Hi Wolfgang,
no, you do not need to use something like a mute block.
The functionality you are looking for is already build into UHD (I'm assuming you are using a USRP as your transmitting device)
The UHD sink in GNU Radio is able to react to some specific stream tags [1] one of which is the length tag.
You can assign a name for this tag in the UHD source block ('Length tag name' field).
Now, if you have assigned a value to this 'Length tag name' field the UHD sink is somehow reacting as a tagged stream block [2].
If samples arrive at the UHD sink, the USRP will only transmit that amount of samples specified in the length tag starting from that length tag.
Therefore, you should not see any 'U' messages no more as the USRP automatically stops transmitting and does not expect any more samples follow.
However, keep in mind that with tagged streams every first sample of a 'packet' needs to have the length tag attached.
E.g consider the following stream of samples where the first sample has a length tag assigned to it with value 10:
1 2 3 4 5 5 7 8 9 10 11
| |
length tag next length tag needs to be exactly here
value = 10 samples
Hope that helps.
Cheers,
Julian