5G is the fifth generation of cellular network technology. It follows 2G, 3G, 4G, and their respective associated technologies, and also addresses specific requirements for new services ranging from industrial IoT to critical communications. There are three major 5G service scenarios: eMBB, uRLLC and mMTC. The eMBB service needs huge network bandwidth, the uRLLC service requires low network transport latency, and mMTC service ask for massive connections. Compared with 4G, 5G can bring a revolutionary network experience and a new business application model as well as unlimited possibilities for operators' future service expansion, while posing new challenges for transport network. As a growing number of 5G services emerge, the network should not only provide ultra-broadband, but also meet multiple demands such as network simplification, flexible connections, latency optimization and E2E slicing to ensure SLA of diverse services.