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So I never knew this till today but I thought you needed CAT5 for VoIP but this business is running VoIP on CAT3.

Don't you need the extra wires for the voice, data and power?

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    There used to be a Cisco product called LRE which was basically in-house DSL.
    – Ronnie Smith
    Commented Jun 9, 2015 at 15:47
  • But there is now a cat 6 AND 6A. I'm currently setting up a VOIP network using cat3 as a backbone. 2 of the 5 100pr home runs are well in excess of the 300' max distance. I'd say they're at least 500' long. So long story short they are running category 5e cables from the IDF to the workstations, using Category 3 cable and Category 3 Jack's to link everything up. And just to add insult to injury they want me to terminate just one pair on the 66 blocks! Yes that's right I said 66 blocks. didn't they get rid of those back in like the 1990s... LOL! But I'm not here to reinvent the wheel I'm just h
    – Mark Smith
    Commented Jul 12, 2019 at 20:17

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Ethernet specifications state four pairs, even if only two are used. The category rating system is a set of signal specifications, independent of the number of pairs. (there are 25, 100, etc. -- even 500 -- pair CAT3 trunk cables.)

As long as there are four pairs, a VoIP phone can be powered and connected over a CAT3 ethernet cable.

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VoIP runs over IP, so it is independent of layer-1. The only sort of restriction based on layer-1 would be the bandwidth required. Voice is not separate from data on the wire.

You may be meaning a VoIP phone, which may be powered by PoE, but most have wall-warts, too. If the Category-3 cable has all four pairs, there is no real reason the it couldn't support PoE, too. Most copper PHY can support 10/100/1000 GB. If you have a PoE 10/100/100 port, you could run it at 10 Mb with PoE.

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  • For a dedicated voip port, absolutely true. However 10/half might be challenging if there was other data that competes with the VoIP traffic
    – Mike Pennington
    Commented Jun 8, 2015 at 22:20
  • I agree, but I think that falls under the bandwidth requirement, not the cable limitations of needing extra wires as suggested by the OP. Way back in the dark ages of WAN, I ran VoIP and data over 128K. The VoIP carrier claimed a 96K requirement for the VoIP alone, and you could certainly tell on the PC when I was using the phone. With easy access to inexpensive 100 Mb WAN circuits, I can't tell anymore unless I use some diagnostic software.
    – Ron Maupin ♦
    Commented Jun 8, 2015 at 22:43
  • it's the half duplex portion I mostly focused on. I don't think QoS is going to manage LLQ on half duplex well just because collisions can defer transmission in unpredictable ways
    – Mike Pennington
    Commented Jun 8, 2015 at 22:56
  • No reason you can't deliver PoE on Cat3 with only two pairs as 802.3af/802.3at using mode A runs the power on the same pairs as a 10/100 connection. I wouldn't recommend it, but it is possible.
    – YLearn
    Commented Jun 8, 2015 at 23:38
  • @YLearn, Yes, from the original PoE standard, power can be over the data pairs, but we have run into different equipment that doesn't seem to follow the IEEE standards. In particular, a major card scanner. It "accidentally" requests PoE over the data pairs and blows up. The vendor blames our Cisco switches, but how they have the pins of their equipment shorted together is non-standard. Needless to say, it is the network's fault...
    – Ron Maupin ♦
    Commented Jun 8, 2015 at 23:56
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Allow me to clarify that It is possible to use cat3 cabling for IP phones in addition to POE.
Since the early 1990s, prior to many IP standards and written documentation for LAN Networks I have connected successfully many computers using only 2-pairs on a cat3 cable. Additionally, you can use 1-cat3 cable split out to 2-data jacks to run 2-computers. But, cat3 is limited to 16mbps, more commonly seen as 10-mbps. A POE enabled switch will provide power on pairs 2 and 3; pins 1,2,3 6. This is known as phantom power POE running on same pairs in which data is transmitted and received.
Let's discuss cat4 cable, very rare to see but supports up to 20mbps only and same configuration as above applies. A POE Adapter provides power in ununsed pairs 1 and 4; pins 4,5,7,8.

Then we come to Cat 5, 5e, 6, and 7 as discussed on the internet.

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  • +1 - there is no need for four pairs to provide PoE - the 802.3af standard has smarts for auto-detecting and then operating on two sets of available pairs
    – Benjamin Dale
    Commented Feb 5, 2017 at 22:42
  • Splitting Category-3 to two different wall plates actually violates the ANSI/TIA/EIA 568 standard. You are required to split after the wall plate. Also, Category-4 and Category-5 no longer exist as cable categories, and Category-7 has never existed, nor does it seem likely to ever be certified. ISO/IEC Class F is what most people mean by Category-7, but the cable categories are defined by ANSI/TIA/EIA, which has so far refused to create Category-7, and has stated that it will not.
    – Ron Maupin ♦
    Commented Feb 6, 2017 at 4:54

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