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SFP transceiver question

by Steven Haninger / August 20, 2009 8:33 PM PDT

Here's the dilemma. I've got a 3Com 10 port managed POE switch were using to connect to wireless APs in a school I give some volunteer support to. We've 8 ports in use with APs and one is used to link with the router. We have a potential need to add 2 more APs but, with 9 in use, we're one port short. I have POE injectors but would prefer to connect to this switch because we turn it on and off separately from the other network devices. This POE switch has an SFP port. BTW, the model # is 3CDSG10PWR-US. Can I buy a module to fit this port that allow me to plug the Ethernet cable from the router into it and gain use of all 10 POE ports? These modules come in many forms and some are quite pricey. The manual for the switch doesn't get into this sort of detail. I guess they expect folks who buy these to know how to use them. TIA

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by R. Proffitt Forum moderator / August 20, 2009 9:32 PM PDT

Or they expect us to call the maker. I know Cisco. I know they have engineers and a better level of technicians standing by to answer technical questions. Did you call and ask?

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(NT) Did I call 3Com? No I did not
by Steven Haninger / August 20, 2009 9:54 PM PDT
In reply to: Or.
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Please do.
by R. Proffitt Forum moderator / August 20, 2009 9:59 PM PDT

Cisco offices seem abundant here in New England. They make a technical product and while I know a little about Cisco gear when it comes down to specific part numbers and does it work with this or that I always call them.

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only 9 ports
by bill091 / August 21, 2009 2:49 AM PDT

Had to check this worked the same as most other vendors. Stupid forum will not cleanly let me post the specs on your switch so.

You only have 9 ports. The last port is shared with the SFP module. This is very common trick vendors do when counting ports. The 10th port can either be the built in port or the SFP. Both are not active.

In your case the last port is not even POE so should put your router in port 10

In general I do not think I have seen a SFP that can do PoE itself. They can do copper 10/100/1000 but not the PoE. Many times in switches like this they do not support the copper SFP module since its silly to do it since the built in port is already 10/100/1000 copper.

I think you are out of luck you are going to have to daisy chain another switch to make this work.

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You could be right
by Steven Haninger / August 21, 2009 5:56 AM PDT
In reply to: only 9 ports

10 seemed like a strange number...some sort of 4-4-2 arrangement. We also have a 50 port switch. 40 are standard 10/100, and 2 are 10/100/1000. It also has an SFP port that, I believe, is shared with one of the gigabit ports. I'll re-read the spec sheet on the POE switch. We originally only expected to need 4 or 5 APs so bought a smaller switch. That became 8 APs very quickly do to some amateur miscalculations on my part. Hey, I'm just a guy off the street and not an IT wizard. We'd now like to add more coverage. We use the management capabilities minimally...mainly to enable/disable ports as needed. You can also enable/disable power to any port. I enabled 1-8 so can easily check to see if I can get power to the other 2. Otherwise, the APs we bought came with injectors and we can use those as needed. You just lose some remote management ability though but that's not critical. Thanks for pointing that out. I'll check to see if you're correct that only 9 ports will available for POE.

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Never mind. It should have 10 available ports
by Steven Haninger / August 21, 2009 6:04 AM PDT
In reply to: You could be right

This is the switch

3Com does list accessories for its products. I should be able to find what I need from them. The SFP port shows as dual purpose...copper or fiber. I need copper and it should work as desired if I just find the right module. Thanks again.

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Arggh...maybe you're right after all
by Steven Haninger / August 21, 2009 6:18 AM PDT
In reply to: only 9 ports

Says it's dual purposed with one of the 10/100/1000 ports. It doesn't say shared or which port but I remember that trick with 5 port hubs. The 5th was just an extension of #4 to be used as an uplink without needing a crossover cable. I suppose that all I need to do is enable the SFP and see if another port gets lost. So much for that idea. Happy

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by bill091 / August 21, 2009 7:19 AM PDT
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Thats the one but I linked to the 3Com site
by Steven Haninger / August 21, 2009 9:25 AM PDT
In reply to: .

It shows the SFP 91 and 92 as options. These aren't dirt cheap and I don't see what would be the purpose of having 10 POE ports when one obviously has a need for some sort of uplink that won't need power. The only SFP connector we have now links the fiber optic broadband system to the LAN and that part is owned by Time Warner. I can see using these for fiber or longer distances than Ethernet but all I'd want is to use it to uplink to the router to gain one more port. That doesn't, at this point, look to be an economical solution. This is only a small K-8 school. We installed wireless for the laptop/netbook use in the upper grade classrooms, library, and common areas. It's been working wonderfully for that purpose and don't want to make trouble. I doubt this is a good idea now but was worth a shot. Thanks again

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Looks like I'm dead in the water
by Steven Haninger / August 22, 2009 5:40 AM PDT

Whether I could find a copper SFP module or not isn't going to matter. Port #10 is shared as earlier noted. Thanks for the response. Nothing learned is ever wasted.

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