Attacking the connectivity bottleneck

The world of optics is in a period of flux, a time of constant and sometimes spontaneous change. A few years ago, there was a movement to make the core of networks optical. Three years later that effort is still under way, as more and more resources are poured into metro optical networks in anticipation of an even higher bandwidth explosion.

This bandwidth demand explosion has already occurred. It has happened at the network edge, but has not reached the network core. Soon there will be large, unused pipes in the metro core--but not because there is no traffic. The traffic exists, but it cannot find its way to the core because the connectivity from the edge to the core is not well developed.

In typical views of metro optical networks, you see three segments--metro core, metro access and metro edge. Each represents parallel efforts to make this an all-optical world where all protocols and network elements can co-exist. But since metro edge, metro access and metro core have not found each other, due to either lack of foresight or simple priorities, the situation has given birth to what is fast becoming an optical dead zone. 

This optical dead zone is a hindrance to the growth of optical networks on two counts:

• It creates a void where it becomes too complicated to manage and sustain networks. 

• It prohibits revenue growth. 

Free-Space Optics A line-of-sight technology that enables optical transmission of data, voice and video communications though the air, allowing optical connectivity without laying fiber or securing spectrum licenses.  The technology was originally developed more than two decades ago and was initially used by the military and space aviation pioneers for secure communications. Recent developments have advanced it from a short-term solution for short-haul bridges to a viable alternative for helping service providers deliver the promise of optical networks.

For carriers to generate revenue, they have to get this high bandwidth to the ultimate end user at the edge and connect them to the core. The only way to achieve this high bandwidth goal is to use an optical technology that is not only cost-effective, but also scalable and quick to deploy. An old technology called free-space optics fits the bill for this modern dilemma.  

This line-of-sight technology technology requires light, which can be focused by using either light emitting diodes (LEDs) or lasers (light amplification by stimulated emission of radiation). 

The use of lasers is a simple concept similar to optical transmissions using fiber optic cables; the only difference is the medium. Interestingly enough, light travels faster through air than it does through glass, so it would be fair to classify free-space optical communications as optical communications at the speed of light. 

As an optical technology, FSO is a natural extension of the metro optical core, bringing optical capacity to the edge of the network by connecting the ultimate end user--cost effectively, reliably and quickly.

The Market

The increasing demand for high bandwidth “now” in the metro networks—as service providers clamor for a wide range of applications, including metro network extension, enterprise LAN-to-LAN connectivity, wireless backhaul and LMDS supplement—has caused an imbalance. This problem is often referred to as the “last mile bottleneck.” 

Service providers are faced with a need to turn up services quickly and cost-effectively, at a time when capital expenditures are restrained. But the last mile bottleneck is only part of the problem. Similar issues exist in other parts of the metro networks. Instead of calling it the last mile bottleneck, it should be called the “connectivity bottleneck,” which better addresses the core of the problem. As any network planner would tell you, the connectivity bottleneck is everywhere in metro networks.

View Figure that compares FSO with other access methods View Table that compares the features of FSO with DSL, LMDS and fiber

There are a few alternatives to address this connectivity bottleneck from a technology standpoint--but most don’t make economic sense.

The first, most obvious, choice is fiber. Fiber, without a doubt, is the most reliable means of optical communications. But the digging, delays and costs to lay that fiber often make it prohibitive. Also, once you lay fiber, it becomes a sunk cost. If the customer leaves, it is almost impossible to recover that cost.

The second choice is a system using radio frequency (RF). This is a mature technology that has been deployed for several years. Although they offer more range than FSO, RF-based networks require immense investments to acquire the spectrum license, yet they can’t scale to optical capacities. The current RF bandwidth ceiling is 622 Mb/s. When compared to free space optics, RF does not make economic sense for providers looking to extend optical networks.

The third alternative is copper-based technologies--cable modems, T-1s or DSL. Even though copper infrastructure is available on a wider scale, and the percentage of buildings connected to copper is much higher than fiber, it is still not a viable alternative for solving the connectivity bottleneck. The biggest problem is bandwidth scalability. Copper technologies may ease some pain, but the bandwidth limitations and regular fees make them a marginal solution, even on a good day.

The fourth--and most viable--alternative is FSO. It is the optimal solution, in terms of technology (optical), bandwidth scalability, speed of deployment (hours versus months) and, above all, cost effectiveness (at least one fifth the cost of installing fiber).

According to RHK, only 5% of the buildings in the United States are connected to a fiber backbone, yet 75% are within one mile of fiber. As technology and demands increase, it is safe to assume that many of these businesses run high-speed local area networks, which makes it frustrating to be connected to the outside world through low-speed connections such as DSL, cable modems or T-1s. Most of the trenching to lay fiber has been done for improving the metro core (backbone), while the access and edge have completely been ignored. 

