Cheap n adapters only handle 2.4g

Not all N adapters are created equal. My three cheap laptops all have N wireless adaptors, they can only receive 2.4g signal and the wireless speed is between 50 to 100 mbp depending on the distances between the router and laptops.

My good laptop can handle both 2.4g and 5.4g.  When using 5.4g the wireless speed can reach upto 180 Mbp