 unlicensed frequency spectrum bands and why are they used? How are personal communications systems regulated? How can telecommunications systems network without the presence of a central node? What are telemetry and sensor networks and how do they communicate? What is visible light communications and its import? Learning Objectives Unlicensed band wireless communications systems technologies Mesh networking systems Wireless local area networking technologies Wireless personal area networking technologies Visible light networking technologies Broadband Telecommunications Technologies and Management, First Edition. Riaz Esmailzadeh. © 2016 Riaz Esmailzadeh. Published 2016 by John Wiley & Sons, Ltd. Companion Website: www.wiley.com/go/BTTM Personal Wireless Communications Systems Historical Note As discussed in previous chapters, the radio frequency spectrum is generally allocated by a national body to provide specific telecom- munications services to end-users. Governments further design policies to ensure the spectrum is licensed to those organisations which can effi- ciently provide these services. The discovery of dielectric heating at microwave frequencies for medical usage (such as in physiotherapy) and industrial use (such as microwave ovens, pictured [1])¹ in the 1940s necessitated frequency allocations for non- telecommunications applications. However, these devices emitted substantial electromagnetic energy which potentially could interfere with nearby telecommunications equipment operating using the same frequency band. To ensure zero mutual interference a number of frequencies were set-aside worldwide for industrial, scientific and medical (ISM) applications by the International Telecommunications Union (ITU) in 1948 [1]. These are known as ISM bands, and devices can operate in these frequencies subject to transmission power constraints to minimise interference. As no government licence is required for devices operating in these frequencies, ISM bands are also known as unlicensed bands. ISM bands were r