ACOs versus DCOs

By Gurinder Parhar, Vibha Goel on December 19, 2016 | Leave a Comment

While both support reconfigurable optical networking, analog coherent optical modules and digital coherent optical modules each offer different benefits.

Back in the days of dial-up Internet, 10 Gbps long-haul transmission seemed more than speedy enough. Fast forward to today, when an online retailer might want to process and backup 1 million online transactions per hour and streaming video companies serve up feature films to hundreds of thousands of subscribers simultaneously. Indeed, studies indicate that global Internet protocol (IP) traffic will triple over the period from 2015 to 2020. To satisfy the voracious demand for bandwidth, the telecom industry has turned to coherent transmission techniques. These methods exploit not just amplitude but phase and polarization characteristics of light to enable data transmission at rates of 200 Gbps and faster.

Speed alone isn’t enough, however. Asset owners need fast network build out and the flexibility to reconfigure once the pieces are put in place. For that, they turn to a new generation of integrated analog coherent optical modules (ACOs) and digital coherent optical modules (DCOs). These devices contain all of the optical components necessary for transmit and receive in a single standardized package. This gives telcos and enterprises alike the freedom to choose the approach that best addresses their needs.

The photo detectors used in optical communications receivers only detect intensity. As a result, for many years, optical communications used amplitude modulation schemes that had a one to one correlation between bit rate and the baud rate (the number of times the modem could turn off and on in a second). A 10 Gbaud modem could only support a 10 Gbps data rate, for example. Baud rates have a practical electronic limit, however, while bandwidth demand does not. To support the kinds of data rates required by current applications, modern communications systems are needed to break through the baud-rate ceiling. Enter coherent detection, which leverages not just the amplitude but the phase and polarization of light to transmit multiple bits of data per baud.

Modulation Schemes

A variety of coherent modulation schemes exist; let’s look at two of the most common.

Dual Polarization Quadrature Phase Shift Keyed (DP-QPSK)
In phase-shift keying (PSK), multiple data bits with different optical phase states propagate down the fiber simultaneously. Various flavors of PSK exist. Quadrature phase shift keying (QPSK) utilizes four symbols out of phase with each other by 90 degrees. This scheme corresponds to 2 bits per baud. Dual-polarization QPSK doubles the data rate yet again by transmitting two quadrature data stream son two orthogonal polarizations for 4 bits per baud.

Quadrature Amplitude Modulation (QAM)
Quadrature amplitude modulation, as the name implies, changes both the phase and the amplitude of the signal to implement more symbols and thus represent more bits. More sophisticated versions of the scheme exist, with integral amplitude shifts and smaller phase differences. These include 16-QAM (16 symbols representing 4 bits per baud), 64-bit QAM (64 symbols representing 6 bits per baud) and 128-QAM (128symbols representing 7 bits per baud). The trade-off – and in engineering, there are always trade-offs – is that as data rate increases, so does noise relative to the spacing between states, which reduces the effective transmission distance.

symbol_graphic

Although encoding a QAM or QPSK signal has its difficulties, the real challenges arise at the receiver.Because photo diodes only measure intensity, the detection scheme needs to include a method for capturing phase difference. The solution is to use an optical interferometer to detect interference between the optical signal and a reference signal. This technique essentially converts phase difference to an intensely variation that can be captured by photo detector. This device is called a Coherent Receiver, and is usually a photonic integrated circuit, or PIC, which is like an electronic IC chip, only for optical signals rather than electronic ones.

The Trend: Your Way, Only Faster

Despite their classifications, ACOs and DCOs have quite a bit in common. To highlight, let’s compare a CFP2-ACO with a CFP-DCO. For starters, they are both built into industry-standard form factors, although the CFP2 package is half the size of the CFP. The compliance ensures that they are both pluggable and interoperable.

Both classes of modules are designed as integrated devices that have several components in common:
• A narrow band laser source-the narrower the line width, the lower the noise and therefore the higher order QAM modulation the device can support.
• A low-insertion-loss modulator-minimizing loss reduces the need for a booster erbium-doped fiber amplifier (EDFA) in the module. This cuts size and cost while improving optical signal-to-noise ratio (OSNR). A stronger signal gives a longer reach.
• A high-sensitivity coherent receiver.

The devices also need digital signal processors (DSPs) for to code and decode the coherent signal to. This brings up the key differentiator between the ACO and the DCO. In the case of the CFP2-ACO, the DSP is located off-module with the rest of the electronics. This gives the CFP2-ACO proper a smaller form factor and lower cost, and also reduces heat generation. It’s a more complex solution, however, that requires the user to interface the optical module with the DSP.

State-of-the-art CFP2-ACOs have demonstrated data rates up to 200 Gbps, with 600 Gbps on the horizon. Because the DSP is located off module, users are free to choose their own DSP. That makes the ACO a good fit for network equipment manufacturers who may wish to incorporate their own proprietary DSPs. The trade-off is greater need for in-house expertise during the design, build out, and maintenance phases.

In the case of the CFP-DCO, the DSP is located within the module. That converts it to a plug-and-play module that offers very efficient deployment. The approach makes it easy for organizations with a range of in-house skills to take advantage of 100 Gbps and even 200 Gbps data rates across DCI and long-haul distances.

In industry, there is always a balance between custom solutions that provide performance differentiation and commodity solutions that simplify integration while minimizing cost. At the same time, the digital signal processors inside a CFP-DCO enable users to configure the module to address their application needs, but also to reconfigure their network in real-time to address changes in demand.The trade-offs are larger size and higher cost, as well as increased heat generation and power consumption from the DSP within the module. The necessity of managing the heat generated inside the CFP-DCO module often means limiting the reach of data rate of the transmission. ACO or circuit board implementations that can use more electrical power, since the heat dissipation can be handled over a larger area, can often achieve higher performance.

The general rule of thumb is that the ACO is the better fit for users requiring highest performance and product differentiation, while the DCO approach is better for organizations wanting to derive benefit in a hurry.The geographic sweet spot for the ACO tends to be the Americas and Europe, while the DCO is more popular in China. It’s also a solution growing increasingly popular for certain DCI applications.

As bandwidth demand grows, service providers seek solutions that deliver the performance they need with the features they want. Whether the application demands top performance or ease-of-use, network designers have their choice of integrated modules that can support the speeds required by the networks of the future.

What Is a CFP2-ACO?
https://www.neophotonics.com/what-is-a-cfp2-aco/

CFP2-DCO and CFP2-ACO Transceivers – Basic Definitions
https://www.neophotonics.com/cfp2-dco-cfp2-aco-transceivers-basic-definitions/

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