In 3GPP Rel-13, a narrowband system based on Long Term Evolution (LTE) is being introduced to provide wide-area cellular connectivity for the Internet of Things. This system, named Narrowband Internet of Things (NB-IoT), can be deployed in three different operation modes - (1) stand-alone as a dedicated carrier, (2) in-band within the occupied bandwidth of a wideband LTE carrier, and (3) within the guard-band of an existing LTE carrier. The design targets of NB-IoT include low-cost devices, high coverage (20-dB improvement over GPRS), long device battery life (more than 10 years), and massive capacity. Latency is relaxed although a delay budget of 10 seconds is the target for exception reports. This paper discusses the design and performance of the downlink and uplink data channels. Although the design of these channels is based on LTE, there are some differences, particularly in the uplink, with the introduction of subcarrier-level transmission. The similarities have enabled rapid specification, while the enhancements in the design enable the achievement of the stated targets for NB-IoT.