INTRODUCTION Optical Wireless Communications (OWC) is a niche technology combining the best of both the wireless and optical worlds, enabling the develop- ment of ultra-broadband, agile and non-obtrusive communication systems [1, 2]. OWC is being applied in numerous commercial indoor and out- door systems that range in scale from short- range/personal-area to access/metro area networks. In particular, OWC is compatible with other well-established technologies like optical fiber and interior lighting, and can benefit signifi- cantly from advances in these industries. As a result, OWC during the last decade has seen a sig- nificant increase in the attained bandwidth and an equally important cost decrease. OWC is expected to keep up these trends, closely following the advance of fiber optics, and provide even more broadband and cost-effective systems in the future. The potential of OWC for broadband, cost- effective, safe and non-obtrusive communication systems has attracted the interest of two major standardization bodies, IEEE and IrDA (Fig. 1). IEEE included an OWC implementation sce- nario of the original 802.11 standard, which has been discontinued. ABSTRACT The application of Optical Wireless Communi- cations (OWC) has grown in recent years that the whole industry would benefit from common stan- dards to which competitive products comply. Standards are essential, particularly when the market expands into high-volume products like home appliances and other consumer goods. This article offers a timely review of standards-writing activity, as OWCs find their way into diverse products varying from TV remote controls to satellite links. This article discusses the most popular standards for optical wireless communications. We outline the IEEE 802.11 standard for optical wireless local area networks, and the ongoing standardization effort by IrDA on personal optical wireless sys- tems. The article concludes with a discussion of the recently announced IEEE 802.15.7 standard on visible light communications. Instead, IEEE continued to pursue OWC standardization. The result is the state-of-the-art 802.15 standard on Visible Light Communica- tions (VLC) that use commercial LEDs to pro- vide broadband indoor communications. IrDA, on the other hand, has maintained a comprehen- sive list of standards that focus mainly on the implementation of OW links for Personal Com- munication Systems (PCS). OW-based personal communications at 1 Gb/s have been demon- strated and a ten-fold increase in bandwidth is expected in the near future. This article is organized as follows: we offer a short overview of the IEEE 802.11 OW standard and the challenges it faced. We discuss the IrDA protocol stack and unique solutions IrDA adopt- ed for implementing a low-cost, high-bandwidth PCS. We present the IEEE 802.15 VLC stan- dard, focusing on the key requirements of the physical and link layers. Finally, we summarize ongoing efforts for the development of future standards. OPTICAL WIRELESS LOCAL AREA NETWORKS Following the introduction of Wireless Local Area Networks (WLANs), in 1997, IEEE released the initial IEEE 802.11 standard for the physical (PHY) and Medium Access Control (MAC) layers [1]. Three choices are standardized in the PHY layer; two utilize radio (Direct Sequence and Fre- quency Hopping Spread Spec- trum modulation) and one utilizes IR signals, although a single product can support only one of them. All three transmission methods provide a basic data rate ( R) of 1 Mb/s and an optional rate of 2 Mb/s. The PHY of 802.11 is split into the Physi- cal Layer Convergence Protocol (PLCP) and the Physical Medium Dependent (PMD) sublayers. PLCP prepares (parses) data units that are trans- mitted (received), whereas PMD performs the data transmission (reception) and modulation (demodulation) under the guidance of PLCP. The IR PHY supports two different Pulse Posi- tion Modulation (PPM) schemes, 16 PPM for 1 Mb/s and 4 PPM for 2 Mb/s. The peak-power wavelength of the transmitter (Tx) is between 850 and 950 nm, while a typical link length L is limited to 10 m. Finally, two main transmission techniques are used: point-to-point and diffused. The MAC layer is based on the CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) technique, where the station that has a frame ready for transmission first senses the channel. If the channel is free for a period longer than the DIFS (Distributed Inter-Frame Spac- ing) duration, the station transmits. If the chan- nel is busy, the station continues sensing until the channel is free for a period longer than DIFS. Afterward, the station employs a con- tention-resolution method prior to sending, namely Binary Exponential Back-off, so as to minimize the probability of frame collisions due to multiple simultaneous transmissions. IR transmission was initially employed in IEEE 802.11 because it provides a number of sig- nificant advantages over radio approaches such S TANDARDS FOR I NDOOR O PTICAL WIRELESS C OMMUNICATIONS The application of Optical Wireless Communications (OWC) has grown so much in recent years that the whole industry would benefit from common standards to which competitive products comply. Standards are essential, particularly when the market expands into high-volume products like home appliances and other consumer goods. This article reviews the existing OWC standards and points toward future directions A. C. Boucouvalas, Periklis Chatzimisios, Zabih Ghassemlooy, Murat Uysal, and Konstantinos Yiannopoulos 24 Service-specific convergence sublayers (SSCS) Service access point (SAP) Optical media MAC MAC common part sublayer (MCPS) MAC link management entity (MLME) PHY PHY layer data (PD) PHY layer management entity (PLME) PHY-switch COMMUNICATIONS TANDARD S S A. C. Boucouvalas and Konstantinos Yiannopou- los are with the University of Peloponnese. Periklis Chatzimisios is with Alexander TEI of Thessaloniki. Zabih Ghassemlooy is with the University of Northumbria at Newcastle. Murat Uysal is with the Ozyegin University. This work is supported by COST Action IC1101 Optical Wireless Com- munications-An Emerg- ing Technology (OPTICWISE). IEEE Communications Magazine — Communications Standards Supplement • March 2015 0163-6804/15/$25.00 © 2015 IEEE