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Tutorial on Bi-Directional Transmission Technology

Migrate to 40G With BiDi Technology

Tutorial on Bi-Directional Transmission Technology

FS Official 2016-08-16

Optical fibre communication has been showing a trend toward higher transmission capacities, larger transmission spans, higher flexibility and lower transmission costs during the past few years. Within this background, we had ushered in the era of bi-directional transmission technology, or BiDi technology for short.

BiDi Technology—Less Fibres for Faster Speed

For bi-directional transmission technology, “bi-directional” is meant in sense of using optical fibre in two directions, similar to the use of single railway track in two directions. BiDi technology allows transceivers to both transmit and receive data to/from interconnected equipment through a single optical fibre. In the early stage of optical fibre communication, one optical fibre transmits signals of only one wavelength, one optical fibre can only transmit signals of one wavelength. That means at least two fibres are needed to accomplish the full-duplex communication with one optical fibre transmitting Tx optical signals, and the other optical fibre transmitting Rx optical signals. This is the conventional two-fibre Bi-Directional communication. However, with the never insatiable need for more capacity and greater performance, the existing network resources are getting harder to manage it. To maximize the capacity and usage of optical fibres, BiDi technology cropped up, which can use one optical fibre to implement the transmitting and receiving of optical signals, thus saving optical fibre resources and doubling your network capacity.

Inside the BiDi Products

Currently, the main BiDi products are BiDi transceivers, which also referred to as WDM transceivers. Different from the common a two-fibre bi-directional optical transceiver, which is composed of TOSA and ROSA, the BiDi transceiver adopts BOSA to replace the TOSA and ROSA, achieving bi-directional transmission over a single optical fibre. BiDi transceivers split light into different wavelengths. They must work as a matched pair having the correct receive and transmit wavelength.

Inside the BiDi Products

Case Study: Migrate to 40G With BiDi Technology

The transition to 40G is in its full force, driven by the increasing demand for unimagined capacity and performance from the servers and data centres. It seems that these severs and applications can never be satisfied. For them, enough is never enough. Even if you have not prepared for a 40G Ethernet, you will need it very soon. However there exist some challenges for it.

Problems Facing 40G Migration

 10G SR transceivers operate over dual-fibre multimode fibre (MMF) with LC connectors. But 40G SR4/CSR4 transceivers operate over MMF ribbon with MPO connectors. As a result, 40G MPO-based SR4/CSR4 transceivers cannot reuse aggregation fibre infrastructure built for 10G connectivity.  The common 40G transceivers require 12 fibre strands in practice which are terminated by MTO-12 connectors. But we can see from the picture below that only 8 fibre strands carry traffic, causing 4 fibre strands unused and wasted.

Problems Facing 40G Migration

Solution With 40G QSFP BiDi Transceiver

To solve above problems, 40G QSFP BiDi transceiver based on BiDi transmission technology had emerged. It can transmits full-duplex 40G traffic over one dual-fibre LC connector OM3 or OM4 MMF cable. So, it has the capability to reuse 10G fibre infrastructure. In other words, it enables data centre operators to upgrade to 40G connectivity without making any changes to the previous 10G fibre cable plant, resulting in zero-cost cabling migration from direct 10G connections to direct 40G connections.

Solution With 40G QSFP BiDi Transceiver

Present Application and Future Prediction of BiDi Technology

Currently, BiDi technology is widely adopted by enterprises to get upgraded to 40G performance, for it can save a huge cost, whether for upgrading the current data centre or building a new one. The table below shows the costs and savings of migration and new deployment of 288 direct connections with 40G BiDi technology.

Present Application and Future Prediction of BiDi Technology

From the table, we can see that with such a huge saving for a data centre migration or expansion, 40G BiDi transceivers based on BiDi technology will continue to gain popularity in 40G Ethernet. Since the market for 40G BiDi is broad, have you ever thought about 100G BiDi? Will there be 100G BiDi transceivers in the future? 100G Ethernet is racing to us, and the discussion of 200G and even 400G Ethernet has started heating up among data centre professionals. Under this circumstance, there has been talk that a 100G BiDi optic will be available soon to the market, and I believe that should happen sometime this year. There is no standard for 100G BiDi right now, but we can still predict that the 100G BiDi will utilize an LC connector running on duplex signals. The unknown is the distance it will run on OM3/OM4 glass.

Application Precautions

 BiDi transceivers must work as a matched pair having the correct receive and transmit wavelength. Pay attention to the colour of pull taps on optical modules. An optical module with the yellow pull tap needs to be used with an optical module with the blue pull tap. Optical modules with pull taps of the same colour cannot be used together.  No standard has been released for BiDi transceivers. The wavelengths supported by BiDi modules from other manufacturers may differ from each other. Therefore, the BiDi transceiver from one manufacture may not interwork with optical modules from other manufacturers.  Before BiDi transceivers are used, it is recommended to make a unified planning, for example, to determine the wavelength for upper layer devices and the wavelength for access devices, so as to prevent inter-working failures because of wavelength differences during implementation.

Summary

The obvious advantage of utilizing BiDi transceivers is the reduction in fibre cabling infrastructure costs by reducing the number of fibre patch panel ports, reducing the amount of tray space dedicated to fibre management, and requiring less fibre cable. Although BiDi transceivers cost more compared with traditional two-fibre transceivers, the cost savings of utilizing less fibre is more than enough to offset the higher price of them. And the 100G BiDi transceiver will run to us sooner or later, which may become the most economical and efficient choice for your fibre optic network. Let’s wait and see.


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OTN Solution Team
Senior Telecommunication Engineers

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