What is CD/PMD Testing? (Chromatic Dispersion & Polarisation Mode Dispersion Explained)
- Paul Forster
- Feb 17
- 8 min read
As fibre networks across the UK and Europe continue to upgrade to 100G, 400G and 800G infrastructure, dispersion testing is no longer optional — it’s essential.
At TNS Comms, we provide professional CD/PMD testing services across the UK and Europe, supporting telecom operators, data centre providers, integrators and enterprise networks that require accurate fibre characterisation before high-capacity service activation.
If you are commissioning new fibre, upgrading an existing route, or accepting dark fibre, CD/PMD testing ensures your infrastructure is ready.
👉 Learn more about our dedicated dispersion testing service: https://www.tnscomms.co.uk/cd-pmd-fibre-testing-uk-europe
What does CD/PMD testing actually measure?
CD/PMD testing is part of fibre characterisation testing. Instead of only checking that light “gets through” (loss), characterisation confirms the fibre will behave correctly at high data rates over the required distance.
Chromatic Dispersion (CD) in plain terms
Chromatic Dispersion happens because different wavelengths (colours) of light travel at slightly different speeds through glass. Over distance, that causes a pulse to spread out and blur into neighbouring pulses, which can create errors and reduce usable reach at higher speeds.
Polarisation Mode Dispersion (PMD) in plain terms
PMD happens because real-world fibre is never perfectly symmetrical. Tiny stresses, bends, manufacturing tolerances, or environmental effects cause light to split into two polarisation states that travel at different speeds. The result is differential group delay (DGD)—another form of pulse spreading that becomes more critical as bit rates increase.
Why we test for CD and PMD
People often ask: “If my OTDR and loss test are good, why do I need dispersion testing?”
Because loss is only one part of link performance. A link can pass Tier 1 insertion loss and still fail at higher capacities if dispersion is excessive for the planned service and distance.
Here are the most common reasons CD/PMD testing is specified:
1) Turning up higher-rate services (10G → 800G)
As you move from 10G/40G to 100G/400G/800G, tolerance to impairments tightens and engineering margins matter more. That’s especially true for:
Metro and core upgrades
Long-haul spans
Data centre interconnect (DCI)
DWDM / coherent optical builds
High-capacity backhaul projects
2) Fibre acceptance, commissioning and handover
For many projects, dispersion results are required as part of network acceptance testing and formal handover packs—particularly where the fibre will carry high-capacity wavelengths or coherent services. TNS Comms supports this with clear, client-ready reporting.
3) Preventing “mystery faults” later
Dispersion issues can present as:
Reduced optical margin
Increased bit error rates
Latent instability that only appears during temperature swings or reconfigurations
Service failures after an upgrade even though the fibre “passed” basic testing
CD/PMD testing reduces risk before the network goes live.
4) Validating fibre type and suitability
In mixed estates, not all fibre is the same. Standards and design assumptions often reference common fibre categories (for example, ITU-T single-mode fibre families), and dispersion characteristics vary by fibre type and wavelength plan.
How CD and PMD impact 10G, 40G, 100G, 400G and 800G circuits
Dispersion becomes more important when any of the following increase:
Bit rate (more bits packed into less time)
Distance (more opportunity for spreading)
Spectral complexity (DWDM/coherent channels, tighter spacing, higher baud rates)
10G and 40G
At 10G/40G, CD/PMD may or may not be required depending on distance, optics type, and engineering rules. But on longer spans or older fibre, testing can quickly highlight whether dispersion compensation or different optics are needed.
100G
100G is a common inflection point. Many delivery teams start requiring dispersion testing more routinely—particularly for DCI, metro builds, or where the service is sensitive to accumulated impairments.
400G and 800G
As you get into 400G and 800G, “unknowns” are expensive. New optics and higher symbol rates raise the bar for link characterisation, and vendors and operators increasingly want documented results as part of acceptance. Field tools designed for modern interfaces help reduce retests and speed up turn-up.
IEC standards commonly referenced for CD/PMD measurement
When clients specify CD/PMD testing, they often want it aligned to recognised international measurement standards.
IEC chromatic dispersion measurement
A commonly referenced standard for chromatic dispersion measurement methods is IEC 60793-1-42.
IEC polarisation mode dispersion measurement
A commonly referenced standard for PMD measurement methods is IEC 60793-1-48.
