  Ultra-reliable low-latency communications (URLLC) has been considered as one
of the three new application scenarios in the \emph{5th Generation} (5G) \emph
{New Radio} (NR), where the physical layer design aspects have been specified.
With the 5G NR, we can guarantee the reliability and latency in radio access
networks. However, for communication scenarios where the transmission involves
both radio access and wide area core networks, the delay in radio access
networks only contributes to part of the \emph{end-to-end} (E2E) delay. In this
paper, we outline the delay components and packet loss probabilities in typical
communication scenarios of URLLC, and formulate the constraints on E2E delay
and overall packet loss probability. Then, we summarize possible solutions in
the physical layer, the link layer, the network layer, and the cross-layer
design, respectively. Finally, we discuss the open issues in prediction and
communication co-design for URLLC in wide area large scale networks.
