Ho Chi Minh City's (Saigon) Tan Son Nhat Airport is Vietnam's busiest international air hub, followed by Hanoi's Noi Bai Airpot. A few international flights also serve Danang. Bangkok has emerged as the principle embarkation point for Vietnam but it's still possible to get direct flights from a number of major Asian cities and a few Australian cities. Buying tickets in Vietnam is expensive. Departure tax is US$10, which can be paid in dong or US dollars. There are currently six border crossings for travellers coming to Vietnam, but more may open soon. All crossing points suffer from heavy policing and often requests for 'immigration fees'. For getting to/from China, it's become very popular to cross the border at Friendship Pass, or Dong Dang, 20km (12mi) north of Lang Son in northeast Vietnam, to get to/from Nanning. There is a twice-weekly international train between Beijing and Hanoi that stops at Friendship Pass. The other popular border crossing with China is at Lao Cai in northwest Vietnam, which lies on the railway line between Hanoi and Kunming in China's Yunnan Province. There's also a seldom used crossing at Moi Cai. It's possible to enter Laos from Lao Bao in north-central Vietnam; there's an international bus from Danang to Savannakhet (Laos). The other crossing is at Keo Nua Pass/Cau Treo, west of Vinh. The only crossing to Cambodia is via Moc Dai; an international bus links Phnom Penh with Ho Chi Minh City.