Learning Objectives

• Understand how DWDM multiplies the capacity of a single fiber pair
• Identify the components of an optical transport network
• Explain the role of ROADMs and coherent optics in modern transport

The Bandwidth Problem

Every byte that leaves a data center, every streaming video, every cloud API call — at some point it travels over an optical fiber. While IP and MPLS routers handle packet-level switching, the physical medium that carries the bits is an optical transport network.

A single modern fiber can carry 100+ wavelengths (colors) at 400 Gbps or more each, yielding over 40 Tbps per fiber pair. This is made possible by Dense Wavelength Division Multiplexing (DWDM) — the technology that packs many independent optical channels onto a single fiber by using different laser wavelengths spaced 0.8 nm (100 GHz) or 0.4 nm (50 GHz) apart.

DWDM in the Transport Network

A transponder at the edge converts the client signal (e.g., 100 GbE from a router) into a specific DWDM wavelength. The multiplexer combines multiple wavelengths onto a single fiber. An EDFA (Erbium-Doped Fiber Amplifier) boosts the optical signal across long spans — typically 80-120 km between amplifiers. At the far end, a demultiplexer separates the wavelengths, and another transponder converts back to the client interface.

The Optical Transport Network (OTN)

OTN (ITU-T G.709) is the digital wrapper that encapsulates client signals for transport over DWDM. Think of OTN as "SONET/SDH for DWDM" — it provides:

• Forward Error Correction (FEC) — Reed-Solomon or LDPC coding that fixes bit errors at the receiver, extending reach by 3-6 dB
• Performance monitoring — Tandem Connection Monitoring (TCM) for fault isolation across multi-carrier paths
• Multiplexing hierarchy — ODU0 (1.25 G) through ODUC4 (400 G+) and beyond
• Protection switching — sub-50 ms linear and ring protection

OTN frames encapsulate Ethernet, Fibre Channel, SONET, or any other client signal with a fixed 4-byte frame alignment overhead. The overhead bytes carry the TCM, GCC (General Communication Channel), and fault indicators that make OTN manageable in a carrier environment.

ROADMs and the Flexible Grid

A Reconfigurable Optical Add-Drop Multiplexer (ROADM) lets the carrier remotely route wavelengths without sending a technician to reconfigure the optical patch panel. Modern ROADMs support CDC (Colorless, Directionless, Contentionless) architecture — any wavelength can be added or dropped on any port, in any direction, without contention.

The flexible grid (Flex-Grid) extends DWDM beyond fixed 50 GHz channels to channel widths of 25, 37.5, 50, 75, or 100 GHz. This is essential for high-baud-rate coherent signals (e.g., 800 GbE using 64 GBaud or higher) that require more than 50 GHz of optical bandwidth.

Coherent Optics

Modern transport networks use coherent detection — instead of directly detecting the light intensity (direct detect), the receiver mixes the incoming signal with a local oscillator laser. This recovers the full electric field (amplitude, phase, and polarization), enabling:

• Dual-polarization QPSK, 16-QAM, 64-QAM — packing more bits per symbol
• Dispersion compensation in software — chromatic dispersion is corrected digitally, eliminating DCMs (dispersion compensation modules)
• Nyquist subcarriers — multiple subcarriers within a single wavelength to handle bandwidth fragmentation

Coherent optics pushed single-wavelength speeds from 10 Gbps in 2010 to 800 Gbps and 1.6 Tbps in 2025.

Key Takeaways

• DWDM multiplexes 100+ wavelengths onto a single fiber, each carrying 200-800 Gbps
• OTN encapsulates client signals with FEC, TCM, and protection — like SONET for DWDM
• ROADMs enable remote wavelength routing; CDC architecture eliminates manual patching
• Coherent detection recovers phase and polarization for advanced modulation (16-QAM, 64-QAM)
• Flex-Grid allows non-standard channel widths needed for high-baud-rate signals