Digital Packet Padding for USRP Inquiry

I am curious as to the purpose of the 0x55 that is used for padding
packets to the USRP (in addition to sticking one on the end of the
to begin with)? Are these ignored and dropped by the USRP, or do they
serve some other purpose? And if they are ignored and dropped by the
is it just by hope that somebody doesn’t send a string of 0x55 values?

Thanks for the help,

Michael B.