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cable sharing isdn/ethernet UTP

Hello experts,

is it possible (and officially supported) do implement "cable sharing" to use ONE (8 wire, 4 pairs) cable to connect Ethernet (using 4 wire, 2 pairs, 100MB only) and IDSN (using the other 4 wire, 2 pairs) at the same time using a unshielded CAT5 cable (~60 ft, UTP).

There are many contradictory statements which can be found in the web
- some Manufactures offering cable sharing adapters exactly for this purpose say 'yes'
- some administrator forums warn of that and say 'never do' (because of NEXT problems)

So whats the official conclusion ?
Where can I find links that prove that ?

Thanks in advance and have a happy new year !

6 Solutions
Both the most popular form of Fast Ethernet (100BASE-TX) and 10BASE-T standard Ethernet use two of the four wire pairs found in UTP Category 5 cable. (These wire pairs are also found in Cat 5e, Cat 6, and Cat 6a cable.) An alternative Fast Ethernet standard called 100BASE-T4 uses all four wire pairs in UTP Category 5 cable, but this Fast Ethernet standard was never popular and is seldom seen today.

Craig BeckCommented:
You 'can' use Ethernet splitters.  I have done this for taking two PSTN lines over one cable, for example.

As long as you split the orange and green, and blue and brown pairs into their own 'links' you should be fine for FastEthernet.  The orange and green pairs should be for the Ethernet link as they are the standard pairs and are twisted at the correct frequency, and the blue and brown pairs should be for ISDN link as it doesn't matter about the twists for this transmission.
Rob WilliamsCommented:
Absolutely not.  You can certainly split the pairs for analog phone line use, but not for Ethernet.

Network cabling is extremely fussy.  Just untwisting the pairs a couple of inches induces cross talk between the pairs, resulting in lost packets, retransmissions, and as a result reduced performance.  

I spent several years as a physical network troubleshooter, and although splitting the pairs will work, you really cannot measure the problems induced.  To do so you need a proper network cable analyzer, about $8000, which will quickly show you all of the problems and lost performance.

You are best just to add a small switch at the remote end to give you additional ports when looking for a second drop, but you are wanting to mix services so that is not an option.

As for links to verify this, they are difficult to find.  It's a bit like trying to find a link to tell you not to hook a power line to your metal sink  :-)   What you can find are standards such as TIA/EIA-568 (https://www.bicsi.org/)  which very carefully define how a cable must be terminated and handled through out its length.  These are very specific and lengthy documents.   To paraphrase the primary limitation; they state that the cable twist must remain within I believe it's 5/8" of the CAT5E/6 wall jack, making it impossible to spit the pairs.
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Craig BeckCommented:
Cat5 Ethernet cables are sometimes manufactured and sold with only two pairs connected.

It's no good for Gigabit Ethernet, but it most certainly will work for Ethernet and FastEthernet.

To paraphrase the primary limitation; they state that the cable twist must remain within I believe it's 5/8" of the CAT5E/6 wall jack, making it impossible to spit the pairs.
That means you can't separate the individual wires within the pair more than that when terminating into the outlet.  That doesn't mean you can't separate the pairs individually.

The pairs are twisted at different intervals to help reduce the amount of interference between the pairs.  Moving the pairs further apart only increases the resilience - that's a good thing.  This is effectively what we do when we install Cat5 in separate containment to power, for example - we separate the sources.  Do not misinterpret this as meaning that we untwist more cable than necessary - that's not what I'm saying.

The pre-standard PoE implementation used to send more power down the blue and brown pairs than ISDN does, and that has never caused any issues on a Cat5 cable.  This is effectively the same thing when all said and done.
Rob WilliamsCommented:
You can split the pairs, connect it any way you like and you will have transmission.  MIT even transmitted Ethernet packets over barbed wire in a demo.  But split the pairs, without even connecting the different signal sources, test it with a Fluke or HP certification meter and it will fail miserably for a multitude of reasons.  Fact.

This is why large companies like ADP will not even install their software on your network until you provide a recent certification report.  Far to many networks work, but perform poorly due to modifications, poor cable placement,  and poor terminations.  In most cases the users don't even realize it.

As for POE, it is a standard with specifications, which do not include pulling apart the cable.
Although not EIA/TIA 568 compliant this will work depending on the length of the cable and care to use the twisted pairs as already stated.  This is mainly for the Ethernet portion of your cable as the ISDN can run 6,000 - 18000 ft. depending on PRI/BRI.
We had a maintenance guy connect a Cat5e cable like an electrician would by matching colors on each end without knowing there was a specific order and twisted pairs.  On the cable map he had the pins 1-8 matching but since he didn't use the twisted pairs to shield out the noise it failed in actual use.  We re-terminated both ends and it worked but this was a 250-300 ft. run...if it had been shorter it may have worked.
We pull T-1 extensions all the time....they are coming in the building on less quality cable than what you are running to them from inside.
I'm guessing you want to do this because your cable leaves the building and it is difficult to pull another run.
Craig BeckCommented:
You don't need to tell me what a Fluke will say.  I know all about specifications, testing and usage.

A Fluke fails the test straight away on the wiremap so your certification fails right there.  You can choose to continue the test if you want, but it is still a failure.

So, technically it is possible and it will work, depending on how it is implemented.  Officially it isn't a supported approach by any cabling vendor or certified installer.  The cabling standards though, as RobWill quite-rightly said, don't allow for any manipulation of the cable aside from a single connection between two dedicated endpoints.

In my experience at around 60ft I wouldn't say this is going to be a problematic link.

My reference to PoE was purely as an example.  It was not meant to infer that the PoE standard allows for splitting cables, but rather that the concept itself is similar to what this question is asking in that two pairs of wires are used for a purpose which is different to the other two pairs inside the same cable.
Rob WilliamsCommented:
I mentioned it will fail for a multitude of reasons/tests, not just wire map.

I agree it will work, but I can only suggest do so, and have it tested.   We could discuss all day how we think it will perform.  I made my living for 3 years resolving such network issues with Fluke cable certification equipment and Fluke network analyzers and it never ceased to amaze me how minor variations from the norm often had drastic effects on a network.

If doing so I would recommend not splitting the cable but rather get a device to do so after the wall plate, which is what the author was asking about.  However I have seen simple devices by respected cable equipment manufactures, such as a simple male to male adapter, drop network performance by 80%.  The concern is they break the standards within the device such as running untwisted pairs in parallel or even using circuit boards with parallel insulated conductors.

My concern is not that it drops performance by 80%, you would notice that, but 15% you wouldn't.  Standards have been developed for a reason.
GKiesslingAuthor Commented:

so I understand it's not easy to give an answer without any doubt.
Anyway that helped.
Thanks Experts.

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