  Biometric research is directed increasingly towards Wearable Biometric
Systems (WBS) for user authentication and identification. However, prior to
engaging in WBS research, how their operational dynamics and design
considerations differ from those of Traditional Biometric Systems (TBS) must be
understood. While the current literature is cognizant of those differences,
there is no effective work that summarizes the factors where TBS and WBS
differ, namely, their modality characteristics, performance, security and
privacy. To bridge the gap, this paper accordingly reviews and compares the key
characteristics of modalities, contrasts the metrics used to evaluate system
performance, and highlights the divergence in critical vulnerabilities, attacks
and defenses for TBS and WBS. It further discusses how these factors affect the
design considerations for WBS, the open challenges and future directions of
research in these areas. In doing so, the paper provides a big-picture overview
of the important avenues of challenges and potential solutions that researchers
entering the field should be aware of. Hence, this survey aims to be a starting
point for researchers in comprehending the fundamental differences between TBS
and WBS before understanding the core challenges associated with WBS and its
design.
