How Does a Coherent Module Differ from a Traditional Transceiver?

The Traditional Transceiver — One Dimension of Information

A traditional transceiver, such as a standard SFP or SFP+, operates on the principle of “on or off”. The laser emits two states of light, which the other end of the link receives as ‘1’ or ‘0’. This solution is elegant in its simplicity and genuinely ubiquitous. Such modules work today in billions of devices around the world.

It has, however, one fundamental problem. When you want to send more data, you have only two options: either switch the laser on and off faster, or lay more fibre. There is no third option. Brightness is one dimension of information, and one dimension of information means one dimension of possibility.

The Coherent Module — Reading Images Instead of Flickering

A coherent module takes a completely different approach. Instead of asking “is it lit?”, it asks “how is it lit?” — and exploits several properties of light simultaneously. Coherent modulation encodes information in the amplitude, phase, and polarisation of the carrier signal. These are three independent dimensions that can be combined in any way.

The result is symbols composed of many points — in practice, images in which an entire set of bits is encoded. How many bits exactly depends on the modulation format. QPSK gives two bits per symbol, 8QAM gives three, and 16QAM gives four. With dual polarisation, each of these figures is multiplied by two. DP-16QAM therefore delivers eight bits per symbol — eight times more information than the simplest “lit, not lit” scheme.

There is one more thing that is truly impressive. The sensitivity of a coherent receiver is so high that reading a single bit of information requires an average of just nine photons. That is not a misprint. Nine particles of light are enough to reliably read a bit of data.

DSP — The Brain That Makes It All Work

Behind this precision is a digital signal processor, the DSP. It is the DSP that in real time encodes outgoing data into complex symbols and decodes incoming ones. It handles FEC error correction, compensates for signal distortions in the fibre, and manages polarisation.

The DSP consumes on average half the energy used by the entire coherent module. That is precisely why coherent modules were for years large and hot. Their designers had to pack enormous computing power into the smallest possible housing. The first coherent solutions were built as line cards mounted in large transmission devices. Pluggable modules — which you slide into a router port like a regular transceiver — only became possible once DSP chips became small enough and energy-efficient enough.

Today, the GBC Photonics 400G OpenZR+ module operates at under 22 W. This is the result of using indium phosphide (InP) as the semiconductor material and integrating multiple functions — the laser, optical amplifier, and modulator — into a single piece of that material.

Where Does the Difference in Range Come From?

A traditional transceiver has a limited range because the signal weakens and distorts over long distances. Coherent modules handle this much better, for two reasons. First, the DSP actively compensates for signal distortions in the fibre — chromatic dispersion and polarisation mode dispersion. In classic transceivers these phenomena must be neutralised by separate devices or simply accepted as a range limitation.

Second, coherent modules work together with EDFA optical amplifiers, which regenerate the signal without converting it to electrical form. This opens the way to hundreds of kilometres of transmission without a regenerator — and with modern coherent modules, even over a thousand kilometres.

IPoDWDM — Where These Differences Matter in Practice

Traditional transceivers connect a router to a transponder. The transponder converts the signal to a DWDM wavelength and sends it onward, with another transponder waiting at the far end. A coherent module eliminates this intermediary. You slide it directly into the router’s QSFP port, configure the DWDM wavelength, and the module communicates directly with the optical system. No more separate transponders, their power supplies, cooling, and management.

That is the idea behind IPoDWDM, and the resulting savings are very concrete. Companies deploying such architectures report a 65% reduction in backbone network capital expenditure compared with traditional structures, up to 80% reduction in data centre floor space, and a drop in power consumption from 70% at the network edge to 90% in the core.

Compatibility — One Module for Everything

Historically, coherent modules could be problematic, as every hardware vendor required modules from their own product line. The standardisation of QSFP and the CMIS protocol changed that. GBC Photonics modules work in routers and switches from all leading network hardware vendors compliant with the OpenZR+ standard. Compatibility is confirmed in the laboratory before every delivery.

Additionally, the SRD (Smart Recode Device) environment allows you to program the module yourself to work with a specific vendor’s hardware — from a computer or smartphone, without sending the module to a service centre. You keep one type of module in stock instead of several versions for different vendors.

When to Choose a Traditional Transceiver and When a Coherent Module?

The traditional transceiver still has its place. Connections within a building, on a campus, short distances up to a few kilometres — wherever simplicity and the cost of classic modules win without question.

A coherent module makes sense when distances exceed several tens of kilometres, when you need high throughput on a single fibre, when you are building DCI between data centres, designing an operator backbone, or want to eliminate transponders from the architecture.

The boundary between these two worlds is constantly shifting, as coherent modules are becoming cheaper and easier to deploy. GBC Photonics already offers a coherent 100G module in a QSFP28 housing drawing under 5 W, designed for access networks — a category that until recently belonged exclusively to classic transceivers.

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