ROADM Architectures and Photonic Switching

A Reconfigurable Optical Add-Drop Multiplexer (ROADM) is the optical-layer routing node: it terminates one or more DWDM line directions, lets any wavelength pass straight through in the optical domain, and lets any other wavelength be added (originated from a local transponder) or dropped (handed to a local transponder) without converting the rest of the line traffic to electrical. The technology that makes a ROADM possible is the Wavelength Selective Switch (WSS). This chapter covers WSS internals, the broadcast-and-select vs route-and-select architectures, the modern CDC-F property set (Colorless / Directionless / Contentionless / Flex-grid), multi-degree mesh wiring with contentionless add/drop banks, and the cascade-narrowing penalty that limits how many ROADMs a wavelength can transit.

Concept	What it says
Wavelength Selective Switch (WSS)	The fundamental optical-layer building block: a one-input-many-output (or many-input-one-output) device that routes any wavelength to any output port independently. Modern WSS technology is LCoS (Liquid Crystal on Silicon) — a programmable phase grating that beam-steers wavelengths to fibre ports with software-tunable port mapping and per-channel attenuation.
Broadcast-and-Select vs Route-and-Select	The two ways to wire a ROADM degree. Broadcast-and-select uses a passive splitter on ingress and a WSS on egress (every wavelength reaches every output, the WSS picks). Route-and-select uses a WSS on both ingress and egress (the ingress WSS pre-selects per output) — better OSNR, better isolation, no power loss for unused outputs, at twice the WSS count.
CDC-F	The four properties of a modern add/drop architecture. Colorless: any wavelength to any drop port. Directionless: any wavelength to any line direction. Contentionless: two wavelengths of the same frequency can drop simultaneously to different ports. Flex-grid: the channel slot is sized in 12.5 GHz increments rather than fixed at 50 or 100 GHz.
Cascade-Narrowing Penalty	Every WSS pass narrows the optical passband slightly because the WSS filter shape is not perfectly rectangular. After 5-8 cascaded WSSes a 32 Gbaud signal in a 75 GHz slot loses enough spectrum to start hurting receiver OSNR — this caps the optical reach in pure-photonic deployments at roughly 5-8 ROADM hops.

Wavelength Selective Switch — The Building Block

A WSS is a free-space optical device that takes one input fibre and routes any spectral slice independently to any of its output fibres (or vice versa). Two physical technologies dominate:

LCoS (Liquid Crystal on Silicon) — a 2D pixel array of phase-shifting liquid-crystal cells over a CMOS backplane. A diffraction grating angularly disperses incoming wavelengths across the LCoS panel; per-pixel phase patterns steer each wavelength to the desired output fibre. Modern WSSes are 1×20 or 1×32, with sub-GHz frequency resolution, software-defined per-channel attenuation (0-25 dB), and full flex-grid support.
MEMS (micro-electromechanical mirrors) — earlier generation, fixed channel grid, lower port count. Largely displaced by LCoS for new builds because LCoS programmability is essential for flex-grid and per-channel power equalisation.

flowchart LR
    IN["Input fibre<br/>96 lambdas"] --> GRATING["Diffraction<br/>grating"]
    GRATING --> LCOS["LCoS panel<br/>per-pixel phase<br/>steering"]
    LCOS --> P1["Output<br/>port 1"]
    LCOS --> P2["Output<br/>port 2"]
    LCOS --> P3["Output<br/>port 3"]
    LCOS --> PN["...<br/>port N"]
    style IN fill:#378ADD,stroke:#185FA5,color:#fff
    style GRATING fill:#BA7517,stroke:#854F0B,color:#fff
    style LCOS fill:#1D9E75,stroke:#0F6E56,color:#fff
    style P1 fill:#7F77DD,stroke:#534AB7,color:#fff
    style P2 fill:#7F77DD,stroke:#534AB7,color:#fff
    style P3 fill:#7F77DD,stroke:#534AB7,color:#fff
    style PN fill:#7F77DD,stroke:#534AB7,color:#fff

1×N WSS: the grating disperses wavelengths in space, the LCoS programmable phase pattern steers each wavelength to one of N output fibres. Per-channel attenuation is achieved by detuning the steering pattern slightly from optimal coupling.