LMDS Local multipoint  distribution system  The broadband wireless technology used to deliver voice, data, Internet, and video services in the 25-GHz and higher spectrum (depending on licensing). --From the IEC WebProForum tutorial on LMDS Read more

Studies have shown that disconnects also occur within the core, primarily due to cost constraints, combined with moratoriums, the deployment of such non-scalable technologies as local multipoint distribution system (LMDS), and the time it takes to bring the technology to market. Needless to say, metro optical networks have not yet delivered on their promise. High capacity at affordable prices still eludes the ultimate end user. This is where free space optics comes in--offering service providers a viable alternative and complement to fiber optics for optical connectivity.

How it works

There is a misconception in the industry that free-space optics is a “wireless” technology; instead, it is clearly an optical technology. FSO involves the optical transmission of voice video and data using air as the medium of transmission instead of fiber optic cable. 

FSO technology is relatively simple. It’s based on two units, each consisting of an optical transceiver with a laser transmitter and a receiver to provide full duplex (bi-directional) capability. Each FSO system uses a high-power optical source (a laser), plus a telescope that transmits light through the atmosphere to another telescope receiving the information. The receiving telescope connects to a high-sensitivity receiver through an optical fiber. 

Unlike radio frequencies, the technology requires no spectrum licenses. It is easily upgradeable, and its open interfaces support equipment from a variety of vendors, which helps carriers protect their investment in embedded infrastructures.

Unlike wireless technologies, FSO enables optical transmission at speeds up to 1.25 Gb/s today, and soon speeds up to 2.5 Gb/s and 10 Gb/s using WDM will be possible. This is not possible using any fixed wireless/RF technology existing today. FSO obviates the need to buy expensive spectrum, which clearly distinguishes it from fixed wireless technologies. Unlike wireless technologies,  FSO’s similarity to conventional optical solutions will enable the seamless integration of access networks with optical core networks and help to realize the vision of an all-optical network

FSO and Fiber

While fiber optic communication has gained acceptance in the telecommunications industry, free-space optical communication is still a relatively new entrant. FSO enables similar bandwidth transmission abilities as fiber optics, using similar optical transmitters and receivers, even allowing WDM-like technologies to work in free space.

While it takes months if not years to enable fiber optic communications, due to the combination of acquiring the physical fiber infrastructure and permits to dig in cities, free-space optical communications can be implemented in a matter of days or months at a fraction of the cost. 

Fiber deployments also represent a sunk cost, which is lost when the customer leaves the building or decides to cancel the service. FSO, on the other hand, can be redeployed. Furthermore, due to its flexibility and ease of deployment in multiple architectures, either behind windows or on rooftops, FSO offers economic advantages over fiber optics.

Fiber deployment in urban areas can cost $300,000-$700,000, given the costs involved in digging tunnels and acquiring right-of-way. By contrast, a short FSO link of 155 Mb/s might cost only $15,000-$18,000.

There also are environmental benefits to the technology. While digging trenches for fiber increases pollution, destroys trees and can lead to the destruction of historical landmarks, FSO is a non-intrusive eco-technology, allowing the co-existence of technology and the environment.

Challenges

In light of the similarities between fiber and FSO, it is important to mention one of the most prominent differences: While fiber is a relatively predictable medium, free space is an open medium that is subject to outside disturbances. Networks must be designed to counter the unpredictable nature of the space, which can affect the system’s capacity. FSO is also a line of sight technology, which means that the points that interconnect have to be able to see each other without interference.

All the factors can be countered through network design and technology, but it is important to be aware of certain conditions that can affect FSO:

Fog. Although rain and snow don’t affect FSO, fog does present a significant challenge. Fog is vapor composed of water droplets, which are only a few hundred microns in diameter, that can modify light characteristics or completely hinder the passage of light through them. This is a combination of absorption, scattering and reflection. The solution is a network design that shortens distances and adds redundancies. FSO installations in such foggy cities as San Francisco and Seattle have successfully achieved carrier-class reliability.

Absorption. Absorption occurs when suspended water molecules in the terrestrial atmosphere extinguish photons. This causes a decrease in the power density (attenuation) of the beam and directly affects the availability of a system. Absorption occurs more readily at some wavelengths than others. The use of appropriate power, depending upon the atmospheric conditions, along with spatial diversity helps in maintaining the required level of availability.

Scattering. Scattering is caused when the wavelength collides with the scatterer. The physical size of the scatterer determines the type of scattering. Simply put, when the scatterer is smaller than the wavelength, then it is Rayleigh scattering. When the scatterer is of comparable size to the wavelength then Mie scattering occurs. Finally when the scatterer is much larger than the wavelength then such scattering is known as non-selective scattering. Unlike absorption, in scattering there is no loss of energy, only a directional redistribution of energy that may have significant reduction in beam intensity for longer distances.

