Singlemode vs multimode optical fibre

Currently, 10 Gbps is the going data rate in enterprise networks, with state-of-the-art data centres introducing 40 Gbps. Ongoing development of high-speed ethernet means development of 100 Gbps is well underway and 400 Gbps options are already being explored.

Today’s enterprise customer is faced with a constant challenge - to build a network not only with the latest capabilities, but with the capacity to adapt to increased data speeds within a reasonable future time frame.

Designing these flexible, budget-wise, future-ready solutions is a core concern to most involved in the industry. In this article, AFC Group answers some key questions.     

What is the difference between singlemode optical fibre (SMOF) and multimode optical fibre (MMOF)?

Singlemode fibres have a small core size which supports a single mode or path of light at any one time. This small core requires precision alignment to inject light from the transceiver and means that SMOF has virtually no limits to potential bandwidth and performs excellently over long distances.

Multimode fibres have larger cores that guide many modes simultaneously and make it easier to capture light from a transceiver. MMOF gives high bandwidth at high speeds over short to medium distances.

Which system can do what we need it to do?

The most crucial aspects are distance and protocol support. SMOF has the distinct advantage over longer distances and should be used for anything requiring 10 Gbps over more than 550 metres. But most enterprise networks do not require this distance, and MMOF can support 10 Gbps up to 550 m and support 100 Gbps up to 150 m.

AS/NZS 3080:2013 Table F.3 ‘Supported applications using optical fibre’ shows every ratified fibre standard to date, with the distances and fibre types over which they can be carried. In addition, active equipment manufacturers specify supported distances on their datasheets that may well improve on those stated in AS/NZS 3080:2013 because of the design of those active components.

Singlemode vs multimode optical fibre.

Can your system adapt to greater speeds as development continues?

SMOF would seem on the surface to be the ‘futureproof’ option, with the allure of ‘unlimited’ bandwidth. Currently, SMOF systems use wavelength division multiplexing (WDM), where four lasers operating at 10 Gbps, and transmitting at four different wavelengths, achieve 40 Gbps. But the reality is, for distances less than 550 m, MMOF offers exactly the same capability. Capability for 100 Gbps over LOMMF (laser optimised multimode fibre) is just around the corner and the development of 400 Gbps Ethernet over OM4 LOMMF has already begun.

What is the most cost-effective system?

SMOF cable and connectors are less expensive than MMOF, due to lower cost of manufacture and higher volumes of production. However, the cost of the transceivers and switches for an SMOF solution typically cost more than twice as much as the comparable multimode optic electronics. Similarly, the precision lasers required for SMOF are much more expensive than the LEDs or VCSELs required for MMOF. This cost differential is predicted to be even greater with 100 Gbps demands/capabilities.

There are also consequential cost differences. MMOF is easier to install and terminate in the field.

SMOF also requires more power - 100 Gbps MM transceivers are expected to draw less than 6 watts, while an SM transceiver draws about 20 W. The reduced power requirement means that more MM transceivers can be mounted into a rack unit. For example, 16 or 32 MM ports vs 4 SM mode ports in a single rack unit.

Summary

Many people will lean towards SMOF in the pursuit of futureproofing their network when, in fact, customers can make their networks ‘future ready’ by using MMOF to achieve the same network performance at lower cost. Too often SMOF systems are specified that will never take full advantage of that fibre type, when an MMOF system could have delivered the same performance at a lower system CAPEX and OPEX. The option of using hybrid MM+SM cables is underutilised and can be an ideal option to cover all potential applications. Correct fibre selection is key to the implementation of a network that is future ready and within budget