This article examines some of the technical challenges of moving beyond 10Gbps in the access network, advantages and limitations of coherent and PAM4 technologies as solutions for high-speed networking and how new innovations in the realm of PAM4 technology can set access network operators up for success for the next decade and beyond.
European access networks are constantly changing to meet the evolving needs of their end users. Over the past several years, network operators have gone from employing separate networks for their offerings to providing multiple services across one network. This large-scale trend toward convergence means that the fiber infrastructure of modern access networks is now supporting multiple technologies like Radio Access Networks (RANs), Wi-Fi, Metro Ethernet, Passive Optical Network (PON) and more.
With convergence and growing end user demands around 5G and the IoT, comes an increased need to upgrade the speed and capacity of Europe’s access networks. Already, the EU has set aside 700 million EUR to accelerate research and innovation into 5G technology that will be used for a myriad of use cases. Upgrading networks, however, is a challenging business.
Technical Challenges of Optical Access Networking above 10Gbps
Distinct from backbone and metro networks, access networks have two key features:
- A wide variety of link lengths that connect multiple locations using outside plant infrastructure. Though an access network can have link lengths that span from 10 to 120km, the majority of links span 40-60km.
- A large number of links that leverage diverse protocols and data rates. This adds to the complexity of the overall network.
As if operating access networks was not already complex, upgrading bandwidth above 10Gbps brings its own set of issues. Here are the top 3:
- Optical Link Budget
When network operators push past 10G per lambda, optical link budget considerations become paramount. Overall, the amount of optical power at the link receiver must be above the receiver’s sensitivity in order to successfully gain a link. As access networks span longer distances, accounting for link budgets becomes increasingly complex. Though one can always add optical gain to a system with amplifiers like EDFAs, additional amplification always brings an increase in noise.
- Optical Signal-to-Noise Ratio
Just because a receiver sees light above sensitivity does not mean data is being transmitted successfully. The optical signal-to-noise ratio (OSNR) is the ratio of the actual signal level to the level of noise in the system. The smaller this number, the more likely there will be bit errors in the transmission. As links move to higher order modulation schemes to get more bits per symbol (i.e., PAM4 or QAM), the better the OSNR needed to distinguish the signal levels. Digital signal processors (DSPs) and Forward Error Correction (FEC) can be used to compensate for bit errors, but they also add complexity, power, latency and general interoperability concerns.
- Chromatic Dispersion
For networks with bandwidths of 10Gbps and above, chromatic dispersion has a negative impact on optical transceiver receiver sensitivity. Chromatic dispersion is an effect on an optical signal that causes the pulses to spread as the signal travels through the fiber. In slower signaling systems, ample time between symbols means that spreading does not create an issue. However, as the baud rate increases, the pulse spreading causes symbols to overlap each other, resulting in inter-symbol-interference (ISI).
At bandwidths of 10Gbps, chromatic dispersion limits network links to approximately 80 km. Past 10Gbps, at bandwidth rates of 25Gbps and beyond, chromatic dispersion begins to limit link distance to 15 to 20km. This does not meet the distance requirements of most access networks today. Though dispersion compensating modules (DCMs) exist, they add higher levels of system complexity.
Solving for Higher Bandwidth in Access Networks
Coherent and PAM4 optics are attractive solutions for evolving access networks for higher speeds and bandwidth (up to 400G and beyond) while addressing the aforementioned challenges. However, each technology has its own limitations.
Coherent Optics – Though access-centric CFP, CFP2 and QSFP56-DD/OSFP coherent optics enable a long link reach of greater than 100km (up to thousands of km for CFP and CFP2) at data rates as high as 400G, they have higher space and power requirements than other types of transceivers used within an operator’s access network. As a result, leveraging coherent optics potentially raises total operator costs, which poses a challenge to network operators with constrained budgets.
PAM4 – The PAM4 standard employs four amplitude channels, each with two bits, which doubles the data rate, making it twice as efficient as legacy binary models. Compared to the space and power requirements of coherent optics, PAM4 leverages components that can fit in smaller, more common form factors such as the QSFP28 model. As such, it delivers significant cost savings advantages to network operators that leverage it.
However, PAM4 optics are not without their limitations. Going past 100Gbps at an optical link reach of 5km requires amplification, dispersion compensation and/or forward error correction (FEC) on the optical line. This means the installation of active equipment and greater power expenditure, potentially mitigating PAM4’s low-cost advantage.
New Technologies: Accommodating the Diversity of Use Cases Within Access Networks
Despite their drawbacks, both coherent optics and PAM4 technology will play a strong role in propelling access and other networks to 800G and beyond. What is missing from the discussion, however, is a solution that can solve for the longer reach and costlier nature of coherent optics and the cost-effective yet constrained reach of available PAM4 solutions.
Modern access networks serve many different types of structures from cell towers, enterprise buildings and multi-dwelling units (MDUs) to data centers, aggregation sites, schools and sports stadiums. The new trend in access networking is a focus on data transmission across shorter distances, often less than 40km. Extremely low latency requirements are also adding a new and interesting dimension into the access network, especially as 5G rollouts continue.
It is quickly becoming clear that access network operators need a solution that achieves the following three requirements:
Allows for 100G DWDM so operators can have the speed and capacity they need to meet current and future end-user requirements
Leverages a common transceiver form factor, like QSFP28, for seamless interoperability and cost savings
Enables data transmission at a reasonable distance of 40km without requiring any external transport equipment, amplifiers, filters, etc.
As we launch into 2021, one thing is clear – the future of the optical access network in Europe is bright. With governments continuing to invest in 5G rollouts and individual network operators jockeying for position, the trend toward convergence and the need for higher speeds and bandwidth is only heating up. Optical network equipment manufacturers have a strong role to play in making the networks of the future a reality. Though existing coherent and PAM4 solutions help operators overcome the challenges of upgrading bandwidth to a certain extent, there is still room for improvement. Fortunately, that time has come, and European network operators can look forward to a bold future as they work to meet the requirements of their end users.