TIM-6301: Principles of Cybersecurity-

Warsinske, J., Graff, M., Henry, K., Hoover, C., Malisow, B., Murphy, S., Oakes, C. P., Pajari, G., Parker, J. T., Seidl, D., & Vasquez, M. (2019). Domain 4 – communication and network security. In The Official (ISC)2 Guide to the CISSP CBK Reference (5th ed., pp. 363-411). Wiley.
Read Domain 4 – Communication and Network Security, pages 363-411.

Surasak, T., & Huang, S. C.H. (, 2019). Enhancing VoIP security and efficiency using VPN. 2019 International Conference on Computing, Networking, and Communications (ICNC), (pp. 180–184).
The author proposes in this paper a method to enhance VoIP security by using a VPN.

Hsieh, W.-B., & Leu, J.-S. (2018). Implementing secure VoIP communication over SIP-based networks. Wireless Networks: The Journal of Mobile Communication, Computation, and Information, 24(8), (pp. 2915-2926). doi:10.1007/s11276-017-1512-3.
This paper proposes a secure implementation of VoIP over a SIP network versus TCP/IP.

Bucșă, R.-C. (2020). Teleworking and securing data with VPN technology. Economy Transdisciplinarity Cognition, 23(1), (pp. 78–85).
In this timely paper, the author discusses VPN security amid the massive work from home due to the COVID-19 pandemic.

Iqbal, M., & Riadi, I. (2019). Analysis of security virtual private network (VPN) using OpenVPN. International Journal of Cyber-Security and Digital Forensics, 1, 58-65.
This paper offers an analysis of VPN security using an Open VPN.

Mike Chapple, James Michael Stewart, Darril Gibson, (ISC)2 CISSP Certified Information Systems Security Professional Official Study Guide, 9th Edition