These standards define measurement approaches so results are consistent and defensible for commercial inspection, commissioning and acceptance contexts.
ITU-T “G-series” standards relevant to fibre characterisation and high-capacity transport
“ITU-G standards” is a phrase people often use when they mean ITU-T recommendations in the G-series, which cover fibre characteristics, test methods, and optical transport frameworks.
Here are a few that commonly come up alongside CD/PMD testing and high-capacity optical delivery:
Fibre characteristics (what the fibre is expected to be)
ITU-T G.652: Characteristics of a single-mode optical fibre and cable (widely referenced baseline for SMF properties).
Test methods and definitions (how we measure)
ITU-T G.650.2: Definitions and test methods for statistical/non-linear fibre attributes including PMD measurement approaches.
Optical transport frameworks (how services are carried)
ITU-T G.709: Interfaces for the Optical Transport Network (OTN).
ITU-T G.959.1: Optical transport network physical layer interfaces.
ITU-T G.694.1: DWDM frequency grid (spectral grids for WDM).
These references help align fibre characterisation results with the expectations of modern transport networks — especially where DWDM, OTN and coherent layers are in play.
Where CD/PMD testing fits within real-world fibre testing (Tier 1, Tier 2, Characterisation)
A lot of projects use a layered approach:
Tier 1: Insertion loss testing (IL)
Tier 1 verifies end-to-end loss against a budget and is a common handover requirement.
Tier 2: OTDR testing
Tier 2 uses OTDR to locate events (splices, connectors, bends) and verify the link’s integrity end-to-end.
Characterisation: CD/PMD and advanced parameters
Characterisation testing (including CD/PMD) answers the next question:Is this fibre suitable for the intended high-capacity service over this distance and architecture?
TNS Comms delivers Tier 1, Tier 2 and advanced fibre characterisation depending on the project scope.
The VIAVI OneAdvisor 800 (ONA 800) test platform in CD/PMD delivery
Modern builds benefit from test platforms that can keep pace with the interfaces you’re commissioning. The VIAVI OneAdvisor 800 (ONA 800) is positioned as a field-ready platform for high-capacity testing, including 800G scenarios.
Why this matters on live projects
On high-capacity rollouts, you typically want:
Fast, repeatable workflows
Clear pass/fail visibility aligned to test plans
Reliable operation in field conditions
Tooling that supports both legacy and next-gen interfaces to avoid “multiple kit swaps” during a turn-up window
Supporting 10G/40G/100G/400G/800G environments
In the real world, upgrades rarely happen all at once. You might be validating:
Existing 10G services on the same route
100G upgrades for DCI
400G turn-ups on new optics
Early 800G readiness in selected corridors
Platforms designed for modern transport testing help engineering teams stay consistent as the interface mix evolves.
Typical CD/PMD testing workflow (what clients can expect)
While every network has its own acceptance criteria, a typical CD/PMD testing workflow looks like this:
1) Confirm the scope and acceptance criteria
This usually includes:
Fibre route(s) and endpoints
Distance and wavelength plan (where relevant)
The planned service rate (10G/40G/100G/400G/800G)
The reporting format required for handover
Any operator-specific thresholds
2) Prepare the fibre and ensure clean connectivity
Connector condition can compromise results, so inspection and proper cleaning practices matter before measurement. (It also protects test equipment.)
3) Run dispersion measurements and validate consistency
Depending on the method, you’ll measure:
CD (ps/nm/km or accumulated CD across the span)
PMD coefficient / DGD characteristics (ps)
4) Correlate with OTDR and loss results (where required)
This is especially useful if results suggest anomalies—OTDR can help identify whether physical events correlate with unexpected behaviour.
5) Produce client-ready reports for sign-off
The value isn’t just taking measurements—it’s delivering results that are easy to interpret and use for acceptance, audit and future troubleshooting.
TNS Comms provides professional reporting suitable for handover packs and compliance documentation.
The Business Case for CD/PMD Testing
Many organisations ask: “Is dispersion testing really necessary?”
Consider the cost of:
Failed 400G activation
Transceiver replacement
Delayed go-live penalties
Customer dissatisfaction
Reactive troubleshooting
Compared to proactive fibre characterisation, the risk-to-cost ratio strongly favours testing.
CD/PMD testing is not just technical assurance — it is project risk management.
