The ever increasing demand for bandwidth has triggered development of numerous wide-area-networking (WAN) solutions. For data center providers, the result is a broad choice of data center connectivity solutions, including Ethernet Line (E-Line), Ethernet LAN (E-LAN) and Virtual Private LAN Service (VPLS). Up to 100GbE is readily available with some of these services, and some carriers are planning to make 400GbE more available soon. For example, check out GeoQuote, a one-stop source for carrier connectivity solutions. But for high-data-rate connectivity, better scalability and low latency, an increasing number of providers are leasing dark fiber.
Leasing, “lighting” and operating a dark-fiber network is a major undertaking that requires a well-trained technical staff. The next few paragraphs review important factors, especially fiber-optic attributes, to keep in mind when planning to lease dark fiber.
Since the deployment of the first commercial telecommunication optical fiber in the 1980s, optical-fiber manufacturing has evolved to match bandwidth’s ever increasing quantity and quality. Bandwidth, achievable distance, uptime, latency, jitter and other important bandwidth attributes are determined in part by the fiber’s quality. In addition, fiber that meets today’s bandwidth needs may fall short when bandwidth requirements change, defeating the whole point of scalability that dark fiber is known for.
Before signing a dark-fiber leasing agreement, network managers should request fiber-optic specifications and the latest cable test results, as well as arrange for their own independent testing of the fiber infrastructure. The most important fiber-optic attributes in long-distance, high-data-rate systems include attenuation, chromatic dispersion (CD), polarization-mode dispersion (PMD) and nonlinear effects.
Attenuation (or Loss)
Attenuation, or loss, is the reduction in signal strength as light propagates through an optical fiber. It’s caused by light absorption, scattering, fiber bending and other mechanisms. It reduces the achievable distance before an optical amplifier becomes necessary. The attenuation coefficient of single-mode fiber in terrestrial long-distance networks has evolved from as high as 0.4dB/km to as low as 0.17dB/km (at a 1,550nm wavelength)—a considerable improvement. The dark fiber available for lease has a wide range of attenuation coefficients, and the network manager must ensure that its attenuation supports current and future network design criteria.
Chromatic Dispersion (CD)
Ideally, laser light that carries communication signals should be monochromatic (containing only one color or wavelength), but practical lasers have a distribution of wavelengths. Each wavelength in a digital pulse sees a different refractive index and therefore travels at a different speed, causing the pulse to spread. As a pulse train travels further down the fiber, adjacent pulses overlap and require dispersion compensation.
The higher the data rate, the more susceptible the system is to chromatic dispersion and the shorter the distance achievable before compensation. Using standard ITU-G.652-compliant fiber, for example, one can achieve a distance of 980km at 2.5Gbps before compensation. At 40Gbps, that distance shrinks to 4km, all else being equal (i.e., using the same modulation and detection technologies).
The chromatic dispersion of the dark fiber should be well known and a determination made whether and where compensation is required. The dispersion’s impact on upgrading the system to higher data rates should also be well understood. Also note that for higher-data-rate systems using coherent systems, chromatic dispersion is handled electronically and therefore may not be an issue.
Polarization-Mode Dispersion (PMD)
PMD is a result of fiber geometric asymmetry, and it causes signal distortion. PMD poses a serious concern at higher data rates, specifically for greater than 10Gbps transmission, but it can easily be mitigated using optical fiber with low PMD. Early fibers installed before 10Gbps systems may have elevated levels of PMD, and again, the onus is on the network manager to ensure the dark fiber is tested and meets the network’s design requirements.
Nonlinear impairments in optical fiber are triggered by changes in the fiber’s refractive index at high power intensity. Many nonlinear effects occur in optical fiber, but in dense wavelength-division multiplexing (DWDM) systems, self-phase modulation (SPM), cross-phase modulation (XPM), four-wave mixing (FWM) and amplified spontaneous emission (ASE) are the most important. In DWDM systems, high power in the fiber (leading to high intensity) is inevitable, as the power in each DWDM channel contributes to the overall power. Moreover, if signals require amplification, it introduces high power in the fiber. Transmission fiber with a large effective area can mitigate nonlinear impairments. The network manager must know the fiber’s effective area and its potential impact on signal fidelity.
ASE is generated in the erbium-doped fiber amplifier (EDFA); it reduces the system optical-signal-to-noise ratio (OSNR). Consequently, ASE reduces the number of amplifiers that can be connected in tandem before optical regeneration is required. In the case of long links (hundreds to thousands of kilometers), the network manager should work with the system vendor and select systems that can handle the required distance.
To ensure the dark fiber suffers from no other issues, performing a bidirectional OTDR test is helpful. The results should confirm the absence of high splice losses, sharp bends in splice trays, incompatible fiber links, fiber breaks and other problems.
For short links—less than 60km and 10Gbps per DWDM channel—the necessary equipment may only be transmission systems at the end of the link. But for longer distances, the network manager must consider optical amplification and dispersion compensation. These matters occasionally come as an afterthought when companies consider leasing dark fiber. Amplifiers, electricity to power them, and the amplifier huts needed to house them can contribute to the cost of leasing dark fiber.
When you lease capacity from a network provider, that provider will most likely guarantee uptime. When an enterprise leases dark fiber, the network manager must ensure that the network is properly designed and maintained to guarantee uptime. Redundancy is an important requirement to guarantee uptime.
In planning the setup of the dark-fiber-based network, allow for an alternative path to reroute traffic in the event of a cable cut or any other network disruption. Alternatively, lease a circuit that will be on standby to transport at least the most critical traffic should the main link fail.
System and Infrastructure Monitoring
Once the network is operational and is tested and commissioned, both the transmission system and optical infrastructure should be monitored for performance. Optical-network monitoring systems (ONMSs) can check the optical infrastructure for signs of problems, which the operator can then mitigate before a network outage occurs. An ONMS typically consists of an OTDR and switches to switch the OTDR signal among multiple fibers. When combined with a mapping system, operation personnel can receive alerts regarding a problem and its location so they can fix it in a timely manner. System-level monitoring employs network-monitoring cards, which are part of the network-management system. Details are available from the system provider.
A detailed look at the lighting of dark fiber appears in the carrier-neutral fiber-optic training course from Optical Technology Training (OTT). The intense five-day course is available around the world through licensed trainers. In addition to covering many optical-networking modules, trainees are required to pass a project assignment on “lighting” dark fiber with a DWDM or CWDM system.
About the Author
Jabulani Dhliwayo is the founder of FiberGuide, an optical-network training and wide-area network (WAN) consulting company. As the demand for bandwidth continues unabated and communications networks become ever more complex, Jabulani trains network planning and design engineers in optical networking to keep them current with emerging technologies. He also coordinates proposal processes for enterprise WAN solutions. Jabulani has over 20 years of experience in the fiber-optic industry and has held a variety of positions including college lecturer, senior research scientist, product manager and market-development manager. He holds a PhD in physics from the University of Kent at Canterbury, UK. Connect with Jabulani on LinkedIn.