function- ality. Such decoupling offers the opportunity to change the current limitations. In SDN, modifications to the control plane can be inde- pendently done, while the switching hardware remain unaltered. In NFV, the separation of software and hardware of NFs allows each part to grow and innovate independently. The modularity design choice in SDN and NFV comes with a number of economic advantages, espe- cially in the following areas: Lowering the entry barriers for control plane product and switching plane gears Independent resource/budget allocation of each module Providing high extensibility because each module has clear interface with the rest Enabling rapid prototyping, instead of waiting for every component to be ready Allowing new players to enter the market. There are mutual bene- fits to both traditional equipment vendors and network operators, enabling small players to grow 1.1 Cloud-Enabled 5G: SDN and NFV Increasing the speed of innovation. One key obstacle to any network innovation deployment is the incremental deployability and incen- tives for the first movers. Seamless interoperability usually slows down the deployment process and decreases the incentives for first adopters. With the modular design, first movers can gain techno- logical advantages more easily when deploying new networks and services. 1.1.2 Challenges While SDN and NFV have great potentials as discussed earlier, apply- ing them in production networks faces several key challenges. Scalability: The SDN and NFV architectures need to scale to sup- port millions of subscribers, increasing traffic demand and future growth. It needs to be able to meet the necessity of adding more con- trollers, switches, and NFs to a network. There is a potential need for a hierarchy of control components, or a chain of NFs, and might take a peer-to-peer approach to further expand the scalability. Performance: Carrier network has high requirement on perfor- mance than enterprise or data center networks. The links are usually tens to hundreds of Gigabi