The cellular wireless systems are gaining more attention among other connectivity solutions for supporting machine-type communications (MTC). The efficient support of MTC relies on incorporating new features in the communication networks. Ultra-reliable low-latency communications (URLLC) is one of the considered features that is essential for the support of mission-critical applications, such as industrial automation, e-health, public safety, and future vehicular communications. Enabling URLLC requires employing enhanced transmission techniques to meet the reliability requirements for both data and control channels. This paper develops general communication models for URLLC in uplink and downlink, considering the errors of data and control channels. The models help in determining the reliability constraints on control information, that should be considered in the design of the future cellular systems. In addition, the paper proposes some enhancement techniques that allow relaxing the stringent reliability requirements of control information.