N 5GHz wide channel wireless extension low throughput, why?

Question

I have 1 Time Capsule and 1 Airport Extreme, both in different to rooms to extend my internet connection across my flat without any cables.
The capsule creates a N-only 5Ghz wide channel network and is connected to a DSL modem in my office room.
The extreme extends that network into the living room, also using wide channel.
For testing I have wired a Mac to the extreme.
When I access from that Mac the capsule disk (so exercising the wireless link between extreme and capsule) I get at most 3.7MBytes/s.
When connecting wirelessly to the extreme it is even worse: 2MBytes/s.
For comparison:
When I wire the Mac directly to the Capsule (so excluding the extreme from the wexperiment) I get about 16MBytes/s.
When connecting wirelessly to the capsule (also exclding the extreme) I get about 9MBytes/s.
It seems that the two base stations do not talk N to each other (and I did not use WDS!). Am I missing the point here? Shouldn't the wireless link between s the N base stations be more than lousy 3.7MBytes/s?
How come?
I'd love to fix this!

Answer 1

William your asnwer sound somewhat standard. We know that WDS halves throughput. We know we can use "extend network" in N networks.
That is exactly why we do that Again: I do not use WDS here (in case I haven't mentioned it explicitely).
I use the N network extension feature and still get these lousy rates.
Could it be that the working principle of WDS and N network extension is very similar?
Does anyone know how both work and what the real difference is? (Except for WDS being for G networks We know that aready.
Another thing:
The antennas in Airport extreme and TC don't seem to be too good: Lifting the station half a meter from the floor up to a table makes a difference of several hundred Bytes/s!