FSO communication links can be deployed in indoor, terrestrial, space, or underwater environments. Depending on the environment, an FSO link experiences different impairments that impact its performance. Even for the same environment, different link configurations can be affected differently by the noise and impairment source, therefore, it is crucial to be able to differentiate link configurations.
This chapter presents a simple, yet powerful classification scheme of FSO technol- ogy. In this scheme, an FSO link can be classified as a combination of four different criteria, namely: Environment (ε), Coverage Type (κ), LOS Availability (α), Mobil-
ity (µ), and link distance (δ). An FSO link can be deployed in an indoor, terrestrial, space, or UW scenario. The link can be either a point or cellular coverage which can be realized using a LOS or NLOS link. Furthermore, a link can be fixed or mobile. Using the discussed four criteria, we were able to develop a generic classification that can be used to categorize different FSO links including recently evolving schemes in which other classifications in the literature fall short. In particular, the proposed classification scheme describes any FSO link configuration as a tuple (ε/κ/α/µ/δ).
We discuss all possible FSO link configurations in the four different environments. We provide examples for each FSO link configuration by listing selected recent ref- erences and related research efforts. Moreover, we briefly discuss the impairments experienced by each link type and their possible solutions.
We also discuss heterogenous FSO link that spans multiple environments. Sev- eral examples including the earth-space communication links have been discussed. A heterogenous FSO link experiences a combinatorial effect due to the different envi- ronments.
Unlike heterogenous FSO link, a heterogenous FSO system might incorporate two or more FSO link configurations in order to improve the system performance combining different links advantages. On the other hand, a hybrid FSO system is a system in which one or more different communication technologies are used along with FSO systems. Examples of each type of systems are provided and discussed.
We use the proposed classification scheme to review existing FSO standards and recommendations. IrDA has produced a set of standards aiming for high data rate short FSO links. JEITA CP-1221, CP-1222, CP-1223, IEEE 802.15.7, and IEEE 802.15.7r1 standards are designed for short/medium range VLC supporting low data rate links. On the other hand, limited efforts are directed towards standardizing terrestrial, space, and underwater FSO links. For example, a single recommendation
for terrestrial FSO links, ITU-R F.2106-1, was proposed by ITU.
We can conclude that the FSO is increasingly becoming an attractive technology for emerging and future communication systems and applications. This holds true for either FSO as a stand-alone technology (as envisioned by NASA in its future space applications), or as a complementary technology (future wireless systems and in UW applications). This chapter presents an attempt to use a simple and powerful classification system to jump-start researchers to tap into the growing and expanding the realm of the FSO technology in indoor, terrestrial, space and UW environments.
Chapter 3
Wireless Data Center Networks
In this chapter, we compare the free space optical (FSO) communication and the 60 GHz radio frequency (RF), the two key candidate technologies for implementing wireless links in DCNs. We present a generic classification scheme that can be used to classify current and future DCNs based on the communication technology used in the network. The proposed classification is then used to review and summarize major research in the area of wireless DCNs.
3.0.1
Motivation and Scope
Most existing DCNs can be classified as wired DCNs in which copper and fiber cables are used for networking. Wired DCNs received an increasing attention in the DCN research community evident by the increasing number of papers and surveys that discuss, analyze, and motivate new developments in wired DCNs (see for example [34, 354–356]).
As discussed earlier, the need for developing adaptive DCNs has motivated the research community to investigate the feasibility of incorporating wireless technologies in DCNs. As a result, several research papers on wireless DCNs have been published.
A few recent survey papers on wired DCNs only briefly discuss the deployment of 60 GHz RF technology in DCNs [34, 354–356]. On the other hand, a recent survey paper that exclusively focuses on the topic of wireless DCNs was published early 2015 [357]. Similar to the survey papers on wired DCNs [34, 354–356], Baccour et al. [357] focus their discussion only on deploying the 60 GHz RF technology in DCNs. In [358], we focus our discussion on DCNs using FSO. We analyze existing indoor FSO standards and the challenges that may face the DCN designers. We also identify standardization needs and opportunities to help accelerate the development of FSO links for DCNs.
From the above discussion, we make the following observations:
1. DCN design space is reshaping as new technologies for networking are deployed, and there is a current need to rethink the design philosophy of DCNs. Therefore, a classification scheme that can formally express the changes in the DCN design space is required to help identify new DCN designs.
2. Deploying 60 GHz and FSO technologies in DCNs encounter different design requirements and challenges. However, as we will show in Section 3.1, there are many similarities between the two wireless technologies. Therefore, we believe that the development of DCNs using one of the technologies can significantly benefit from the other.
In the absence of a systematic description of the DCN design space evolution, it can be difficult for researchers to fully explore the DCN design space and identify potential designs. This motivates us to develop a new survey to collate and present current advances in wireless DCNs in a systematic fashion to facilitate the sharing of knowledge among researchers using different wireless technologies to develop wireless DCNs. We propose a classification that can be used to classify existing and emerging
wired and wireless DCNs. Based on this classification, we survey current state of the art of wireless DCNs. We review the requirements, challenges, and trends using 60 GHz RF and FSO technologies. The proposed classification leads to a nearly complete picture of the design space for DCNs. This help us to identify potential unexplored solutions for next-generation DCNs.