Cisco Cisco DWDM Transceiver Modules 白皮書

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Another important parameter of an optical fiber is the size of its core, which is the cylindrical

section made of glass that serves as the waveguide for optical information propagation. In

enterprise networks, there are two main types of MMF: one with a core diameter of 62.5µm, and

the other one with a core diameter of 50µm. When an optical signal is transmitted over multimode

fibers, it spreads into multiple modal components as it enters the medium and travels along the

path. These components need to be recombined in a timely manner at the other end of the

transmission link in order to be recognized and processed by the detection subsystem converting

the optical signal into an electrical signal. The larger the size of the core, the easier it is for modes

to be spread, and the harder it is for them to be recombined.

Figure 2 describes examples of multimode and single-mode fibers, as well as the typical channel

impulse responses showing how the pulse spreads depending on the fiber type.

Figure 2. Pulse Spreading Severity on Various Types of Optical Fibers

Fiber types known as OM1 or FDDI were the first ones to populate campus and building

backbones, connecting servers to hubs and early-generation switches. These fibers have a

62.5µm core with a typical modal bandwidth of 160 to 200MHz*km. As glass processing

techniques improved, newer types of 50µm MMF with higher modal bandwidth were easier to

manufacture. Nevertheless, these new types of better performing fiber did not immediately replace

the installed base, but continued to expand the enterprise network with the latest technology

available. As a result, enterprise networks are currently populated with a mix of all types of MMF.

Traditional OM1 and FDDI-grade fiber types still represent more than 50 percent of the new

deployments and will be accounting for more than 40 percent of the installed base by 2010. This

defines the urge for identifying a 10 Gigabit Ethernet transceiver solution that will be compatible

with the existing infrastructure.