This is largely due to the design of this fiber which was
optimized for lower throughputs. Contrary to the active
components of the network, for which upgrades can be
carried out relatively easily through a software or hardware
upgrade, the optical fiber cabling infrastructure has
a performance envelope that previously could not be
increased without physically changing the cable that
carries the information.
THE ISSUE OF MODAL DISPERSION
IN MULTIMODE OPTICAL FIBERS
Optical fiber is widely used to carry information due
primarily to its small size, low linear losses, and insensitivity
to electromagnetic disturbances. However, optical
fiber possesses intrinsic performance limitations which are
inherent to its physical properties. For multimode optical
fibers installed in the 1980s and 90s, such as OM1
(62.5/125 μm) and OM2 (50/125 μm), high-speed transmission
is limited as a result of modal dispersion. Modal
dispersion is a distortion mechanism occurring in multimode
fibers. Due to the different velocities of the modes,
the signal is spread out and often deformed over time
during the propagation in the fiber as shown in Figure 2.
January/February/March 2020 I 19
IP Phone
Individual
Heterogeneous
Servers
1—100 Gb/s
LAN
Switches
100M—10 Gb/s
1—10 Gb/s
FIGURE 1: The growing demand for increased
bandwidth in LANs.
A GROWING DEMAND FOR INCREASED
BANDWIDTH IN LOCAL NETWORKS
Over the past several years, a lot of effort has been put
into the development of optical transmission systems
at 10 Gb/s in LANs. A growing number of applications
FIGURE 2: Modal dispersion affects all multimode fiber types, thereby
degrading the bandwidth capacity of the multimode optical fiber.
require high bandwidth in order to
ensure ongoing transfer and streaming
of large volumes of data, audio
files and videos. In addition, the
multiplication of mobile terminals
and online devices has contributed
to the need for higher bandwidth.
Therefore, bandwidth-intensive
applications and latency-aware
traffic types are becoming ubiquitous
in the LAN. As a result, it is
necessary to be able to transport
these data streams with a high
quality of service (QoS) and reliability.
Nearly 75 percent of the optical
fibers deployed in the LAN are
multimode fibers. A large majority
of deployed multimode optical
fibers do not support throughput
of 10 Gb/s over campus-wide links.
Multimode Fiber
Conventional
multimode input
Stimulation
modes
Propagation
Degraded signal
velocity
Wireless
Access Point
10—50 Gb/s