Confused RE PoE and Gigabit Ports

[Shutterman]

I’m wanting to add an AP (or router configured as such) to an existing wired network in my home. The AP is for a new laptop that has an Intel 5300 adapter installed. No other wireless devices (yet).

The AP will connect into an unmanaged (i.e., cheap) existing 4 port gigabit switch. The switch connects back to an older gigabit wired router. Two computers on the network are also gigabit capable and wired directly to the router. All the above can handle jumbo frames.

This will be my first foray into the world of wireless computing, so the reviews and articles on SmallNetBuilder have been an invaluable resource.

One such review that caught my eye was on the D-Link AirPremier N Dual Band PoE Access Point (DAP-2553).
Two features that I found attractive about the unit are: 1) it has a gigabit LAN port, and, 2) it “can be powered by power wart or 802.3af PoE”. This last feature would make installation a ton more convenient, and would also provide me a great deal of flexibility in placement.

I was all set to pull out the plastic and place an order, when I noticed this comment at the end of Tim's review from a user who reviewed it:

“Two setbacks, first is that the Ethernet port is gigabyte, but when running POE max is 100 megabit (not DLINK fualt as most POE devices are this way). Second is that it will only run either 2.4 or 5ghz at one time”

I don’t care about the second so-called setback, but I was surprised by the first.

Is this correct…would this unit’s gigabit port only run at 100 megabit speeds if I supplied power via PoE? Is this user correct in that most PoE devices, particularly AP/routers, work this way? If so, what do I need to do to keep my gigabit and jumbo frame capabilities in tact? Do I need a PoE switch?

Any advice would be appreciated.

 

[stevech]

My Opinion:
100BT is fine for realistic WiFi, because the net IP layer yield, again IMO, won't reach 100BT. The air link speed is (WiFi speed reported) is about 30% faster than the net IP yield. Very short range and ideal conditions are needed to exceed 100Mbps at the IP layer with WiFi. Some may claim that over 100BT rates can be achieved with 11n's channel pair bonding (40MHz mode), with ideal signals. But the need? Even streaming HD doesn't consume 100Mbps at IP. Moving lots of large files? Do it by wired ethernet.

So I suggest not paying a premium for a WiFi router with a built-in 1000BT switch. Use an external gigE switch. Same for WiFi APs and bridges.

Others will disagree.

Gigabit WiFi looks good on the marketing slicks.

 

[Shutterman]

My Opinion:
100BT is fine for realistic WiFi, because the net IP layer yield, again IMO, won't reach 100BT. The air link speed is (WiFi speed reported) is about 30% faster than the net IP yield. Very short range and ideal conditions are needed to exceed 100Mbps at the IP layer with WiFi...

Thanks, stevech. I more or less figured this might be the case, but given all the discusson one sees about gigabit and jumbo frame capabilities, I just wasn't sure where it all fit in the wireless world...especially for home users that don't plan to move a lot of data over the air. What you're saying make sense and jibes with the info I've been reading in Tim H's reviews and articles.

All that said, I am curious from a technical point of view to know why this happens with PoE. Is it simply a matter of not enough power? As someone who's not in the technical field, I don't know enough to be able to interpret the PoE specificatons. I therefore simply assumed the spec called for enough power to be delivered to the connected device to do 10/100/1000. What about the injector devices I've seen reference to? Would the situation be the same here as well? (I realize in commercial gear things may be different. So just for the sake of clairity, my questions are mainly consumer gear oriented.)

 

[thiggins]

Is this correct…would this unit’s gigabit port only run at 100 megabit speeds if I supplied power via PoE? Is this user correct in that most PoE devices, particularly AP/routers, work this way? If so, what do I need to do to keep my gigabit and jumbo frame capabilities in tact? Do I need a PoE switch?

The 802.3af spec, which the DAP-2553 spec says is used, uses a phantom power technique, which supports 10, 100 and 1000 Mbit Ethernet connections. You need a 802.3af switch or PoE injector to properly power the DAP-2553.

PoE used to use one or two of the four twisted pair in a CAT 5/5e/6 cable to carry power. This was fine with 10 and 100 Mbps Ethernet which only use two pair. Gigabit Ethernet uses all four pair.
Cheapo "PoE injector" kits don't use phantom power and so aren't compatible with Gigabit Ethernet.

As stevech noted, however, Gigabit for 11n is mostly for marketing. But some N APs can provide > 100 Mbps throughput, in which case the Gigabit port would be needed to not limit throughput.

Unless you have a very busy WLAN, I wouldn't worry about it, though.

 

[Shutterman]

Thanks for taking the time to write up your explanation, Tim. I'm sure others like myself will find your comments interesting and useful as well.

I can't begin to tell you how helpful your reviews and articles have been. Same goes for stevech's posts as well as many, many other regular contributors here on the forum. I've been lurking and reading for quite some time now and feel I've learned quite a bit. Thank you all.

-Shutterman