Achieving 100 Gb/s Using O-band Technology

Migration to 400 Gb/s and 800 Gb/s is one of the hottest topics in telecoms at present. However, most operators are still largely powered by 10 Gb/s or 25 Gb/s technology, especially in access networks and LTE/5G base station uplinks. To ensure networks are prepared for the next wave of transmission, operators need to build wave multiplexing systems that will allow connections to migrate to 100 Gb/s.

Here, two telecommunications network specialists from Salumanus explain what devices to use to run N × 100 Gb/s Ethernet in the urban or access infrastructure using O-band transmission.

100 Gb Ethernet transmissions are becoming increasingly popular in applications such as 5G networks and data centres. One way of ensuring operators can successfully migrate to 100Gb Ethernet is by using O-band transmission. O-band, or original band, was the main band used in telecommunications, due to its zero chromatic dispersion. With its spectrum width between 1260 nm to 1360 nm, O-band was the basis for creating lasers and detectors.

Over time, C-band became the preferred choice for operators due to the high attenuation rate of O-band in long distance applications. However, the increasing bit rates, forced further changes. The 100G transmission in the C-band could work only for 2–3-kilometer (km) distances for NRZ/PAM4 modulation. To send the data farther, operators need to compensate for the chromatic dispersion or use more expensive coherent optics.

How to Run 100 Gb/s?

There are several ways to run 100 Gb/s links. The most conventional 100 Gb/s transmission solution is using grey LR4 or ER4 modules. The limitation of this technology is the number of parallel transmissions that can be run. We can run a maximum of one 100 Gb/s transmission on one fiber.

The second option is to run N x 100 Gb/s using a DWDM system based on transceivers that use PAM4 technology. Because of the way the modules work, the DWDM solution requires, apart from multiplexers, the use of chromatic dispersion compensators and optical amplifiers, which effectively increases capital expenditure (CAPEX).

The third method is the use of coherent modulation, which allows us to implement connections without the need to use compensators. Due to the power consumption of currently available coherent modules, this solution requires the use of classical architecture with transponders, because 100 G coherence modules are in the form of CFP/CFP2 interfaces.

Figure 1. The chart of chromatic dispersion. (Image: Salumanus)

GBC Photonics offers another solution that allows operators to run N x 100 Gb/s. This solution is based on a 200 GHz grid in the O-band and allows users to work at up to 30 km distance. Operations in the O-band enable the elimination of chromatic dispersion compensators. According to the chart of chromatic dispersion (Figure 1), for the most popular fiber (G.652) the dispersion is almost equal to 0 at around 1300 nm. Thanks to the use of a 200 GHz grid, we can create up to 16 independent transmission channels.

PAM4 and Direct Detect

One of the greatest advantages of O-band solutions is the use of PAM4 and Direct Detect modulation, which allows the use of GBC Photonics modules for transmission on one and two fibres. The patented nCP4™ processor based on the PH18 Silicon Photonics Tower Semiconductor platform was used to implement the correct PAM4 modulation. The nCP4™ processor allows operators to convert N electrical lines with a 56 baud stream into N optical lines at a speed of up to 800 Gb/s. The integration of several optoelectronic elements offers better parameters compared to conventional bonding of discrete elements.

The PH18 Silicon Photonics Tower Semiconductor solution is a parallel technology development trend aligned to indium phosphide technology. Additionally, the improvement of the receiving sensitivity was obtained by using the APD receiving diode. As a result, the main advantage of combining PAM4 and Direct Detect modulation is the ability to implement modules in both single and double-fiber applications.

O-band Multiplexers

Figure 2. Some of the transceiver equipment offered by GBC Photonics. (Image: Salumanus)

To run several transmissions on the same pair of optical fibres, network operators need to use multiplexers. Using O-band in this case, only the distance between the channels and their number changes.

O-band multiplexers from GBC Photonics allow operators to run 16 channels with a channel spacing equal to 200 GHz. Each port carries one specific channel, the width of which is ± 0.12 nm from the central wavelength. GBC Photonics has also developed a slightly cheaper, 8-channel version. It uses the same modules, while the multiplexer itself has half the number of channels with a spacing of 400 GHz, thus reducing the cost.

Passive O-band Track

The multiplexers themselves are 100 percent passive devices, requiring no power or software connection. By linking two multiplexers, thanks to a special cascade of filters on each channel, we have the same attenuation, which is about 4 dB. In the case of O-band WDM technologies, this attenuation is the most important parameter as it limits the distance at which we can run the transmission.

Each optical module has its own power budget, which is the difference between the power of the transmitted signal and the sensitivity of the receiving diode. The O-band modules have a power budget of 15 dB. Based on this, we can calculate that the module itself can provide services up to 30 km. However, when building a wave multiplication system, in the calculations we must take into account the attenuation of all passive elements, such as the fiber optic line and multiplexers. In this case, we can run services at a distance of up to 25 km.

Simplex and Duplex Solutions

Figure 3. A chart comparing energy savings by number of transmissions between DWDM, DWDM Coherent and Gray 1310 nm solutions. (Image: Salumanus)

Wavelength division multiplexing systems can realize transmissions using either one or a pair of fiber connecting edge locations. The duplex system uses the same wavelength for transmitting and receiving, carried by different fibres. However, in the case of implementing such a system using one optical fiber, we use two different wavelengths, one for transmission and one for receiving. This way, the number of services we can run is reduced by half.

Thanks to O-band technology, we can increase the capabilities of a pair of optical fibres by running up to 16 x 100 GbE on them. This system on them. This system can provide services up to 25 km. The entire solution is fully passive and does not require chromatic dispersion compensators. Optical modules can also be installed directly in network devices such as routers or switches.

Lower CAPEX and OPEX

Compared to DWDM solutions, we eliminate 10 ~ 20W amplifiers and a 40 ~ 150W chassis on each side of the transmission line. Additionally, compared to coherent technology, we eliminate the need to use transponders/ muxponders and grey optics to make connections between network devices. Such a procedure allows operators to reduce electricity consumption by up to 80 percent.

O-band solutions provide an easy and economical method for migrating to 100 Gb/s and is dedicated for access and mobile base station uplink network. Using O-band-based multiplexers and modules allows operators to cover distances of up to 30 km, while maintaining their required passive infrastructure for easier management and lower energy consumption.

This article was written by Jakub Kolasiak, and Michal Owca, Product Managers for the Transmission System and Optical Transceivers Departments for Salumanus. For more information, visit here .