  With the ever-growing demand for low-latency services in machine-to-machine
(M2M) communications, the delay performance of random access networks has
become a primary concern, which critically depends on the sensing capability of
nodes. To understand the effect of sensing on the optimal delay performance,
the challenge lies in unifying the delay analysis of sensing-free Aloha and
sensing-based Carrier Sense Multiple Access (CSMA) with various design features
such as backoff and connection-free or connection-based. In this paper, based
on a unified analytical framework, the mean queueing delay of data packets with
Aloha and CSMA is characterized and optimized, with which the upper-bound of
sensing time for CSMA to outperform Aloha in terms of the minimum mean queueing
delay is further obtained. The analysis is also applied to the Random
Access-Based Small Data Transmission (RA-SDT) schemes in 5G networks to
investigate when and how significant their delay performance can be improved by
sensing, which sheds important insights into practical access protocol design.
