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Why the road to 400Gbps is not as straight as you would expect

Staff Reporter
July 13, 2023

Due to the incompatibility , anyone looking to USE 400Gbps will need to consider replacing their transceivers.

One of the biggest hurdles for the UK’s step towards digital transformation is the upgrade of its digital infrastructure says Rob Pocock, Technical Director, of cyber security consultants Red Helix

Last year, the UK government published its digital strategy with a heavy focus on positioning the country as a ‘Global Science and Tech Superpower’. Not a bad idea when you consider that, according to a report by Public First, digital technology could carry the potential to grow the UK economy to over £413 billion by 2030. There is, however, a lot of work that needs to be completed before reaching that point.

One of the biggest hurdles for the UK’s step towards digital transformation is the upgrade of its digital infrastructure – which is needed to accommodate ever-increasing data demands.

New technological advancements – such as 5G, machine learning (ML) and artificial intelligence (AI) – are being incorporated into the likes of smart networks, advanced manufacturing and autonomous transport, with almost limitless potential. But for these technologies to work efficiently, high-speed connectivity must be available across the country.

These demands, amongst other factors, have led to a jump from a 100Gbps infrastructure to a 400Gbps infrastructure, somewhat bypassing the 200Gbps milestone. The issue here is that the previous speeds all used Non-Return-To-Zero (NRZ) modulation and this isn’t capable of processing a 400Gbps network speed. As a result, this means that an entirely different form of modulation will be required in order to make this speed function within the network.

The incompatibility issue

In order to achieve the faster data speeds of 400Gbps, more data needs to be transmitted down the cable at the same time, which is where Pulse Amplitude Modulation 4 Level (PAM4) comes into play. While NRZ modulation uses two levels to represent binary data (with one level representing a ‘0’ and the other representing a ‘1’), PAM4 uses four different levels (representing ‘0-0’ ‘0-1’ ‘1-0’ and ‘1-1’). This allows PAM4 to transmit twice as much data in each clock cycle, with each level representing two pieces of data instead of one, effectively doubling the rate of data being passed over the same physical medium.

Having these two distinct modulation schemes in place, however, means that they aren’t capable of interoperating. PAM4’s four-level system will not be understood by equipment designed to read the two-level system of NRZ, and vice versa, the NRZ two-level system will not be understood by the PAM4 four-level system. Therefore, for network operators looking to upgrade to 400Gbps, they need to ensure that both ends of a link are using the same modulation.

Pocock: NRZ modulation and this isn’t capable of processing a 400Gbps network speed

This issue really comes into effect when using breakout cables; (for example, breaking out 400Gbps into 4 x 100Gbps links) as most of the 100Gbps optics in today’s networks are running NRZ modulation and are therefore incompatible. Users would, therefore, need to change their 100Gbps optics to ones using PAM4 modulation. This is assuming the device the optics are inserted to has been certified to work with PAM4-100Gbps optics.

It must be noted that not all of these issues are insurmountable; however, it is another factor that network architects and engineers need to consider on top of everything else before jumping in and upgrading their networks.

The true cost of moving to 400Gbps

Due to the incompatibility between PAM4 and NRZ, anyone looking to start using 400Gbps will need to consider replacing their existing transceivers. 400Gbps transceivers are expensive, as is often the case with new technology, and whilst it is true that the price will eventually come down after a period of time, customers need to be aware that they won’t be as cost-effective as the older transceivers which were previously used.

There are two reasons for this. Firstly, these new transceivers need to maintain compatibility with existing Quad Small Form-factor Pluggable (QSFP) form factors and, as such, they need to fit into the same physical space as the previous, NRZ-based transceivers. Developing a transceiver that is able to transmit and receive PAM4 signals, while keeping the size the same as previous transceivers, involves considerable costs in development– making the new transceivers more expensive.

Secondly, because the 400Gbps transceivers generate more heat than their 100Gbps counterparts, there will need to be improved methods of cooling and heat transfer. As the size of a transceiver is the same, there is more heat to disperse, increasing the risk of the transceiver becoming distorted as a consequence. Not only does this add to the cost of research and development, with additional testing required to understand the temperature thresholds, but it also means that further investment is needed in cooling and power infrastructures.

Understanding the requirements before upgrading

If we look back at the moves from 10Gbps to 40Gbps and from 40Gbps to 100Gbps respectively, it was relatively easy due to the fact they all, for the most part, used the same modulation. This means that the use of breakout cables to split or combine different speeds worked well, providing us with a simple route to move between speeds which eased the migration. Now, with the move to PAM4, we just need to work out what can and can’t interoperate within the newly expanded limits.

There is, of course, an eagerness to upgrade to 400Gbps networks. But before starting, it is important to understand exactly what is required for a 400Gbps transmission to work and how much it is going to cost. As we transition to higher speeds, it is no longer just about considering the type of fibres being used (single mode or multimode) and the transmission length of a particular technology. We must also consider how devices capable of running at these faster speeds are going to interoperate with existing networks. It is important to note that interoperability is key as it enables consistent, smooth and efficient performance when transferring data across a company.

Network operators who recognise these challenges can avoid costly mistakes and make the most efficient use of the resources, not only now, but also during future upgrades – as it is highly likely we will run into similar challenges when upgrading to 800Gbps, or even 1.6Tbps, networks.

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