Abis is the link between BTS and BSC.
This link is usually a radio link, or an optical link. In some cases (distant BTS), the only way for the BTS to communicate with the BSC is by satellite. In this case, you need special VSAT equipment, of course, but by selecting this option the BSS will automaticaly adapt some timers to take into account transmission delays due to satellite (RTT = 500ms).