Abstract
Free Space-Optical networks deployed as a mesh topology are well suited for ultra broadband last mile business and residential services. Considered are several capacity and mobility management aspects of such networks.

Introduction
Free Space Optical (FSO) networks are capable of providing low cost fiber-like quality, reliability, and capacity, without requiring buried optical fiber cabling. Accordingly, they are well suited for ultra broadband last-mile business and residential services, and for cellular and WiFi backhaul. An FSO network architectures based on a mesh topology is presented and discussed in considerable detail. Despite the notorious effects of fog attenuation, such a network is capable of providing carrier grade 99.999% availability by virtual of its physically short links (less than several hundred feet), yet is able to span a small-to-medium sized city. Considered are some traffic handling and (when used for cellular or WiFi backhaul) some mobility management aspects of such networks

Free Space Optics Fundamentals
An FSO link appears much like a fiber optic link in that the transmit end uses a laser diode and the receive end uses a photodetector. Missing is the optical fiber. This is replaced by a pair of lenses, one at each end of the link. The transmit lens collimates the light produced by the laser into a narrow cone; the receive lens captures a portion of the transmitted light and focuses this on the photodetector. Path loss (proportional to 1/R2) and atmospheric loss (e-aR) limit the amount of light available at the detector and, consequently, the maximum length of the link, R. Also, servo motors are often needed to keep the transmit and receive lenses aligned.
Although clear air, rain, and snow all contribute to the atmospheric attenuation coefficient, by the far the most notorious impairment is fog. Since the presence and density of fog are random variables, it is common to speak of the availability of an FSO link as a function of link length. Sought is the length limit at which a certain availability is assured; this maximum link length is a function of the link margin and geography, being longer where dense fog occurs less frequently. Using a 20 mw laser diode and lenses each with a diameter of 2 inches, a maximum length of 100 meters is achievable at 622 Mb/s (OC-12) with an availability of 99.999% in most world cities, with a significantly greater length possible in many of these cities. Of course, considerably greater link length is possible if the availability requirement is relaxed.

Free-Space-Optical Mesh
To provide high reliability, it is imperative that the FSO link length be kept short. A mesh topology, shown in Figure 1, achieves this objective. Here, each network node serves one client (building, wireless access point, etc.) and consists of up to four FSO transceivers plus a high speed packet switch to collect and distribute traffic to the client and to route traffic between the client and the fiber infrastructure. Since each node regenerates and repeats traffic, the links closest to the fiber will congest well before those further away. Nonetheless, it has been shown that if the capacity of each FSO link is, say, C units, and if no client generates or terminates more than C units of traffic, then at least 5C units of traffic can always be delivered to the fiber infrastructure.
In this talk, we generalize upon this result to show that a total of at least KC units of traffic can always be delivered to the fiber infrastructure, where K is a positive integer greater than or equal to 5, provided that no node generates or terminates more than BC units of traffic, where B>0 and
B = 4/(K-1), K odd B = 4(K-1)/(K2-2K), K even
Thus, for example, at least 10C units of traffic can always be delivered to the fiber infrastructure if no client generates or terminates more than 0.45C units of traffic. Furthermore, we show that at least these levels of traffic can be carried bi-directionally, as long as every client in the network terminates more traffic than it generates, or vise-versa.
When used for cellular backhaul, if each access point can support C units of traffic, then up to 5C units of traffic can be generated collectively by all mobile units, with connection dropping overwhelmingly dominated by the unavoidable instance where too many users try to connect via the same access point. Thus, under these conditions, connections will rarely be dropped because of congestion of an FSO link.

References:
[1] Acampora, Boom, and Krishnamurthy, IEEE Jsac, Aug.
[2] 1998; Acampora and Krishnamurthy, IEEE Personal Comm, Oct. 1999;
[3] Acampora, Scieentific American, July 2002