Building future pathways for next generation data centre infrastructure
Thu 6 Apr 2017 | Rick Pimpinella
At a recent 400G Light Speed event in London, Dr. Rick Pimpinella, a leading researcher and innovator in the field of optical fibre cabling, provided an insight into how innovations in optical fibre are helping to provide the next steps in capacity and through-put planning…
Just as the storage memory in your computer, USB drive, or smart phone rapidly reaches its storage limit with photographs, music, and other data, the data center operator must plan for the unrelenting demand for more data as it is the foundation for business continuity.
To meet future needs, it is important to focus on next generation network infrastructure. A critical component of the infrastructure is the structured cabling, and its crucial to know if it can support the next generation network resources and transceiver technology designed for higher speed data rates and maximum channel reaches.
Consider the use of 10Gb/s switch to switch connections today, and the need for 40Gb/s tomorrow. For this scenario, to upgrade from 10GBASE-SR to 40GBASE-SR4, the duplex fibre pairs must be replaced with four duplex pairs (eight fibres) terminated with MPO connectors. In addition, only eight of the 12 fibres in a typical 12-fibre cable are utilised leaving four dark fibres. To fully leverage all fibre strands, the data center infrastructure must be carefully designed with an upgrade path in mind.
A more efficient upgrade path would be to replace 10GBASE-SR transceivers with next generation 50GBASE-SR transceivers (to be ratified in 2018), with the same duplex fibre structured cabling. The higher speed 50Gb/s solution is projected to be lower cost and utilises an advanced modulation scheme which can be extended with future 200Gb/s transceivers. Focusing on next generation data center infrastructure will provide a cost effective flexible infrastructure for years to come.
Advances in fibre
Fibre technologies have to evolve in response to the applications’ bandwidth needs. To support higher data rates over short channel reaches (< 100m), multimode fibre will continue to offer the lowest cost optical solutions. However, for future Terabit per second data communications, single-mode fibre (SMF) will ultimately be required.
As network equipment data rates increase, fibre types must also evolve to support the optical transceivers. Consider the evolution of multimode fibre. Categories OM1 and OM2 MMF provided the necessary bandwidth and channel reach for 50Mb/s and 200Mb/s optical data links introduced in the early 1980s. These early optical data links used light emitting diodes (LEDs) operating in the 1300nm window and supported reaches up to 2km. However, 20 years later when data rates increased to 10Gb/s, LEDs and OM1/OM2 MMF had to be replaced with VCSEL technology and laser optimised Categories OM3 and OM4 MMF operating in the 850nm window, supporting reaches up to 300m and 400m respectively.
There is a great deal of ‘specmanship’ and misinformation regarding the best fibre type
Although MMF is more expensive to manufacture than SMF, single-mode transceivers are significantly more expensive. Traditionally, SMF transceivers have been at least three times more expensive. For short reach applications, less than 500m, the transceivers dominate the total channel cost (see Figure 1).
As data rates increase beyond 400Gb/s, newer MMF technology capable of supporting multiple wavelengths must be deployed. Panduit’s Signature Core fibre and the recently standardised Wide Band MMF (OM5), developed within a TIA Task Force Chaired by Panduit, are designed to support multiple wavelengths. However, these wideband MMFs support relatively short channels (<200m) compared to SMF, and therefore, the demand for SMF in large and hyper scale data centers will increase.
Making an informed decision
One major hurdle facing the data center operator is understanding what fibre type to install; single-mode or multimode and if multimode, which Category: OM3, OM4, or OM5. There is a great deal of ‘specmanship’ and misinformation regarding the best fibre type, maximum channel reach, optical power margin, and overall installed cost.
To achieve higher data rates beyond 50Gb/s, one must employ multiple fibre pairs (parallel optics), or multiple wavelengths (wavelength division multiplexing, WDM). For the transmission of Ethernet over multimode fibre, the IEEE 802.3 Standards body specifies parallel fibre pairs for aggregate data rates up to 400 Gb/s.
Figure 1. Channel cost versus reach for single-mode 100G PSM4 (parallel SMFs) and 100GBASE-SR4. Although SMF is significantly less expensive, the transceiver cost dominates the total cost of the channel. The cost of a multimode channel exceeds that of a single-mode channel at 555m. The data is based on the cost to distributors.
Multimode transceivers utilising multiple wavelengths for transmission over a single duplex fibre pair is defined in Multi-Source Agreements (MSAs) between transceiver manufacturers in order to attain market acceptance and to ensure interoperability. For data rates up to 400Gb/s over MMF, the IEEE specifies parallel optics, but beyond 400Gb/s both parallel optics and Short wavelength WDM (SWDM) must be employed.
The challenge for selecting a fibre type depends on cost, future data rate needs, and maximum channel reach. SMF will support all future data rates and channel reaches, but the performance comes at a cost premium on the order of three to five times that of multimode channels. For SWDM, wide band fibre such as Signature Core or OM5 will be required to guarantee performance over the specified channel reaches with a high confidence level.
All specified channel reaches are based on minimally compliant optical transceivers and the bandwidth of OM3 and OM4 laser optimised multimode fibre. Transceiver manufacturers often provide enhanced transceivers that can support longer reaches. For example, 40GBASE-SR4 has a standards specified reach of 150m over OM4, while an enhanced version of this transceiver can support a reach of 400m.
A primary objective in developing the next generation data rate is maintaining the structured cabling infrastructure. To this end, 40GBASE-SR4, 100GBASE-SR4, and the future 200GBASE-SR4 currently under development, all use the same four fibre pair structured cabling. In addition, for each of these solutions the four parallel fibre pairs can be ‘broken out’ (separated) into four discrete 10GBASE-SR, 25GBASE-SR, and 50GBASE-SR channels respectively. It is important to note SWDM transceivers do not support breakout.
Cabling standards
The development of standards must precede the adoption of application standards and future network infrastructure. Only fibres specified by leading standards development organisations such as TIA or IEC are included as media type options in application standards such as Ethernet and Fibre Channel.
The development of cabling standards and future higher speed infrastructure applications are coordinated by means of liaison letters and active participation from multiple standards bodies. Deploying standards compliant data center infrastructure will ensure a reliable data center network.
