July 2001

Seeing The Light

BY TONY RYBCZYNSKI

FIBER DIET
There are two kinds of fiber optic cables: multimode fiber (used predominantly within buildings) and single mode fiber (used in the metro and wide area). Multimode fiber has a relatively thick core of glass with a diameter of 50 to 162.5 microns allowing the light beam to bounce around a fair amount. Its attraction is that it is easier and cheaper to couple the light source to the fiber because of the larger core diameter. Single mode fiber has an 8 to 10 micron core that more tightly guides the light, with resulting lower loss of energy. In both cases, this glass core cylinder is encased in a cladding glass tube, commonly resulting in a strand of 125 micron diameter. The refractive design of the core and the cladding glass is designed to keep the light in. Bending radius specifications have to be followed if breakage is to be avoided.

Fiber optic cables have multiple wavelength windows; these wavelengths exhibit substantially lower attenuation than other wavelengths. Multimode transmission often takes place using the 850 nm window, while today's single mode systems use 1310 and 1550 nm. A convention has emerged to describe optical systems in terms of these wavelengths (measured in nanometers), rather than frequency, which is the speed of light divided by the wavelength. Visible light has a wavelength of from 400 to 700 nm, while the above wavelengths fall into the near-infrared band.

Within some buildings, structured copper and fiber cabling may have been installed when the building was originally built, though typically constrained to risers (vs. every desktop). In others, fiber cables may have to be pulled, which may or may not be an issue depending on the age and function of the building. Outside, fiber optic cables are generally buried, with the cost of trenching, conduit installation, and fiber pulling ranging from $20 to $150 per meter (trenching costs may represent 90 percent of the installation cost). While these costs may seem high, once installed, optical cables have economic lives measured in decades. In either case, since the cost of the actual cable is measured in cents per meter, multi-strand cables are used with anywhere from a handful to many hundreds of strands per cable, not all of which would be lit. A market has emerged for unlit strands being swapped between service providers to improve market coverage. The costs of buried fiber has led to alternatives such as using utility poles, and to explorations of innovative approaches such as shallow "trenches" just under the road surface and using sewer systems.

SEEING THE LIGHT
Lighting up the optical pipes is done using light sources such as light-emitting diodes and lasers, the former for shorter distances. Separate fibers are normally used for each direction, though there are low capacity systems that are bi-directional. These optical sources run on a single wavelength of light and are pulsed at up to 10 Gbps. They can be integrated into a vast array of enterprise networking devices, including Ethernet switches, routing switches and routers, ATM switches, storage and application servers, video codecs, and SONET muxes. SONET muxing was designed to combine circuits operating at a broad range of speeds onto a single wavelength operating at bit rates of up to OC192 or 10 Gbps. Enterprises have used SONET ring technologies for a number of years as an efficient way of connecting a large number of metropolitan sites.

Another form of multiplexing is called Dense Wave Division Multiplexing (DWDM). This form of multiplexing logically creates multiple parallel lanes on the optical highway. In carrier backbone applications, DWDM allows fiber capacities of 1.6 Tbps by multiplexing 160 wavelengths, each carrying up to 10 Gbps of SONET, IP, and ATM. This has dramatically decreased the cost per fiber bit-mile in these core network applications. SONET and DWDM systems can be standalone or combined in various ways to define a range of communications solutions. For example, in metropolitan applications, payload independent DWDM systems have been developed which simultaneously support traditional SONET, IP, and ATM, and also various forms of storage networking and mainframe channel extension traffic. A large number of enterprises deploy these systems because of their cost effectiveness and because they will accept any optical payload, thus allowing the enterprise IT organization to transparently handle increases in Ethernet speed (e.g., going from 100 to 1,000 Mbps) as well as the evolution of storage networking and mainframe channel extension protocols.

GOING THE DISTANCE
The extension in reach of optical systems has been equally dramatic with state-of-the-art capabilities allowing light transmission of 2,500 miles without regenerators, using instead amplifiers that boost the signal every 50 to 80 miles!

But first some physics. Reach is dictated by three main physical characteristics of optical systems: attenuation, dispersion, and non-linearities of the fiber (at various wavelengths). Optical systems are all-digital, sending digital pulses representing ones and zeros at an extremely high rate. Ideally, the pulses that are transmitted are received as sent at the other end of the optical pipe. Attenuation decreases the strength of the signal, ultimately making it hard for the receiver to find the signal in the noise. Dispersion spreads the nice clean pulses, ultimately corrupting adjacent pulses. Non-linearities particularly come to play in DWDM systems, since these support multiple wavelengths, each of which is impacted differently by the transmission media.

