[SoCoSA/discuss] Two 100BASE-TX over one cable

[Trevor Benson]

Can definitely be done.  the JC used to run 1 CATx cable and split use it for ethernet, appletalk and FXS (phone station ports) to each desk.

For what you want to do there are custom "splitters" you can buy.  They plug in like a phone splitter giving 2 ports from a single port.  The splitter provides pins 1,2,3 and 6 are mapped to port 1 (pins 1,2,3 and 6), and 4,5,7 and 8 are mapped to the second ports pin 1,2,3,6.  You need to implement these on BOTH sides of the cable obviously.  

Newer splitters have an ethernet cable built in to extend the unit.  I suggest these units highly because otherwise the old school splitters would be too dense to plug into a patch panel if cables were above and below as well as on both sides. Making space for 2 ports when you only took up 1 becomes difficult in high density patch panels.

Here are some examples around $10.  They used to be much less expensive.
StarTech RJ45SPLITTER Cat 5/5E 2-to-1 RJ45 Splitter Cable Adapter - M/F StarTech RJ45SPLITTER Cat 5/5E 2-to-1 RJ45 Splitter Cable Adapter - M/F
CABLES TO GO 37133 5.9 in 2-Port RJ45 Splitter/Combiner Cable CABLES TO GO 37133 5.9 in 2-Port RJ45 Splitter/Combiner Cable

WARNING:  Depending on your ethernet switch model and speed as well as if the computer is equipped with a 1000Base-T NIC you may have additional problems.  1000Base-T specification requires all 4 pairs to be present, and some interfaces are very touchy about auto negotiation (even at 10/100Base levels) if any of the pairs are missing.  Depending on the card and/or switch you may require to hardcode the connection link and duplex on both sides.  If the switch and the NIC in the computers are both limited to 100Base-TX you are less likely to run into issues with auto negotiation of link.  However I have seen issues of cards refusing to come up in any way even when configured for 10Base-T Half Duplex, so I would suggest testing this out with your equipment and verifying NIC speeds for each desktop.  Testing the 1000Base-T NIC's first with a pair of these would be prudent before spending money on a batch of them and finding problems.  However the same goes for rewiring the cable by hand to multiple RJ45 ports and skipping the splitters so test thoroughly before you spend a bunch of money or time attempting to go this route.

Trevor Benson
A1 Networks  |  Network Engineer
dCAA, dCAP, Network+, CLA, CNA, MCP, LPIC-1
DID (707)703-1041
Fax (707)703-1983 
tbenson at a-1networks.com








On Jun 2, 2011, at 4:55 PM, Jordan Erickson wrote:

> I saw this kind of cabling job to simply hook 2 PCs up to a Linksys
> router, so it is possible...
> 
> 
> gandalf at sonic.net wrote:
>> This doesn't sound impossible. I'd like to run two different networks over the same cable. We've got a VOIP solution that is proving to be clean bandwidth demanding. I'd like to move it to our already existing backup internet connection. However some of the offices only have a single CAT5 jack. So I was thinking a simple solution would be to transmit both connections on the same cat5 connection to the offices with only one plug and then split them off at both ends. A little custom cabling should do it. Of course I couldn't run gigabit over this setup, but most of our machines don't dabble in that anyway. 
>> 
>> Hmmm, another solution might be to just use the internal network and then separate it in the phone room with the VOIP phones configured on one network and the computers configured on another, something like 198.162.0.1 and 198.162.0.2 with each aDSL connection configured as a separate network. With the main infrastructure running at 100mbps and each internet connection running at 3.0mbps or less there shouldn't be any real fight for bandwidth until the actual gateway. 
>> 
>> I understand that each phone connection uses 64kbps but both upload and download meaning that a 384 u/l internet connection can only theoretically support 6 simultaneous calls and probably only really support 5. 
>> 
>> Questions? Comments? Laughter?
>> 
>> 
>> (imagine a cute or dirty tagline here)
>> 
>> 
>> 
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> 
> -- 
> Jordan Erickson - LNS
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