OM5
광케이블
소개: Wideband Multimode
Fiber
Data centers are constantly moving toward faster speeds and higher
densities – especially in hyperscale, Web 2.0 companies, such as Amazon, Google,
Facebook, Apple and Microsoft. Speeds as high as 25G and 100G Ethernet have
already become mainstream in data centers, and the industry is working
collaboratively on next-generation Ethernet development, such as 200G and 400G
Ethernet.
Multimode
fiber
(MMF) is a cost-effective solution in these environments, thanks to its high
tolerance for fiber misalignment and relatively low connection loss at each
connector interface. Multimode fiber cabling systems – combined with LEDs and
VCSEL (vertical-cavity surface-emitting laser)-based optical transceivers – are
ideal for short-reach optical interconnects.
The channel capacity of multimode fiber has multiplied by using
parallel transmission over several fiber strands (four or 16, with collections
of 25 Gbps lanes carried in each direction). But this method raises cabling
system prices.
The Problems with Multimode Fiber
Initially
developed to support Fast Ethernet (100BASE-FX and 1000BASE-SX), OM1
and OM2 multimode
fiber cable can no
longer support 10 Gbps and 25 Gbps data
transmission speeds. In the ANSI/TIA-568.3-D Standard, OM1
and OM2 multimode
fiber
types have been “grandfathered” in, and are not
recommended for new installations.
Until
recently, OM3 and OM4 (laser-optimized
multimode fiber [LOMMF]) were the mainstream multimode fiber cabling choices
to support 10G, 40G and 100G Ethernet,
InfiniBand
and Fibre
Channel
protocols.
As bandwidth requirements increase much faster than the VCSEL-based
transceiver technology curve, however, it’s becoming more costly for optical
fiber cabling systems to support next-generation Ethernet speed migration.
For example, in the IEEE 802.3bs Standard draft, 400GBASE-SR16 has
been specified to reuse 100GBASE-SR4 technology, but it calls for a new MPO-32
connector instead of an MPO-12 connector.
A Potential Alternative: Wideband Multimode
Fiber
Wideband
multimode fiber (WBMMF) is an ANSI/TIA development that can deal with escalating
data rates and the infrastructure required to support higher bandwidth. It uses
wavelengths to increase each fiber’s capacity by at least a factor of four,
which allows at least a fourfold data-rate increase (or a fourfold reduction in
the number of fibers required to achieve a given data rate. Instead of using
four separate fibers to transmit four optical signals, the signals can be sent
down one fiber over four separate operating windows.
ANSI/TIA-492AAAE,
the new wideband multimode fiber standard,
was approved for publication in June 2016 after a 20-month, industry-wide study
carried out by a special TIA taskforce within TR-42.11 (Optical Systems
Subcommittee) and TR-42.12 (Optical Fibers and Cables Subcommittee).
The
International Organization for Standardization/International Electrotechnical
Commission (ISO/IEC) has recently decided on the nomenclature for wideband multimode fiber cable:
OM5.
This new fiber cable standard has
already been referred to by the IEEE 802.3 working group for next-generation
Ethernet standard development.
Please
note that the TIA-492AAAE document specifies the raw glass fiber performance for
the wideband operation, while ISO/IEC OM5 and TIA 568.3-D specify the cabled
fiber performance containing the wideband multimode fiber.
Multimode Standard Specifications (Note: Standard In
Progress)
The table above represents different multimode fiber standard
specifications and their supported link distances for IEEE 802.3 Ethernet
applications.
Wideband
multimode fiber can support:
- Wavelength division multiplexing (WDM) across the 840-953nm
wavelength range
- Backward compatibility with OM4 multimode fiber at
850nm
The effective modal bandwidth (EMB) – the maximum signaling rate
for a given distance – of wideband multimode fiber has been defined to not only
support the bitrate of 25.78125 Gbps, as specified in IEEE 802.3bm 100GBASE-SR4,
but also to support 28.05 Gbps, as specified in the 32G Fibre Channel (32GFC)
standard, both at 100m minimum reach across the entire wavelength
range.
Minumum effective modal bandwidth for wideband mulitmode fiber as
specified in ANTI/TIA-492AAAE
The figure above shows the minimum required EMB as specified in
ANSI/TIA-492AAAE. In a multimode fiber link, data rate and maximum reach are
limited by:
- Fiber cable attenuation (reduced signal strength) and connection
loss
- Chromatic dispersion in the fiber (spreading out of light pulses
over time due to different wavelengths traveling at different
speeds)
- Modal bandwidth of the fiber
Because
multimode fiber’s cable attenuation and chromatic dispersion are lower at higher
wavelengths, the required minimum EMB is relatively lower at 953nm than at the
840nm end. The minimum EMB of wideband
multimode fiber
at 850nm is specified as the same value as for OM4 (4700MHz/km) to guarantee
backward compatibility.
Other link impairment factors are also
involved:
·
VCSEL spectral width (Δλc) and rise-fall
time
·
Transmitter emission power
·
Optical modulation amplitude (difference between two optical power
levels)
·
Signal-to-noise ratio
·
Sensitivity and bandwidth of the
photo-detector
·
Crosstalk from adjacent channels
These specifications are typically developed by IEEE 802.3 to
ensure a technically feasible transceiver product with enough mass-production
margin.
Short-Wavelength-Division-Multiplexing (SWDM)
Applications
The parallel multimode fiber MPO cabling (pictured below in Figure
3) is considerably more costly than the multimode fiber LC-duplex patch cord
(pictured below in Figure 4).
-Left:
100G QSFP28-SR4 Transceiver, Right:
MPO-12 Fiber
-
Left: 100G QSFP28-SWDM4 ,Right: LC-Duplex Fiber
To reduce costs in direct point-to-point connections, it’s more
desirable to use a single pair of fiber instead of MPO
trunks.
Using a single fiber to carry multiple wavelengths (wavelength
division multiplexing [WDM]) isn’t a new concept, and has been widely used in
the telecomm world to reduce singlemode fiber numbers. In short-reach datacomm
applications,
Cisco BiDi (bidirectional optical technology) and Arista
Universal transceiver solutions using two and four wavelengths are also been
proven to be market successes.
In 2015, the SWDM Alliance was formed by a
group of transceiver, fiber and system vendors to develop a multisource
agreement (MSA) for SWDM transceivers. Because OM5 wideband multimode fiber
permits the use of a much wider wavelength range of 850nm to 953nm, it’s
desirable to reduce fiber count by transmitting multiple VCSEL wavelengths in
the same multimode fiber.
Potential wavelength grids are defined as 850nm
(λ1), 880nm (λ2), 910nm (λ3) and 940nm
(λ4), with a spacing of 30nm. Both 40G and 100G QSFP SWDM4
samples have been demonstrated; the target release date will be in early
2017.
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