SWDM and WMMF’s True Colors
Did you know?
Multimode optical fibers are the popular choice for the relatively short distance runs in data centers. That’s because the transceivers and connectors are less expensive than the components used for single mode fibers. Typically, most transmission link distances in data centers are less than 150 meters. The Bandwidth of a low-cost Vertical Cavity Surface Emitting Laser (VCSEL) laser, coupled with 50 micron laser optimized multimode fiber will adequately support duplex transmission at 10 Gbps over these distances.
However, the requirement for more information capacity over each multimode fiber is pushing data rates from 10Gbps (G) to 40G using cost-effective short wavelength division multiplexing (SWDM) VCSEL transceivers operating in the range of 850nm to 950nm wavelengths.
There is also a growing interest to further reduce costs for 100G and 400G links by using less fiber lanes with multiple SWDM channels over multimode optical fibers as well.
The “Cabling Evolution Map” shows how to increase the aggregate speeds of the 10G and 25G transceivers from to 40, 100 and 400G using parallel optics with four channels of SWDM on each fiber. The next generation of transceivers will be modulated at 25G rather than at 10G (two columns on the right of the map). By integrating SWDM into the 25G transceivers, one can see how a significant savings in the number of required connected fibers can be achieved.
More options to get to 40 and 100 Gbps:
Cisco has developed a 20G QSFP transceiver operating with two 20G wavelengths (840nm and 900 nm), bi-directionally over two multimode fibers to achieve 40 Gbps transmission. This BiDi system employs the existing legacy duplex fiber cables without the need to add more parallel fiber lanes.
SWDM4 transceivers are also available with four wavelengths operating over a pair of LC duplexed fibers. The 40G SR4 QSFP+ uses four 10G wavelengths on OM3 MM fiber over 100meters and 150 meters on OM4 fiber. (The 100G single mode LR4 QSFP+ version uses four 25G wavelengths).
Labor and material cost savings are realized by being able to employ the existing LC multimode duplex cabling plant.
So, it can be cost advantageous to consider SWDM to reduce the use of more expensive MPO connectors with parallel optical cables and to also extend the reach of multimode cabling systems. WBMMF is a new type of multimode fiber with optical properties tailored for SWDM applications in the range from 850 to 950nm and capable of more than doubling the reach for 40G and 100G applications.
WBMMF also supports legacy OM3 and OM4 fiber applications with backwards transceiver compatibility for Ethernet and Fiber Channel links.
The TIA942AAAE WBMMF fiber specification is expected to be ratified shortly, and the TIA-568.3-D specification is expected to include WBMMF as a recommended multimode fiber media. In addition, the draft of the ISO 11801 (edition 3) specification also includes WBMMF.
What to consider next?
Most 100G based Ethernet switches allow each port to be configured as not just a single 100G parallel optical port but also as individual duplex 10G ports by physically splitting out the pairs. One thing to consider is that the use of some types of transceivers may not easily allow for the simple migration from the lower switching speeds of an installed 10G infrastructure to 40G and 100G fiber networks. For instance, duplex 40G WDM transceivers cannot be broken out into separate 10G fiber connections.
Therefore, it is important to know what’s required for a particular fiber network installation and to plan ahead for future needs. Taking a complete view of the system (Switches + transceivers + connectivity + fiber cabling) is key to utilizing the existing cabling infrastructure and to plan a migration design strategy for the next 3-5 years.