Issue link: https://www.e-digitaleditions.com/i/1192126
20 I ICT TODAY TABLE 1: Performance of multimode optical fibers: throughput versus distance. Maximum distance of link 100 Mb/s (100BASE-FX) 1 Gb/ (1000BASE-SX) 10 Gb/s (10GBASE-SR) 40 Gb/s (40GBASE-SR4) OM1 (62.5/125) 2000 m 400 m 33 m N/A OM2 (50/125) 2000 m 550 m 82 m N/A OM3 (50/125) 2000 m 575 m 315 m 100 m OM4/OM5 (50/125) 2000 m 600 m 500 m 150 m FIGURE 3: The problem of multimode fiber transmission capacity has led to the creation of technologies that upgrade existing cabling infrastructure. This leads to inter-symbol interference and the inability to retrieve the data that was transmitted. The higher the transmitted throughput, the greater the distor- tion. All multimode fibers are affected by modal disper- sion, including OM5. Certain optical fiber properties, especially the refractive index profile of the fiber, strongly impact the velocity of the different modes of propagation, thereby causing modal dispersion. As glass transformation techniques have evolved over the years, there are different generations of graded-index multimode fibers (OMx, x = 1, 2, 3, 4 or 5) that decrease the differential mode delay between modes according to the fiber type and, therefore, the impact of modal dispersion on bandwidth. To sum up, modal dispersion degrades the width of the bandwidth capacity of the multimode optical fiber. For a given throughput, it reduces the distance that can be reliably attained between the transmitter and receiver. As a result, standard transmission over multimode fiber within 850 nm operating wavelength with 10GBASE-SR is limited to distances of up to 33 meters (36 yards) on conventional 62.5 μm OM1 fibers and 82 meters (90 yards) over conventional 50 μm OM2 fibers. Table 1 provides an overview of the performances of multimode optical fibers types. A typical solution that prevents throughput and distance limitations due to modal dispersion in legacy multimode fiber cabling is to upgrade to a newer-genera- tion multimode fiber type or to singlemode optical fiber, which does not experience modal dispersion. However, redeploying the fiber cabling is frequently a difficult task. It depends upon the distances involved and the difficul- ties in drawing the fibers. An audit of the cabling infra- structure is necessary in order to verify the availability of the cable duct as well as its condition. Depending on the case, civil engineering may be required to deploy new cable sheaths. For these reasons, the complexity and costs of redeploying new cable can be exorbitant. Following this approach may result in a complex installation with a significant impact on the business and its budget. SPECIFIC LIGHT LAUNCHING CONDITIONS TO OVERCOME OPTICAL MULTIMODE FIBER BANDWIDTH LIMITATIONS The problem of multimode fiber transmission capacity as it relates to the need to increase the throughput was discussed as far back as the late 1990s. To try and over- come the multimode bandwidth limitation, a new type of 10 Gb/s interface was introduced within the IEEE 802.3aq standard, referred to as long reach multimode (LRM). These interfaces are specifically designed for backward compatibility with OM1 and OM2 fibers. The LRM inter-