Physical obstructions. Birds can temporarily block the beam, but this tends to cause only short interruptions and transmissions are easily resumed. Multi-beam systems can be used to address this issue and other atmospheric conditions.

Building sway/seismic activity. The movement of buildings can upset receiver and transmitter alignment. A divergent beam can be used maintain connectivity; tracking devices also are being developed.

Scintillation. Heated air rising from the ground creates temperature variations among different air pockets. This can cause fluctuations in signal amplitude that lead to “image dancing” at the receiver end. 

Refractive turbulence. Refractive turbulence causes two primary effects on optical beams. They are beam wander and beam spreading.

The first effect of refractive turbulence to consider is the wander of an optical beam in the atmosphere. Beam wander is caused by turbulent eddies that are larger than the beam. The next effect of refractive turbulence is the spread of an optical beam as it propagates through the atmosphere. Two types of beam spread:  long-term and short-term.

Safety. To the uninitiated, the safety of FSO is often a concern, since it uses lasers for transmission. This challenge has more to do with perception than reality, but nonetheless it warrants consideration. 

Laser safety and the proper use of lasers have been a source of discussion and standardization efforts since the devices first began appearing in laboratories more than two decades ago. The two major concerns typically expressed involve questions about human exposure to laser beams (which present much more danger to the eyes than any other part of the human body) and high voltages within the laser systems and their power supplies. 

Several standards have been developed covering the performance of laser equipment and the safe use of lasers. Safety of the lasers does not depend on its frequency, but rather on the classification of the laser. There are two primary classification bodies, the CDRH and the IEC. 

Economics

Free-space optics offers service providers the following benefits

• Quick customer acquisition:

  Metro service providers can acquire target customers very quickly in comparison to fiber installs. Easy upgrades will allow service providers to retain customers as their bandwidth requirements change.

• Increase network footprint:

  FSO can be used to bring multiple off-net buildings on-net, thus increasing the reach of a service provider at a fraction of the time and cost.

• Access to new markets:

  Service providers can access and acquire customers that otherwise would not be accessible. 

• Increase profit level on existing capital:

  FSO allows service providers to extend their existing (LMDS or fiber) networks without adding additional training, equipment or licensing costs.

• Leverage existing capital budgets:

  Carriers can gain significant leverage in their capital by lowering the cost of “wiring” buildings with high-bandwidth access. This allows them to gain an acceptable return on their capital investment on a lower rate of monthly telecommunications revenue.

• Complement to Fiber:

  On average, it takes 14 months to buy fiber and six months to get construction permits, if construction is allowed by a city. With an increasing number of cities imposing moratoriums, fiber deployment is growing more and more time consuming.  FSO eliminates the wait to acquire fiber facilities. 

• Eliminate Stranded Capital:

  Ease of installation and flexible re-use or redeployment eliminates the risk of stranding capital for fiber installation to a particular building. Once a customer leaves, the same equipment can used to provide service to a new customer.

Applications

FSO offers many applications beyond the traditional “last mile” market:

• Metro network extensions:

  FSO can be deployed to extend an existing metro ring or to connect new networks. These links generally do not reach the ultimate end user, but are more an application for the core of the network.

• Enterprise:

  The flexibility of FSO allows it to be deployed in many enterprise applications, including LAN-to-LAN connectivity, storage area networking and intra-campus connections. FSO can be deployed in point-to-point, point-to-multipoint, ring or mesh connections.

• Fiber Complement:

  FSO may also be deployed as a redundant link to back-up fiber. Most operators deploying fiber for business applications connect two fibers to secure a reliable service plus backup in the event of outage. Instead of deploying two fiber links, operators can deploy an FSO system as the redundant link.

• Access:

  FSO can also be deployed in access applications such as gigabit Ethernet access. Service providers can use FSO to bypass local loop systems and to provide FSO-based high capacity links to businesses

• Backhaul:

  FSO can be used for backhaul such as LMDS or cellular backhaul as well as gigabit Ethernet “off-net” to transport network backhaul.

• DWDM Services:

  With the integration of WDM and FSO systems, independent players aiming to build their own fiber rings may use FSO to complete part of the ring. Such a solution could save rental payment to Innncumbent local exchange carriers, which are likely to take advantage of this situation.

Breaking the Bottleneck

The increase in high-bandwidth applications at the edge of the network, coupled with the lack of a high-speed infrastructure connecting the edge to the core, has turned the threat of a connectivity bottleneck into reality. This threat not only affects end users, but also affects the service providers who face delays in laying fiber and building optical infrastructure. Time and cost are playing against service providers, resulting in incomplete networks, lack of revenue and increased competition.

Service providers need a way to accelerate the completion of their optical networks and access traffic at the edge to start generating revenue quickly. Free-space optics helps provide this connectivity in the easiest possible way. 
Baksheesh S. Ghuman is Chief Marketing Officer for LightPointe, San Diego, CA. He can be reached at bghuman@lightpointe.com.

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