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We run a Netgear GS724Tv3 switch and part of the configuration per port is an option for "Lower Power Mode". The documentation for the switch lists this feature as a "Green Ethernet Configuration" feature:

A cable test is performed when this is enabled and the port link is up at 1 Gbps: if the cable is less than 10m, PHYs are placed in low power mode (nominal power)

We have purchased a set of short patch cables of lengths < 1m and we might possibly be having issues with a few of them (testing is still underway). We have this feature disabled at the moment.

Should this feature be enabled if the cables between a NIC / SWITCH are < 10m where power consumption is not a concern? As a follow-up question is there a minimum cable length requirement for CAT6 gigabit?

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I'll be interested to hear the answer to this one. I got into an argument with a former IT vendor for one of my Customers who swore that there was a 1M minimum length limitation for gigabit Ethernet but couldn't provide any documentation therewith. FWIW, this "lower power mode" sounds dodgy and I'd avoid it. What are the possible cost savings from an Ethernet switch, per year? Is it worth the risk? (I can't say I'd be using a "Netgear" switch in any kind of corporate capacity either, but...) –  Evan Anderson Feb 1 '10 at 20:52
    
Minimum cable length sounds vaguely familiar to me too. However, I was thinking that it was for fiber optic. Hmmm. Trying to think of a part of the spec where a minimum might be required. –  Brian Knoblauch Feb 1 '10 at 21:03
    
If I had my network gear handy I'd just make a patch lead as short as my crimpers (and fingers) would allow and see what's what. Although I guess if it's too short you'll loose the T part of UTP. –  Mark Henderson Feb 1 '10 at 21:37
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The minimum cable lengths are only relevant for half duplex mode where CSMA/CD comes into play, so they are relevant for 10Mbps and 100Meg half duplex. If you are running your GigE network Full Duplex they shouldn't be relevant. I'm looking for a definitive reference for this but haven't found one yet, when I do I'll answer. –  Helvick Feb 1 '10 at 21:43
    
@Helvick. I think you are right, the wikipedia article has a mention about this, but it doesn't say what the minimum length is. It only says that it inserts blank frames in half-duplex mode (GigE only) –  Scott Lundberg Feb 1 '10 at 22:48

6 Answers 6

up vote 3 down vote accepted

According to: http://www.ctrlink.com/2006_07_01_archive.html

there is no minimum length when using a Star topology. (one node connected to each port of a switch). In the older times, when you had an Ethernet ring, there had to be minimum distances between devices to prevent impedance problems on the ring.

Concerning the low power mode, are you sure that isn't related to POE? I don't see how you can lower the power and still meet the guideline for voltage. Based on the impedance of the cable, the power consumed will be directly related to the voltage put on the cable by the NIC... See Ohms law, which isn't perfect when you are talking high-frequency waveforms, but the same principles still apply.

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Thanks for the info - POE doesn't exist on this switch. This is an energy savings feature according to the manual. downloads.netgear.com/files/GS716Tv2_GS724Tv3_usermanual.pdf –  Geoff Dalgas Feb 2 '10 at 0:37
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Per this link, it's only powering down when the port is idle. Makes sense... lightwaveonline.com/equipment-design/transport/news/… –  Scott Lundberg Feb 2 '10 at 2:19
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Apparently this is part of an upcoming spec for Ethernet. 802.3az spec. ieee802.org/3/az/public/index.html –  Scott Lundberg Feb 2 '10 at 2:21

This question is still missing an authoritative reference, so here it goes:

There is no minimum length specification to a 1000BASE-T segment.

Charles E. Spurgeon - Ethernet, the Definitive Guide, p. 163.

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I've never seen a minimum cable length recommendation. This feature seems like a fairly straightforward power-saving feature. Shorter cable runs incur lower power loss.

According to the NetGear documentation, there are actually two separate power modes, both of which can be enabled. One of the modes is for cable less than 10 meters.

http://kb.netgear.com/app/answers/detail/a_id/11701

"What are the Green Ethernet Features?

"GS716Tv2 and GS724Tv3 support two power saving modes: Auto Power Down Mode and Short Cable Mode. Both features can be enabled at the same time. When Auto Power Down mode is enabled, power will saved for ports that have no link. When Short Cable Mode is enabled, the device will check the cable length and reduce the output power if the cable length is less than 10 meter."

Here is a fairly good overview of the TIA-568-B recommendations and practices:

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I can't find a link to the spec (which isn't suprising as TIA sells it) however seimon has a pretty good selection of summaries. There is no mention of a minumum length but there is one since the minimum bend radius dictates it for the cable (4 times the cable diameter for horizontal UTP cables under no load conditions- I'll let some math wizard figure that arc length out). It's certainly less than 1M. Here is the summary of cable practices here: TWISTED-PAIR CABLING INSTALLATION PRACTICES

There is a guideline in ANSI/TIA/EIA-568-B.1 that says the consolidation point should be located at least 15 meters away from the telecommunications room to reduce the effect of connectors in close proximity.

Also note that (particularly older cat6 cable) some cat6 cable has splines that will prevent the cable from exceeding the minimum bend radius and will most certainly not be useful shorter than about 2 ft. I haven't seen much of that out there but it's out there.

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Yeah, for just the low low price of $354 you download the pdf! –  einstiien Feb 1 '10 at 21:43

I've often wondered about this issue myself. Everything I read online is speculation pointing either to no minimum or 1m. I once had an electrical engineer tell me that from an electrical standpoint you would want some distance (he said > 3ft) to allow for the twisted pairs to operate efficiently, I am by no means an electrical engineer though so i can't explain it beyond what he told me.

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To the best of my knowledge there is no minimum cable length beyond the practical (how closely can you get the connectors to each other and successfully terminated).

I have personally used Cat5e cables in the 4 and 6 inch range at Gig speeds without any issues. (This was with Cisco 3500/3550/2960 switches it should be noted).

This is pure speculation but, the "Lower Power Mode" sounds like the switch will energize the conductors/pins with less electric power at shorter distances. I don't know how much power savings this could possibly be -- perhaps their literature has projections?

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