Multimode Fibers: The Choice for Local Area Networks
Optical fibers are extremely thin strands of silica (glass), which transmit light from one end to the other with minimal loss. At one end, light is transmitted by a laser or a Light Emitting Diode (LED) and at the other end a light-sensitive receiver converts the signal back into a digital transmission.
There are two primary types of glass optical fiber: single-mode and multimode. In premises cabling, multimode fibers (MMF) are typically used because they can be powered by low cost optical sources such as LEDs or Vertical Cavity Surface Emitting Lasers (VCSELs) which operate in the 850 nm transmission window. Multimode fibers support applications from 10 Mb/s to 10 Gb/s over link lengths of up to 550 meters, more than sufficient for the majority of premises applications. In fact more recent grades of multimode fiber are optimized to work with VCSELs, extending the link distances and boosting bandwidth. In addition, a new wide band multimode fiber is now available that supports Short Wavelength Division Multiplexing (SWDM), transmitting four wavelengths of fiber simultaneously over a single strand of fiber.
Single-mode fibers (SMF) are typically used in long-distance telephony and cable TV applications, which must support high bandwidth applications and span transmission distances of hundreds of kilometers. These fibers operate at the 1310 nm or 1550 nm transmission windows, which require higher-cost electronics. Occasionally, single-mode fibers are installed in premise backbone cables or used in data centers were very high bandwidth applications are running. Although singlemode fibers offer almost unlimited bandwidth, the cost of SMF optics often makes a SMF solution more expensive than MMF.
How Fibers Work
Although optical fibers are very thin - slightly smaller than a human hair - they are composed of two kinds of glass: the core of the fiber is where light is carried and the cladding is the glass that surrounds the core. The cladding glass acts as a mirror; as light pulses propagate down an optical fiber, the cladding reflects the light pulses and keeps them contained in the fiber core. This phenomenon is known as total internal reflection. This is possible because the core has a higher index of refraction than the cladding. Surrounding the cladding glass in an acrylate coating that protects the fiber.
In multimode fibers, many modes of light are transmitted down a fiber simultaneously. In fact it gets its name because its core size (50 microns or 62.5 microns) allows light to take multiple path, or modes, down the fiber. Single-mode fiber, as its name implies, allows only one mode of light to travel down the core at a time. Single-mode fibers have a much smaller core diameter, approximately 8 microns. The small core size means that connecting single-mode fibers requires much more precision to connect them.
Data travels down an optical fiber in much the same way as a copper cable - in a series of on and off pulses that signify whether a bit is a 1 or a 0. However, in an optical fiber, this occurs as the light source pulsates on and off. The faster a light pulses, the more data that can be sent down the fiber.
Multimode Fiber Types
Multimode fibers are described by either their core/cladding diameter (for example 62.5/125) or by using a system of classification determined by the ISO 11801 standard -- OM1, OM2, and OM3, OM4 and OM5.
The predominant type of multimode fiber used in premises applications today is Laser Optimized 50/125 mm Multimode Fiber(OM3). This fiber type provides sufficient bandwidth to support 10 GbE and beyond with cable lengths up to 550 meters. Laser Optimized Multimode Fiber (LOMMF) is designed for use with 850 nm VCSELs, rather than the LEDs that were used for lower speeds. Its development extended the distances over which multimode fiber cable can transmit high data rates.
Initially 62.5/125 mm (OM1) and conventional 50/125 mm multimode fiber (OM2) were widely deployed in premises applications. These fibers easily support applications ranging from Ethernet (10 Mb/s) to Gigabit Ethernet (1 Gb/s) and, because of their relatively large core size, were ideal for use with LED transmitters.
The migration to LOMMF/OM3 has occurred as users upgrade to higher speed networks. LEDs have a maximum modulation rate of 622 Mbps so they could not be turned on/off fast enough to support higher bandwidth applications. VCSELs are a cost-effective transmission source, but to obtain the necessary performance, fiber manufacturers had to modify their multimode fiber designs.
VCSELs power profiles along with variations in fiber uniformity can cause differential modal delay (DMD), an effect that causes the speed of individual light pulses to spread over time and make it difficult for transceivers to identify the individual 1's and 0's. This effect reduces transmission capacity as link lengths increase. To combat DMD, LOMMF is manufactured in a way that eliminates variations in the fiber which could affect the speed that a light pulse can travel. The refractive index profile is enhanced to control VCSEL transmission and prevent the pulse spreading As a result the fibers maintain signal integrity over longer distances, thereby maximizing bandwidth.
LOMMF/OM3 is a good choice right now for all local area network (LAN) infrastructure applications and for the data center.
- It is backward compatible with LED signaling technology, so you can use it today to support slower rates and then upgrade to VCSEL based 1, 10 and 100 Gb/s transceivers
- It offers the most bandwidth with cost effective transmission technologies
- It ensures reliable transmission through advanced technologies
- It enables easy migration to higher speeds, and
- It is fully recognized and specified by standards bodies.
Bandwidth is specified as a bandwidth-distance product with units of MHz?km. The bandwidth needed to support an application depends on the data rate. As the data rate goes up [MHz], the distance that rate can be transmitted [km], goes down. Thus, a higher fiber bandwidth enables you to transmit at higher data rates or for longer distances. Conventional 50 mm multimode fiber (OM2) is typically rated at 500 MHz?km; 62.5 mm fiber (OM1) is rated at 200 MHz?km at 850 nm; and 50 mm Laser Optimized Multimode fiber is rated at 2000 MHz?km. Other "flavors" of 62.5 and 50 mm multimode fiber are also readily available, with varying degrees of bandwidth values.
Migrating from an Installed Base
Many users have an installed base of either 62.5 mm fiber or conventional 50 mm fiber. FOTC members are frequently asked about migration strategies for these users.
First, it's important to look at the needs of a specific network. Since these fibers can comfortably support data rates of up to 1 Gb/s over 300m, there is no need to upgrade the fiber plant until higher data rates are required.
Once additional capacity is needed or if end-users want to plan for future network speed upgrades, then there are several solution options. In general, the most cost-effective solution is to deploy OM3 fiber with 850nm 10GbE VCSELs.
Alternatively, while conventional fibers will not perform as well as LOMMF with VCSELs, it is possible to increase data rates using them. Another solution involves Wavelength Division Multiplexing (WDM). Standard multimode fibers can support 10 GbE over links up to 300m using wavelength division multiplexing of four x 2.5 mbps lasers. Currently this solution is expensive, but may come down in price. To help maximize the utility of the installed base the IEEE developed the 10GBASE-LRM standard as a means for running 10 GbE over 62.5 mm fiber up to a maximum link length of 220 m, using single-mode lasers.