if this is the case, then you're barking up the wrong tree when discussing using Cat3 cabling for LAN/data communication.
it is highly discouraged to use such cabling as it cannot efficiently or reliably support 100MB operation and definately cannot support 1GB operation.
VoIP requires quality of service which not only entails prioritizing the voice traffic and signalling on a data network but also dictating the type of media to be used for the expected voice quality.
cat3 is prone to crosstalk at 100mb full/duplex speeds as well as imposing distance limitations before the 100mb signal is degraded. (well before ethernet standards stated signal degradation due to media distances)
the above along with the fact that you're thinking of actually putting Cat3 into your existing data network; i think you'll be looking at many weird, intermittent, continuing symptoms if you use Cat3
(along with the fact that the cable plant would not be usable for the future growth of the network)
if you want a VoIP/data network that you can manage and that does not have the users complaining to you every day, i would recommend making the investment involved in installing the correct cable plant. it will save you more than just money in the long run.
I works fine. I have done a couple deployments (against my wishes) but what you do is force 10megs everywhere. At the switch port, on the 79xx SW port and PC port and at the PC NIC card. POE works over CAT3 fine also. I have had 60+ on one floor with it running. A few issues, but it was more bad patch cables. We are going to install about 130 on a temporary floor next week. Same premise... Force 10F everywhere. On the phone, at the switch, etc. Keep your distances low (less than 200) and you should be fine.
I had a customer several years ago that had to install data and voice in their hotel, but couldn't replace the existing, really horrible, cat 3 wiring. So, we installed the Cisco LRE product. That seem to work fairly well, it still had its moments, but it seem to work better than the customer using ethernet over cat 3.
I haven't chacked for a while if it has been EOL'd (wouldn't break my heart if it was) but you may be able to take advantage of that.
What made the customer eventually buy it, was Cisco did a demo of it with voice, data and video over a spool of cat 3 cable, we were never able to get it to work as well as the demo (no surprise there) but the customer was still happy
Just to add a little to the excellent answers you have already received (from Greg,Tommer et.al) We all would prefer to use Cat6 or Cat5e, but sometimes this is impossible due to some restriction (financial,leasehold agreement or something else). There is limited support from Cisco for using Cat3, have a look;
Q What type of Ethernet cables can/must be used?
A We support all standard cable types.
CAT3 Supported for 10Mbps
CAT5 and above Supported for 10/100/1000Mbps
The link between a bus and a drop cable that connects a workstation to the bus.
Category 5 (CAT5) cables support frequencies up to 100 MHz, while Category 3 (CAT3) cables support frequencies no higher than 16 MHz. For this and other reasons, CAT5 is an increasingly popular choice, especially in networks that carry both data and voice traffic.
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