Common scenarios where CD/PMD testing is essential
Here are situations where CD/PMD testing is frequently requested or strongly recommended:
Data centre interconnect (DCI)
DCI routes often run high capacity, tight delivery timelines, and low tolerance for uncertainty. Documented fibre characterisation reduces risk during service activation.
DWDM / coherent optical deployments
Dense wavelengths and coherent modulation formats place more emphasis on understanding fibre behaviour across the span.
Long-haul and metro upgrades
Older fibre routes may have unknown history, multiple repair events, or mixed fibre types—characterisation brings clarity before you commit to new optics.
Dark fibre acceptance
If you’re leasing or handing over dark fibre, CD/PMD testing helps validate what’s actually being delivered beyond just “continuity and loss.”
TNS Comms specifically highlights support for DWDM/coherent, long-haul/metro links, and DCI in its CD/PMD service scope.
Why TNS Comms for CD/PMD testing and fibre characterisation
CD/PMD testing is specialist work: the equipment is advanced, the interpretation matters, and the results often become contractual acceptance evidence.
TNS Comms provides CD and PMD fibre testing services across the UK and Europe, and supports broader fibre testing delivery for commissioning, audits and fault diagnosis.
Experience that translates into smoother project delivery
On live projects, the best testing partner isn’t just someone who can press “start”—it’s a team that understands:
How fibre routes are actually built and maintained
The realities of access, time windows and multi-party sites
What “good” reporting looks like for client handover
How to reduce repeat visits by aligning testing to the end goal (acceptance, upgrade, troubleshooting)
TNS Comms positions itself as a specialist integration and fibre testing provider with decades of industry experience.
Qualified engineers and professional reporting
TNS Comms notes specialist fibre characterisation capability, trained engineers, and clear reporting aligned to recognised standards and operator criteria.
Typical CD/PMD Testing Workflow
Scope Confirmation
Route endpoints
Distance
Service type
Required reporting format
Fibre Preparation
Connector inspection
Cleaning verification
Measurement
Chromatic dispersion accumulation
PMD coefficient and DGD
Correlation
Cross-reference with OTDR where required
Reporting
Clear client-ready documentation
Suitable for acceptance and audit
Our engineers ensure every report supports operational and commercial requirements.
Our Final Word: Future-Proofing European Fibre Infrastructure
As bandwidth demand accelerates across the UK and Europe, dispersion behaviour becomes a defining factor in network reliability.
Professional CD/PMD testing ensures:
High-capacity readiness
Compliance with recognised standards
Reduced commissioning risk
Long-term scalability
Confidence in infrastructure investment
TNS Comms delivers expert fibre characterisation services that support modern optical networks — from 10G legacy systems to 800G next-generation infrastructure.
Frequently Asked Questions
How do I know if I need CD/PMD testing?
If you’re commissioning or upgrading capacity (especially 100G, 400G or 800G), building DWDM/coherent layers, accepting dark fibre, or working on long/critical routes—CD/PMD testing is often worth including.
Is CD/PMD testing only for long-haul?
No. It’s common in long-haul, but also highly relevant in metro, DCI, and any environment where higher capacity optics and tight margins are in play.
Can I rely on vendor assumptions instead?
Vendor guidelines are useful—but they don’t replace measuring the actual fibre you’re putting into service, especially in complex or older estates.
Can coherent optics compensate for dispersion?
Coherent systems provide electronic compensation, but excessive dispersion still reduces margin and performance stability. Testing ensures levels are within acceptable thresholds
Next steps: book CD/PMD testing with TNS Comms
If you’re planning a new build, an upgrade, or a service turn-up where fibre performance must be proven — not guessed — CD/PMD testing gives you confidence before the network goes live.
TNS Comms delivers specialist dispersion testing services across the UK and Europe.
TNS Comms can support:
CD/PMD fibre characterisation
Tier 1 insertion loss testing
Tier 2 OTDR testing
Jointing, splicing, verification and handover support
Integrated Fibre Capability
We do not operate in isolation. Our wider services include:
Fibre optic installation
👉 https://www.tnscomms.co.uk/fibre-optic-installation-contractor
Fibre optic jointing and testing
👉 https://www.tnscomms.co.uk/fibre-optic-jointing-and-testing
Fibre optic maintenance
👉 https://www.tnscomms.co.uk/fibre-optic-maintenance
Data centre fibre services
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