The evolution of the 2G/3G to 4G LTE mobile data network focuses on high data-rate, low-latency and packet-optimized architecture integrated with multi radio access technologies. At the heart is the all-IP Evolved Packet Core (EPC) and voice-over-LTE (VoLTE) reuses the IP Multimedia Subsystem (IMS). The control plane of the EPC utilizes Diameter as its base and introduces multiple additional interfaces and reuses the IMS interfaces. The standards bodies have largely ignored the carrier grade aspects of Diameter signaling, e.g., scalability, reliability, redundancy and management. This whitepaper describes the role of the Diameter Signaling Controller (DSC) to address these issues while also serving as a natural host for Diameter-based, value-added applications.