Routing Speed Wireless to wireless

Hi,

when I’m routing wireless through a RB600 performance is half the performance
I expect.

Situation:
(A)RB500 - (B)RB600 - (C)RB600

Bandwidth Test UDP Send
A->B 30MBit
B->C 20MBit
A->C 11MBit

A->B is Nstream
B->C is normal 802.11a

All Cards are R52. Frequency A->B = 5600 B->C=5500
Normal 5GHz (no Turbo).
Distance A->B = 10km, B->C = 1km

What is my problem??

Thanx,
Stefan