As new technologies emerge, they often face unforeseen challenges created by the growth that they create. Data transmission in the enterprise is no different. Presently, most multimode fibre (MMF) typically carries data traffic at Ethernet data rates up to 1.25Gbit/s (Gbit Ethernet or 1GbE). As enterprise applications evolve and the demand for bandwidth grows, there is a strong need to support higher data rates.

The enterprise space has a significant base of installed fibre optics cables, and practical economics suggest that leveraging this existing infrastructure to carry 10.3Gbit/s traffic (10Gbit Ethernet or 10GbE) makes sense. A significant challenge to be considered is that installed legacy fibre was not designed to support higher data rates, and thus maximum link distances are more limited for 10GbE than for 1GbE, due to increased levels of modal dispersion.

The emerging IEEE 10GBASE-LRM standard (IEEE P802.3aq) provides the possibility to achieve 10GbE data transmission over legacy MMF to the distances required by enterprise applications.

Advancing beyond 10GBASE-LX4

This is not the first time that the IEEE has worked on standardising a new technology to address similar applications. The 10GBASE-LX4 optical interface was a part of the original 802.3ae standard, and it is already deployed in the field. LX4 is not comprised of just a single 10Gbit/s serial link – it is a combination (using CWDM) of four 3.125Gbit/s data streams that achieve a total throughput of 10Gbit/s when optically multiplexed on the same fibre.

To support LX4, an optical module is required to transmit four different wavelengths at 3.125Gbit/s each. This makes the module design more challenging, and in order to achieve CWDM operation, requires the use of four different lasers and four receivers in a single module. This is expensive to accomplish and the modules are complex to manufacture in very high volumes.

Additionally, the large number of components required in an LX4 module makes it difficult to support the small form factors suitable for higher port density applications, like XFP or the emerging SFP+. This need for less complex, lower-cost modules that are smaller in size and simpler to manufacture is what prompted the current LRM standard.

With these requirements in mind, the IEEE P802.3aq task force has targeted the following objectives for 10GBASE-LRM:

1) leverage the existing 10GBASE-R PCS (physical coding sublayer)

2) support a bit error ratio (BER) lower than or equal to 10-12

3) support fibre media selected from IEC60793-2-10

4) provide a physical layer specification which supports link distances of at least 220m on 500MHz-km multimode fibre.

Existing fibre links in the current enterprise infrastructure range up to 300m in length, although fibre lengths of 220m encompass the significant majority of all applications. Additionally, simulations performed by the standards task force indicate that the new LRM standard is expected to have sufficient margin to support more than 90 per cent of installed fibres up to 300m in length. This provides good coverage of the existing fibre base using LRM technology.

Distinguishing features of the LRM standard

A 1310nm laser is used for transmission over multimode fibre. Before the wide adoption of lasers for communication applications, LED sources were used. Previously-installed multimode fibres were thus optimised for 1300nm operation with these sources. The selection of this laser source would therefore make the longer transmission distances possible due to this optimisation. The use of 1310nm Fabry-Perot (FP) lasers is permitted by the LRM standard, and these lasers are less expensive than 1310nm distributed feedback lasers (DFBs).

LRM optical receivers must support a multimode input and deliver high responsivity at 1310nm. The output of the receiver must be linear in order to preserve the input waveform shape.

This is important in equalising the waveform with the receive chain. LRM modules should include electronic dispersion compensation (EDC) to equalise the modal dispersion present in the multimode transmission medium. EDC is an indispensable part of LRM design. This technology is based on channel optimisation fundamentals used successfully in the past for lower data rate domains.

Effects of dispersion

Dispersion occurs when components of a signal travel at different speeds over a transmission medium and arrive at the receiver at different times. The different spectral components in an optical signal can create chromatic dispersion and the different transmission modes can cause modal dispersion.

Another element of dispersion is polarisation mode dispersion which occurs when signal components with different polarisations have different transmission speeds. Any form of dispersion can cause pulse broadening and inter-symbol interference, which appears as an eye closure of the optical signal.

In order to achieve a specific end-to-end bit error ratio (BER), a higher signal-to-noise ratio (SNR) is required in the optical link to compensate for the eye closure caused by dispersion effects. This higher SNR is the dispersion penalty. When fibre optic systems are designed, the dispersion penalty must be considered in the power budget along with other penalties such as connector loss and attenuation within the fibre cable. This becomes even more critical when considering higher data rates such as 10Gbit/s transmitted over legacy multimode fibre.

For most enterprise applications, dispersion is the most significant factor that must be considered, because attenuation is negligible over the relatively short distances commonly serviced by multimode fibre.

The dominant form of dispersion in multimode fibres is modal dispersion, which reduces the effective transmission distance across all data rates.

Electronic dispersion compensation

In order to reach transmission distances of up to 220m at 10Gbit/s, special design techniques are required to compensate for the effects of modal dispersion. Electronic filtering (also known as equalisation) can be included in a communications channel to compensate for signal degradation caused by the medium. The use of filtering to compensate for dispersion in an optical communications link is known as EDC.

There are two primary approaches being considered for EDC applications at 10Gbit/s. The first is a combination of feed-forward and decision feedback equalisation. The second is a maximum likelihood sequence estimator (MLSE) that uses DSP techniques to determine the most likely bit sequences that have been transmitted using a decision tree and error estimation of the input data signal.

Future preferred MMF transmission technology

The 10GBASE-LRM standard is strongly supported by the supplier community, and many silicon vendors are developing ICs with EDC functionality for LRM applications. In addition, optical sub-assembly designs that support LRM are currently under development by multiple optics suppliers.

The formal ratification of the 10GBASE-LRM standard by the IEEE is anticipated in a few months, and yet LRM has already created a niche for itself in the legacy multimode fibre space with network system designers now looking for samples of LRM-based transceiver modules.

With all of its inherent advantages, 10GBASE-LRM is expected to become the industry’s preferred multimode transmission technology for enterprise applications in the very near future.

Christian Urricariet is director of marketing, high-speed optics, and Sunil Sharma is senior application engineer at Finisar