As a result, it's quite common to transmit signals over single-mode fiber for distances up to 25 miles at 1,310 nm, and 50 miles at 1,550 nm, typical metropolitan distances. These distances are achieved by using the highest power sources that can be sent without interfering with other signals. One way of going further is to insert regenerators in the path, but this typically goes beyond what enterprises are willing to do themselves. In carrier networks, these regenerators can be standalone, as you might have in an undersea cable, or they can be integrated into add-drop muxes, which add new traffic to the fiber or extract it from the cable to deliver to a user, somewhat analogous to a bus stop (as opposed to terminal muxes, such as on the enterprise premises, which might be analogous to bus terminals where all traffic is terminated).

However, regenerators are expensive and difficult to manage, while add-drop muxes may not be functionally required. There are a number of sophisticated techniques that have been developed to extend the reach of optical systems. Examples include:

• Special, more expensive non-dispersion and/or low attenuation fiber.
• Externally modulated transmitters using a very high-speed physical shutter in front of a continuously running laser, rather than turning the laser on and off quickly.
• Inserting a length of fiber, called a Dispersion Compensation Module, which distorts the pulse in the opposite way of the distortion the pulse suffers on the fiber.
• Programmable control of the shape of the pulse (particularly its width) to enhance reach performance.
• Optical amplifiers that magnify the signal; two competing/complementary technologies are called Erbium Doped Fiber Amplifies (EDFAs) and Raman optical amplifiers.
• A high-tech technique called Solitons, which generates specially shaped pulses which interact with each other as they go down the fiber to compensate for dispersion in the fiber and other non-linearities. This is a critical technique to achieve ultra-long transmissions.

These are definitely interesting, but beyond the scope of most enterprises, who prefer to turn to experts to meet their long haul needs.

BEYOND FIVE NINES RELIABILITY
According to Meta Group, the average loss of revenues due to a one-hour outage is $1,000,000, though this varies by company size and industry. That's the measurable loss. Outages often mean loss of customer contact or diminished ability to serve the customer well, potentially resulting in loss of a profitable customer to the competition. Understanding how optical networks have been designed to even exceed the reliability of the phone network can be instructive, particularly if it means bringing this level of reliability to IP networks.

Fiber optic networks can be configured as mesh, ring, and star networks, the latter two in fact being cases of partial meshes. When running Ethernet over dark fiber, reliability is achieved either by running protocols such as Multi-Link Trunking (whereby multiple point-point fibers are treated as a trunk group) or by higher-level mechanisms (e.g., dynamic IP routing). This is very much business as usual in that the optical links are treated as any other physical media. The development of SONET has established a new level of reliability through Layer 1 mechanisms, including end-to-end path protection and node-by-node link protection with very fast recovery (i.e., 50 msec).

Because of their inherent simplicity, SONET rings are used in one of two primary modes:

• Dedicated path protection, whereby data to be protected is sent simultaneously on both sides of the ring and the receiver chooses the best signal (termed Uni-directional Path Switched Ring).
• Protection is shared on all fibers on the ring by reserving half the line capacity between adjacent nodes for protection (termed Bi-directional Line Switched Ring).

That being said, what the enterprise sees is ultra-high level of service. There is another implication. Lessons learned from designing highly resilient SONET rings are being applied to the design of metro DWDM solutions and towards the standardization of Resilient Packet Rings for IP and Ethernet traffic. These latter two areas are of direct interest to enterprises, since many are deploying private solutions based on these solutions. The next level of opportunity is to evolve IP routing systems to fully leverage these optical developments.

THE LAST PHOTONIC WORD
The improvements in optical systems have been dramatic, doubling every nine months in price performance. This contrasts with Moore's Law, which states that processing improvements are doubling every 18 months. We expect this to continue with more bandwidth per wavelength, more wavelengths per fiber, integration of optics with IP and Ethernet, and photonic switching using sophisticated technologies such as tiny mirror-based Micro ElectroMechanical Systems (MEMS), and bubble inkjet/waveguide technologies.

These developments in optical technologies challenge the assumptions that have lead to the widespread distribution of server, storage, and routing intelligence across the network. Rethinking enterprise networking in light of optical technologies can lead to dramatic simplification, lower total cost of ownership across IT, and freeing up of resources for strategic investments.

Tony Rybczynski is director of strategic marketing and technologies for Nortel Networks' Enterprise Solutions unit. For more information, visit the company's Web site at www.nortelnetworks.com. E-mail questions or comments to tonyryb@nortelnetworks.com.

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