DATA CENTER NETWORK Tech Note: Measuring Fiber Optic Link Loss The link loss “budget” defines the amount of acceptable loss between nodes on a fiber link—important because operating within the parameters is critical for establishing a reliable, stable network The fiber optic link loss budget, also known as the “link margin” or the “power budget” of the link, is a measure of signal power gain or loss expressed in decibels (dB). Link loss can be affected by both the active and passive optical components in a link. There are a number of reasons why understanding and “budgeting” potential link loss in a data center network is important: • Operating with certain parameters is critical for establishing a reliable and stable network. • The link loss measurement can assist with the task of selecting transmitter wavelength (850 nm, 1300 nm, and so on), cable type (MultiMode Fiber or Single Mode Fiber), and other network components. • It can help you determine the maximum physical distance of a link. • It is a means of estimating whether or not current and future equipment can be supported over a cabling system. • Looking into the future, link loss is impacted by higher bandwidths and the evolving structured cabling systems. The following factors can impact signal power loss: • Number of paired connectors in the link • Number and type of splices in the fiber • Stressing of the cable • Extending the length of the cable in the link • Misaligned optical couplers • Change in temperature • Degradation in end equipment signal transmitter • Aging of passive components DATA CENTER NETWORK Tech Note PARAMETERS AND FORMULAS The following table details parameters for estimating the link loss budget. For more information about link lengths with multiple types of fibers at different rates, see Annex D of FC-PI-4 from the T11 organization. Component Metric Description Transmitter power -- dBm (decibel referred to one milliwatt) Receiver sensitivity -- dBm Minimum power that can be detected at the target device. Rating noted in device specification such as SFP. Use average value if minimum value is not available. Cable attenuation dB/km Rating varies with cable type, bandwidth, cable length, and source wavelength. Use the typical dB loss values. Connector pair (tx/rx) Count Where two fibers are joined using connectors (e.g. LC). Use typical dB loss values. The connector at the end device is not included in the equation. Cable splice Count Where two fibers are joined together mechanically or by fusion (heat and electricity) to extend the fiber distance or to repair a fiber. Use the typical dB loss values. dB Extra loss allocation for future repairs, aging of emitters, and component additions. Use the typical dB loss values. Safety margin Typical laser power output rating at the source device. Rating is noted in device specification such as SFP. The following formulas are used to calculate link loss. Formula Metric Link budget: Transmitter power – Receiver sensitivity -- dB Description Acceptable optical power/significant loss between two nodes Total link loss: Cable attenuation + Loss from connector pairs + Loss from splices dB Estimated optical power/signal/light loss as it passes through the cabling system between two nodes Link loss budget: Link budget – Total link loss – Safety margin dB/km Estimated optical poser/signal/light loss allowed after factoring in the known cabling and cabling maintenance NOTE THE FOLLOWING: Total link loss must not exceed Link budget Measuring Fiber Optic Link Loss Page 2 of 6 DATA CENTER NETWORK Tech Note CALCULATING THE LINK LOSS BUDGET Here are the steps for calculating a value for the link loss budget: 1. Calculate the typical allowed dB loss between two networked nodes (Link budget) 2. Calculate the typical loss expected from the physical cabling, including a margin of loss due to repairs (Link loss) 3. The Link loss budget is the difference between the step 1 and the step 2 values. The following table provides representative values for cabling link loss. Cabling Link Loss Note Cable attenuation Optical Multimode 3 (OM3) Laser Optimized Multimode Fiber (LOMMF), 850nm laser, 50/125µm 3.5 dB/Km max 3.0 dB/km OM3 LOMMF, 1300nm laser, 50/125µm 1.5 dB/km max 1.0 dB/km 0.75 dB (0.5 dB IOS) max 0.3 dB for LC (Lucent connector), 0.5 dB for MPO (Multifiber Push-On) Connector pair TIA-586B Maximum Loss Typical Loss Cable splice Mechanical 0.3 dB max 0.2 dB Cable splice Fusion 0.05 dB max 0.05 dB 3.0 dB max 0.7 dB (2 splices + 1 connector pair) Safety margin Measuring Fiber Optic Link Loss Page 3 of 6 DATA CENTER NETWORK Tech Note Figure 1 shows an example of calculating link loss budget in a representative network. A End device interface transmitter power (850 nm laser) -- 2.5 dBm B, C, D, F Connector pairs, each @ 0.5 dB loss 2.0 dB E Mechanical splice, each @ 3.0 dB loss/km 0.2 dB Cable type & distance OM3 850 nm, 300 m @ 3.0 dB loss/km 0.9 dB G End device interface receiver sensitivity (850 nm wavelength) -- 15 dBm Figure 1. Calculating the link loss budget 1. Calculate the Link budget: Transceiver power – Receiver sensitivity = (A – G) = (-- 2.5) – ( -- 15.0) = 12.5 dB 2. Calculate the Total link loss: Total connector pair loss + Total splice loss + Cable attenuation = (B + C + D + F) + E + (3.0 / 1000 * 300) = 3.1 dB 3. Calculate the Link loss budget: = 12.5 – 3.1 – 0.7 = 8.7 dB Based on this calculation, 8.7 dB additional loss is tolerable on the link after factoring in the known parameters. Measuring Fiber Optic Link Loss Page 4 of 6 DATA CENTER NETWORK Tech Note OM3 GRADE FIBER: CABLE OF CHOICE FOR FC OM3 or LOMF general specification: • 50µm core / 125 µm cladding OM3 (2000 MHz-km bandwidth @ 850 nm wavelength) • 290 – 380 m @ 4 Gbit/sec transmission (after 1.5 – 2.4 dB connection loss) • 110 – 150 m @ 8 Gbit/sec transmission (after 1.5 – 2.4 dB connection loss) OM3 “plus” (4700 MHz bandwidth @ 850 nm wavelength) • Up to 550 m (may be too early for distance/transmission industry specifications for this cable type) MEASURING FIBER OPTIC LINK LOSS Two devices you can use to measure the actual link loss are described below. An Optical Time Domain Reflectometer (OTDR), shown in Figure 2, accurately measures power requirements in a given transmission line. An OTDR can measure loss across an end-to-end link or across each cabling section. It is also used to locate cable cuts or section of degraded cable within a link. Figure 2. Optical Time Domain Reflectometer (OTDR) and optical test kit The optical test kit, shown in Figure 3, is an Inexpensive kit with a stable optical light source and an optical power meter. The optical power meter can be operated with the existing laser source. Figure 3. Optical test kit Measuring Fiber Optic Link Loss Page 5 of 6 DATA CENTER NETWORK Tech Note BEST PRACTICES FOR MINIMIZING LINK LOSS • Stay well within the maximum cable distance calculated for the link. • Apply typical or worst-case values during loss calculations. • Use the highest grade cabling components for the application to be supported. • Match the cable type with the wavelength, bandwidth, and distance to be supported; do not mix cable types within a link. • Inspect loss ratings of all cabling components during the selection process. • Record loss measurements for horizontal and vertical cable runs during installation. • Become familiar with how to quickly determine the link budget and link loss of selected sections of the cabling. • Account for power loss associated with future repairs and expansion. • Try not to stress the cable. • Prototype a link with anticipated maximum cable distance and selected components—and then take measurements to calculate the actual link loss. © 2007 Brocade Communications Systems, Inc. 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Contact a Brocade sales office for information on feature and product availability. Export of technical data contained in this document may require an export license from the United States government. Measuring Fiber Optic Link Loss Page 6 of 6
