question on iSCSI and NBD

["sunzen.w" <wangxz@ict.ac.cn>]

I have a question on iSCSI.It is originated from contrast of iSCSI with
NBD.

As all know, NBD also support block storage over IP. So maybe we can
also consider it as a technology of IP storage,which originated even
earlier than iSCSI.

Besides it's simplicity (which may bring less powfulness. ?)
Another feature i think more is that it intercepts on the request level.
while iSCSI intercepts on more lower level,it is on SCSI command level.
This shows a different computing distribution strategy.

But with the prevalence of virtual device(such as LVM ,...)
what are the results of the two different technology?
In order to access the virtual device, the low level request of SCSI
command which are carried by iSCSI should be transformed to upper level
request,while the additional transformation is not needed when using
NBD,where just a simple routing mechanism is needed. 

Do i understand them correctly?
As a technology of IP storage ,iSCSI provide apparent advantages(not all
the aspects) over FCP 
What advantages over NBD does iSCSI provide?  

need all your point and help.

with my best regards.