Block 1 contains 15 modules that are continued in thread from Block 0. Three new modules are introduced in Block 1 and three modules from Block 0 that may be completely implemented in the Block 0 timeframe are not upgraded in Block 1 and are therefore not continued as a thread. Essential modules are introduced in Block 1 and they are presented in the graphic below. ICAO has defined these modules as essential as they provide substantial contributions towards global interoperability, safety or regularity of flight. These modules are also considered as providing a prerequisite step towards the aviation system performance objectives of the GANP. The ASBU Minimum Path (Essential, Desirable, Specific and Optional modules) is presented on the following pages.