hi pix
In an extended range cell, the timing advance corresponding to the real signal delay (measured in the BTS) is called Virtual Timing Advance (VTA). VTA is the timing advance that would have been used if the air interface protocol had allowed a
TA larger than 63. When VTA is larger than 63, the time slot next to the one assigned is also needed for reception. At
immediate assignment the BSC extracts the VTA value from the
random access delay information it receives from the BTS.
When the call is established, VTA is regularly sent from the
BTS to the BSC.