SWDM AND OM5-WIDEBAND MULTIMODE FIBRE

With the increasing demand for higher data rates (25G, 40G, 100G and 400G) and their continued expansion within data centre infrastructures,  SWDM and the recently ratified OM5-WBMMF is set to become a very important consideration for fibre optic infrastructures.

AN INTRODUCTION

Since 2010 with the advent of the IEEE 803.3ba 40G and 100G multimode standard, Ethernet based parallel optics have typically been deployed in short reach Data Centre applications.

The fibres are spatially multiplexed using Tx/Rx optical transceivers to achieve high data rates. The data is thensimultaneously split between 4 or 10 optical fibres at a wavelength of 850nm in the first operating window.

This solution uses laser optimised OM3 and OM4 and VCSEL Ethernet type lasers and the signal is duplex. One fibre is required for Tx and one Rx so that a 40GBASE-SR4* systems use a total of 8 fibres (4 x pairs each with 10G) and 100GBASE-SR10 uses a total of 20 fibres (10 x pairs each with 10G).

There are however numerous challenges with splitting the signal down parallel channels. For example, if the fibre paths differ appreciably in length the issue of “skew” or “time of flight” errors or delays may occur (IEEE 802.3ba lays down a requirement that skew between channels in parallel optics be <75ns).

But all is not lost, in the background a new technology called WBMMF or OM5 as it recently acquired nomenclature suggests has gradually been progressing and which at its heart allow a multimode solution to split the core signal into 4 different wavelengths which can then utilise a single pair of fibres for transmission.

40G Parallel

4 X 10G & 850nm (Requires 8 fibres)

Becomes

40G Serial

10G Over 4 X Wavelengths @ 850nm (Requires 2 fibres)

100G Parallel

10 X 10G & 850nm (Requires 20 fibres)

Becomes

100G Serial

25G Over 4 X Wavelengths @ 850nm (Requires 2 fibres)

THE BASICS OF SHORT WAVE DIVISION MULTIPLEXING (SWDM)

Ethernet is principally operated at 850nm over multimode optical fibres.

Imagine now if we are able to operate at 4 wavelengths simultaneously, say 850nm, 880nm, 910nm and 940nm. Instead of using one laser, four VCSEL lasers are tuned to these four different wavelengths.

This is possible because it gives sufficient channel spacing to avoid any undesirable interference effects as the VCSEL pulse disperses (spreads over the wavelength range) and attenuates (loses optical power with transmission distance) as it travels along the fibres.

If each individual wavelength based channel operates at 10 Gbp/s, the data capacity of the fibre is 40Gbp/s.

In terms of hardware, the four tuned VCSEL lasers are multiplexed at the transmit Tx end using an optical combiner then de-multiplexed at the receive Rx end using an optical splitter into the individual Rx modules. This is very good news indeed as the system uses single Tx/Rx modules and only a single fibre for transmit and another fibre for the duplex return.

The scalability of this becomes apparent very quickly. For example, if 25 Gbp/s is transmitted then 100G is possible over four wavelengths whilst still only using two fibres.

There are however some factors and challenges that need to be considered. These include the performance of the fibre cable and connectors and the overall network topology.

WIDEBAND MUTLIMODE FIBRE

Transmission of four wavelengths has been demonstrated over OM4 cable. However, the distances achieved are limited. For SWDM to work within complex data centre type networks with multiple connections and cross connects, a higher grade of fibre is required – enter OM5-WBMMF.

Optical fibre manufacturers are in the process of producing this new wideband optimised (50/125) OM5 fibre. The principal change over existing OM4 fibre is that through graded refractive index profile engineering they are developing high performance VCSEL laser optimised 50/125 glass fibres cores that have flatter attenuation, an increase in bandwidth and modal dispersion characteristics in the range of 850nm to 950nm.

This ensures the fibre performance is less “peaky” and the same level of network performance is achieved for each of the 4 channels across the wavelength range.

wideband mulitmode fibre wbmmf

RATIFIED STANDARDS AND THE SWDM ALLIANCE

The SWDM Alliance is a collection of companies who believe that the adoption of shortwave wavelength division multiplexing (SWDM) is a cost-effective means of transmitting multiple channels on one duplex multimode fibre pair. The goal of the organization is to create and promote an industry ecosystem that fosters the adoption of SWDM for cost-effective data centre interconnections over duplex multimode fibre at data rates at or above 40 Gbp/s.

The SWDM Alliance is focused first on 40 Gbp/s (using four 10 Gbp/s wavelengths) and then on 100 Gbp/s (using four 25 Gbp/s wavelengths). The alliance have initially demonstrated

  • 100 Gb/s (4 x 25 Gb/s) using 4 wavelength SWDM over 100m using standard OM4 fibre
  • 100 Gb/s (4×25 Gb/s) using 4 wavelength SWDM over 300m using a new Wideband OM4

This would imply that current OM4 would support SWDM. Whilst this may be true for a simple 100m link, performance over a move complicated link in a working environment with multiple mated pairs of connectors and cross connects has yet to be demonstrated.

The system has also been shown to work with low power ≤1.5W VCSEL lasers. Higher power VCSEL lasers may not be suitable.

INTEGRATED DATA CENTRE FIBRE INFRASTRUCTURE OFFERED BY COMPLETE CONNECT

Supporting all requirements up to 200G Ethernet

MX Series MTP MPO 24 Fibre OM3 Modular Cassette

1G to 10G to 25G

Duplex fibre networking

MPO MTP M310 Series OM4 12 Fibre Patch Cord

40G SR4 and 100G SR4

Using parallel transmission

cisco cpak sr10 compatible mpo cable

100G SR10 and LR10

InfiniBand fibre networking

THE FUTURE

SWDM serial transmission over multimode is really only at its beginning. It is expected that 25 Gb/s over duplex fibre is a reality. This means that a 100GBASE-LR4 (4x25GB/s) standard will offer 100 GB/s data carrying capacity within Data Centre multimode applications.

The future for the technology looks very bright indeed. Initially with SWDM transmission over existing OM4 networks (because SWDM fits perfectly with existing cabling types, terminations and fibre storage equipment) and then with the on-going development development of new OM5-WBMMF networks.

This gives the following benefits: –

  • 25G and 100G over duplex fibres
  • Reduction in fibre infrastructure
  • Increased port density
  • Increased transmission distance
  • Increased data rates as the norm: 25G, 40G, 100G, 400G and 1600G

To conclude serial optics in the Data Centre offer Complete Connect customers the chance to upgrade their network by increasing data carrying capacity fourfold through greatly improved cable and fibre effectiveness in the network, using single VCSEL lasers, simple Tx and Rx ports all “skew” free and easy to test.

Complete Connect solutions have been designed to incorporate this new technology and ensure networks are future proof and upgrades are simple and offer the best ROI.

Complete Connect will keep you informed about exciting new products to deploy this new and rapidly developing technology.


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