Urban air mobility (UAM) is a concept for creating an airborne transportation system that operates in urban settings with an on-board pilot and/or remote pilot in command (RPIC), or with a fully autonomous architecture. Although the passenger traffic will be mostly in and near urban environments, UAM is also being considered for air cargo, perhaps between cities. Such capability is pushing the current communication, navigation and surveillance (CNS) / air traffic management (ATM) systems that were not designed to support these types of aviation scenarios. The UAM aircraft will be operating in a congested environment, where CNS and ATM systems need to provide integrity, robustness, security, and very high availability for safety of UAM operations while evolving. As UAM is under research by academia and government agencies, the industry is driving technology towards aircraft prototypes. Critical UAM requirements are derived from command and control (C2) (particularly for RPIC scenario), data connectivity for passengers and flight systems, unmanned aircraft systems (UAS) to UAS communication to avoid collision, and data exchange for positioning and surveillance. In this paper, we study connectivity challenges and present requirements towards a robust UAM architecture through its concept of operations. In addition, we review the existing/potential CNS technologies towards UAM, i.e., 3rd generation partnership project (3GPP) fifth generation (5G) new radio (NR), navigation detect & avoid (DAA), and satellite systems and present conclusions on a future road-map for UAM CNS architecture.