (or at least were) many installations and adapters that would split a four-pair cable into two independent 10/100 links (for cost, cabling restrictions, ...). That won't work with 1000BASE-T and faster though - the faster Ethernet variants split traffic into four full-duplex lanes to reduce the cable's frequency requirements to a quarter, so they require all four lanes.

If we wanted to increase the speed of ethernet further in future could adding more pairs of cables help?

It could, in theory, use more than the current four lanes. But nobody's going to redeploy their whole cable plant. By various standards (e.g. TIA-568), horizontal/tertiary cabling is commonly deployed using twisted-pair cabling (good for up to 10 Gbit/s, depending on category and length), but vertical/secondary cabling and upwards uses fiber anyway.

40GBASE-T and 25GBASE-T using category 8 cabling (30 m max) are already having a hard time competing with fiber and likely we won't get faster TP standards ever. Fiber is already at 400 Gbit/s and ready for much more using WDM or multi-lane fiber which is much more practical than with copper, so that's the future.

EDIT As has been pointed out in the comments, you could also add pairs for more speed using another cable. That is actually common practice called link aggregation (LAG).

However, LAG performance can differ substantially from an equally fast multi-lane link. Multiple lanes across a single cable are irrelevant to performance because data distribution across the lanes is very finely grained - usually on a multi-bit level, depending on the PCS line code.

An aggregated link distributes data coarsely on the frame level. You really have to avoid changing the frame order, considerably hurting overall performance, so traffic isn't distributed dynamically (by link utilization) but statically - by L2 addresses, L3 addresses, L4 ports, ..., depending on the switches and their configuration. The higher the layer, the better.

At best, you distribute traffic on the L4 connection level - but still, no single flow can ever exceed the bandwidth of a single link. With L3-based distribution, that limit applies to any host-to-host connection. Much worse even, L2-based traffic distribution puts that limit on the sum of all flows between any two routers, hosts or mix. Accordingly, interconnecting 10G switches with aggregated 1G links is a bad idea mostly.

Aggregating multiple links is generally limited as well. The most common LAG protocol LACP allows a maximum of eight active links and all switches I've seen limit the number to eight (or less) for static LAG as well. Ethernet's speeds usually increase tenfold between grades, so that's more than you can gain by LAG. Correspondingly, link aggregation is often just a stop-gap measure before upgrading the actual link speeds.

  • Thank you. That does of course makes sense that earlier networks did use a different cable. It's obvious now that you point it out that it should use telephone cabling. I'd upvote, but can't do so currently. Sep 29, 2021 at 12:06
  • And if you are really going to invest in extra cable, you can just use link aggregation. Sep 29, 2021 at 20:20
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    @user1937198 It is a common misconception that an aggregated link performs the same as a single link with the same bandwidth. It doesn't. Also, Ethernet speeds increase tenfold between generations (mostly) and you can only aggregate up to eight links.
    – Zac67
    Sep 29, 2021 at 20:38
  • 2
    @Zac67: Why can you not aggregate more than eight links?
    – psmears
    Sep 29, 2021 at 21:17
  • 3
    Building installations that used one 4 pair cable for two ethernet links, or adapters for doing so, existed.... Sep 29, 2021 at 21:54

There's nothing to stop a new twisted-pair standard using a new connector in the design, with more wires.

If a physical site had to choose between using existing 8-wire / 4-pair and recabling everything to use a new standard, then the additional costs would push most sites to stay with existing wiring for as long as it fills the need.

One half-way point might be to use two RJ45 plugs, with the expectation that forward-planning sites have more than one socket in every desk location. Downside is that this will halve the effective port density in racks and switches, requiring more space in server/distribution rooms.

Ultimately, technologies survive for long after a replacement arrives. Many old timers have likely seen 10base2 still in use in the 2000's, far after cat5 cable "replaced" it. I had a 10base2 link from a router to a firewall for years, because the link was 2 Mbit and 10 Mbit was ample bandwidth, while the LAN was 100 Mbit and gigabit.

I've also seen a POC/demo of 100 Gbit fibre that used ten separate 10 Gbit links combined. However the cost was significant.

  • 1
    Multi-cable approaches as you suggest at the end of the first part of your answer are actually a thing. The general name for such a setup is ‘link aggregation’, and there are a bunch of ways to do it, most of which work pretty well. However, the usual reason to use it is not bandwidth, but redundancy (properly handled link aggregation is resilient against failures of all but one of the cables or NICs). Sep 29, 2021 at 22:29
  • @AustinHemmelgarn yes you're right, but I'm alluding to a theoretical "new connector" where there are more pins as per OP's question, and one cost-effective way to get them would be to use two existing cables into some adapter. This would not be link aggregation/bonding, it would be one interface and function as one.
    – Criggie
    Sep 30, 2021 at 1:51

To answer the part of your question "Were there some cables that only had two pairs?":

I worked at a very large multi-building site that had 8wire/4pr twisted pair in the walls terminating with and split between two RJ13* jacks at each wallplate. (2 pairs going to each) One RJ13 was used for POTS phone, and the other was later used for 10baseT (and later 100baseT even though that's not supposed to work on Cat3) with the two services sharing the single UTP cable in the wall.

On the other end, in the telecom closet, the network parts of the circuit was split out to a large patch panel of RJ13s. Custom 4wire/2pr RJ13 to RJ45 patch cables were used on both ends. We used that successfully for almost 15 years until each building was rewired with new CAT5/6 throughout.

So yes, the fact that only 2prs were needed until gigabit came along allowed us to use cables that only provided that many (as patch cables, and to repurpose of the unused parts of the phone cabling)

EDIT: As has been pointed out, this is not an approved standard

*RJ13: I'm am more of a networking person than a telecom person, so you'll have to forgive me if I've used the incorrect RJ designation for 6P4C

  • That is actually against the standard. Splitting a cable for compatible applications is required to be done outside the wallplate. The horizontal cable is terminated on a single 8P8C connector, and any modifications are external to that.
    – Ron Maupin
    Oct 1, 2021 at 19:30
  • @RonMaupin yes, that may very well be against industry standards, (edited answer to reflect that). However, If you're a large enough organization you can pretty much set your own internal standards (as long as you're not violating some safety code) especially if you can use an existing huge cable plant with 10000+ endpoints for another 15 years. It also may have well been largely installed before there were clear standards for UTP cables. Am not an expert.
    – user46053
    Oct 1, 2021 at 20:05
  • No, the point is that deviating from the standards can cause the applications using the cabling to fail. That is the entire reason for the ANSI/TIA/EIA 568, Commercial Building Telecommunication Standard, which has been around for much longer than 15 years. If each cable in the cable plant can pass the test suite for the cable category, then the cabling can be used for any application that works on that cable category.
    – Ron Maupin
    Oct 1, 2021 at 20:11
  • @RonMaupin TIA586 became a standard in1991 and was used in new construction soon afterward. Cable I'm talking about was installed in the mid 80's, repurposed where already existing for ethernet in the early 90's and used into the early 2000's Right or wrong, it served its purpose. Just pointing out to OP a real world example in which 2pr/4cond cables were used (as patch cords in this case - with necessary explanation about the building wiring) with success.
    – user46053
    Oct 1, 2021 at 20:18
  • Yes, 1991 when ethernet (10Base-T) on twisted pair did not even exist until 1990, and that was on Category-3 telephone cable that will not even run 100Base-T as asked in the question. Category-5 cabling needed to be installed after the standard was published in 1995 to run that.
    – Ron Maupin
    Oct 1, 2021 at 20:39

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