Although 3G networks have been largely deployed to cope with the increasing demand of wireless data services, little is known on how these networks are used from the network perspective. In this paper, we present analysis of data services based on a nation-wide 3G network trace collected from one of the largest cellular network service providers in North America. Our work differentiates from previous studies by examining data service usage and mobility patterns from various dimensions including application breakdown, user roles, device types and diurnal characteristics. We also look into various access issues such as termination failures and frequent registrations to better understand how the network performs. Our results are important for cellular network operators and protocol designers to improve data service performance and user satisfaction.