AP Cable length issues - Will PoE injectors act as a repeater?

[Greg White]

Hello,

We have a site utilizing the Catalyst 3650 switches with integrated controllers for WiFi.  Unfortunately due to a union/political situation we're not able to work closely with cable designers and installers to ensure cable length restrictions aren't exceeded and often have to get creative with remediation for such problems.

I have 5x3700 APs that are hung in an extremely high ceiling with directional antennas that just can't boot up and connect to the integrated controller from the nearest closet.  The cable length is likely close to 200m.  Unfortunately, installing a 3650 closer to the APs is not an option due to architectural and environmental factors.

I'm wondering if a power injector(AIR-PWRINJ5=) will act as a simple repeater bump in the wire and effectively extend the possible cable length if inserted half way between the AP and the Switch?

We've tried using the 3560C compact switches for this problem which works great for WLC based installations but breaks the integrated controller scenario as all APs must be connected directly to the switch for the integrated controllers to work.

Thanks,

Greg

[Freerk Terpstra]

I never tested this myself, but if I look at Cisco's documentation you are not allowed to double the cable length just because of the PoE injector.

Older documentation states that "The power injectors provide up to 15 watts (depending on the Cisco power supply model) over the unused wire pairs of a Category 5 Ethernet cable, supplying enough power to provide for up to a 100-meter cable run.". I think that this statement is still true today for newer types of injectors / PoE+. The injector does only have to touch 2 wire pairs and if it does nothing else with the others pairs, the maximum length is still 100 meter.

[Ravi Singh]

No it cannot work as repeater. This power injectors provide up to 15.4 W over the unused wire pairs of a Category 5 or better Ethernet cable, supplying enough power for a distance of 328 ft. (100 m) on the Cisco Aironet 1040, 1140, 1260, 2600, and 3500 Series Access Points. While it can also support the Cisco Aironet Series 3600 Access Points when modules are not being used, it is recommended to use the AIR-PWRINJ4= power injector instead for the 3600 series, as it has sufficient power to supply to the AP and any installed modules.

[Greg White]

Thank you for your responses guys.  Can you think of anything creative I can do to solve this?

[Freerk Terpstra]

I can think of one "hack" you can test, but if I would run into into that configuration at a customer site in production I would run away. screaming. And usually that does not happen :-) This hack would involve copper to fiber converters and a local power (or power injector) at the end, but still no guarantee..

I'm afraid that you are going to need to face the politics and re cable stuff or install a extra switch in the middle.

[Greg White]

You're probably right.  Thanks FT.

[Nigrofasciatum]

You can try to introduce a power injector between the switch and the AP, at less than 100 meters from the A . You´ll guarantee the AP receives PoE correctly. However , the cable distance facing the 802.3 Ethernet protocol remains of 200 meters , and this will , if that works, that there are many collisions in the medium due to the timing of the ethernet protocol.

If is possible, you can enter a small switch between the 3650 and the AP, as transceiver "copper-to-copper", without vlans , no routing, nothing ... just making functions of "repeater". Even there are catalyst switches of the Compact series, with inline-outline power (2960CPD-8PT-L,...). They could receive PoE of the 3650 and gives PoE to the AP, without aditional electrical wire.

[Leo Laohoo]

Are you using converged wireless design?  If not, then 5 x 3700 is not much.  You can get a 3560CG-8P to power up the 5 APs.  The 3560C is a compact switch and is FANLESS.  Put the 3560CG closer to where the APs are and run fibre optic back to the 3650 switch.

[Greg White]

I love the 3560C for these situations in WLC based designs but unfortunately in this case we we are using converged wireless design which prevents us from using the compact switches in between AP and 3650 switches.

[Leo Laohoo]

Well, Greg.  You're left with little to no choices.  You can't use power injector and deploy them mid-way.  You can't put a switch nearby.  No other ideas.

[Greg White]

Hi Guys,

I just wanted to share a tip that I received which may allow me to use a compact switch in order to extend the cable length to these APs.

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Somewhere in the network a device has been designated as the Mobility Controller (MC). The MC is capable of supporting directly connected APs as well as traditionally deployed APs.

What you can do is:

1. Install any switch that can provide PoE to the APs.
2. Configure a DHCP scope for the VLAN the APs are connected to that includes option 43 that would include the IP address of the Wireless Management interface of the MC

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I'll update the thread if I get the chance to confirm this.

Regards,

Greg

[Freerk Terpstra]

Hi Greg,

Please do, as far as I know that was not possible with 3650/3850. I guess it is time for me to do some some CA deployments.. :-)