By Tony Irujo
Multimode is usually best for most situations, but new singlemode technology offers long-distance punch.
Applications such as Voice over IP, video streaming, and teleconferencing are pushing data communications rates to 10 Gigabit Ethernet and beyond in enterprise networks. While these higher speeds might lead system designers to believe that singlemode fiber enjoys an increasing advantage over multimode in premises applications, higher Ethernet speeds do not automatically mean singlemode fiber is the best choice.
These size comparisons among multimode (62.5- and 50-µm) and singlemode fibers gives a visual indication of the alignment and connectorization challenges inherent in singlemode systems because of the small core size.
Although singlemode fiber has advantages in terms of bandwidth and reach for longer distances (greater than 1 kilometer [km] at 1 Gbit/sec), multimode easily supports most distances required for premises and enterprise networks. In fact, multimode fiber can support 10-Gbit/sec transmission to 550 meters for long backbone and even short campus runs. Furthermore, the optoelectronics used with multimode fiber are generally less expensive than those required for a singlemode system. Multimode fiber is also easier to install and terminate in the field-important considerations in enterprise environments with frequent moves, adds, and changes.
The multimode/singlemode difference
The two fiber types get their names from the way they transmit light.
Generally designed for systems of moderate to long distance (e.g., metro, access, and long-haul networks), singlemode fibers have a small core size (less than 10 µm) that permits only one mode or ray of light to be transmitted. This tiny core requires precision alignment to inject light from the transceiver into the core, significantly driving up transceiver costs.
By comparison, multimode fibers have larger cores that guide many modes simultaneously. The larger core makes it easier to capture light from a transceiver, allowing source costs to be kept down.
Similarly, multimode connectors cost less than singlemode connectors due to singlemode fiber’s more stringent alignment requirements. Singlemode connections also require more care and skill to terminate, which is why components are often preterminated at the factory. Multimode connections, on the other hand, can be easily performed in the field, offering installation flexibility and cost savings.
Enterprise environments provide particular network challenges, including limited spaces and tight bends, high connection density, and components that get handled frequently. Multimode fibers are ideally suited for these conditions, especially since distances within a premises system rarely approach 550 meters.
Beyond 550 meters at 10 Gbits/sec (or beyond 1 km at 1 Gbit/sec), it is necessary to use singlemode fiber. New choices for singlemode fiber are now available, including a bend-insensitive, full-spectrum singlemode fiber that provides more transceiver options and more bandwidth, and is less sensitive to handling of cables and patch cords than conventional singlemode fiber.
The network designer or end user who specifies multimode fiber for short-reach systems must choose from two types:
• 50-µm multimode fibers were first deployed in the 1970s for both short- and long-reach applications.
• 62.5-µm multimode fiber, introduced in 1985, supported campus applications up to 2 km at 10 Mbits/sec.
Traditional multimode fiber cannot take full advantage of VCSEL technology, but laser-optimized multimode fiber�s refractive index profile minimizes differential mode delay (DMD).
The mid-1990s, with the introduction of the vertical-cavity surface-emitting laser (VCSEL) light source, saw a shift back to 50-µm fiber. Today 50-µm laser-optimized multimode fiber (also known as OM3) offers significant bandwidth and reach advantages for most building applications, while preserving the low system cost advantages of 850-nm-based multimode fiber.
For 10-Gbit/sec capability, 50/125-µm OM3 fiber offers better modal dispersion and more bandwidth than 62.5/125 fiber.
Because optoelectronics represent a large percentage of total system cost, the most economical solution for 10-Gbit/sec transmission in the enterprise is 50-µm OM3 fibers that have been designed and manufactured specifically for use with inexpensive VCSELs. This cost advantage is likely to hold true at higher speeds because future transceivers will likely be manufactured to take advantage of the technology that can support 10-Gbit/sec transmission.
With an eye to the future, OM3 fibers have the capability to support higher transmission rates using low-cost parallel optics transceiver arrays, or a combination of parallel optics and coarse wavelength division multiplexing (CWDM). For example, 10 OM3 fibers each operating at 10 Gbits/sec can be aggregated into a 100-Gbit/sec system (10 x 10 array). Or, two OM3 fibers, each carrying four wavelengths (CWDM) at 12.5-Gbits/sec apiece, can provide 100-Gbit/sec speed.
Why not use singlemode fiber with a single laser, in what is called serial transmission, operating at 100 Gbits/sec? That type of laser is not commercially available today, and probably will not be for a long time. In fact, it will be challenging to cost-effectively develop and produce such a laser. Therefore, achieving higher speeds on singlemode fiber will require the same parallel (multiple-fiber) and CWDM (multiple-wavelength) technologies discussed previously. But now you are faced with the same transceiver- and connector-alignment challenges that can drive up the cost of these components when used with singlemode fiber.
Going the distance
In general, multimode fiber continues to be the most cost-effective choice for enterprise applications up to 550 meters. Singlemode fiber is best used for distances exceeding 550 meters. If the network’s transmission distances dictate the use of singlemode fiber, consider specifying the recently introduced bend-insensitive zero-water-peak (full spectrum) fibers, which are designed to provide long-term reliability in applications with tight bends and small enclosure areas.
Increased network speeds need not preclude cost-effective multimode fiber solutions. Multimode fiber has the advantages of lower-cost electronics, fewer alignment and connectorization issues, and ease of handling-all of which will continue to be factors even at higher speeds.
TONY IRUJO is manager of customer technical support with OFS (www.ofsoptics.com).