Key Insight

A WSS is the optical analogue of an Ethernet switch — but at the wavelength level. It has no digital plane, no packet buffer, no MAC table; it is pure optics steered by a microprocessor that programs the LCoS pixel pattern.

Broadcast-and-Select vs Route-and-Select

A ROADM degree is one connection to a line direction (one fibre out, one fibre in). Within a degree, the ingress and egress sides are wired in one of two ways:

flowchart TB
    subgraph BS ["Broadcast-and-Select (B&S)"]
        BSIN["Line in"] --> BSPL["1xN passive<br/>splitter"]
        BSPL --> BSWSS1["WSS<br/>(other degrees)"]
        BSPL --> BSWSS2["WSS<br/>(drop bank)"]
    end
    subgraph RS ["Route-and-Select (R&S)"]
        RSIN["Line in"] --> RSWSS_IN["Ingress<br/>WSS"]
        RSWSS_IN --> RSWSS1["WSS<br/>(other degrees)"]
        RSWSS_IN --> RSWSS2["WSS<br/>(drop bank)"]
    end
    style BSIN fill:#378ADD,stroke:#185FA5,color:#fff
    style BSPL fill:#BA7517,stroke:#854F0B,color:#fff
    style BSWSS1 fill:#1D9E75,stroke:#0F6E56,color:#fff
    style BSWSS2 fill:#1D9E75,stroke:#0F6E56,color:#fff
    style RSIN fill:#378ADD,stroke:#185FA5,color:#fff
    style RSWSS_IN fill:#1D9E75,stroke:#0F6E56,color:#fff
    style RSWSS1 fill:#1D9E75,stroke:#0F6E56,color:#fff
    style RSWSS2 fill:#1D9E75,stroke:#0F6E56,color:#fff

Property	Broadcast-and-Select	Route-and-Select
Ingress side	Passive splitter	WSS
Egress side	WSS	WSS
Wavelengths reaching each port	All	Only those selected for that port
Per-port insertion loss	Splitter loss + WSS loss (high)	2× WSS loss (lower)
Filter rectangularity (passband)	One WSS pass	Two WSS passes — slightly more narrowing
Channel-isolation	Modest (single WSS extinction)	Excellent (two WSS extinctions in cascade)
Cost / part count	Lower (splitter is passive)	Higher (extra WSS per degree)
Used in	Older generation, low-degree metro	Modern multi-degree backbone

Rule of Thumb

Modern (2020+) backbone ROADMs are universally route-and-select. The extra WSS pays for itself in OSNR, isolation, and the ability to trim per-channel power on both sides of the node — important for SRS-tilt management on long C+L lines.

CDC-F — Modern Add/Drop Property Set

A first-generation ROADM (circa 2005) was C-D-D-F all-failed: each drop port was wired to a fixed wavelength, a fixed direction, and a fixed channel slot. Adding a new wavelength meant a truck roll. Modern designs add four properties one at a time:

Colorless

A drop port can be tuned to any wavelength via the WSS — no per-port optical filter, no fixed wavelength assignment. The transponder’s own laser tunes to the desired wavelength; the drop port carries whatever wavelength the WSS routes to it.

Without colorless	With colorless
Each drop port = one fixed wavelength	Any drop port = any wavelength
Adding a new wavelength = adding a new port (truck roll)	Adding a new wavelength = software command

Directionless

A drop port is not wired to a single line direction. Any wavelength arriving from any line direction can be routed to any drop port. Implemented by feeding all degrees’ WSSes through a shared add/drop bank.

Without directionless	With directionless
Drop ports grouped per direction	Drop ports shared across all directions
Restoring a failed link requires re-patching	Restoration is a software re-route

Contentionless

Two wavelengths of the same optical frequency arriving from two different directions can both drop, simultaneously, to two different drop ports without colliding. Without contentionless, two same-frequency drops collide because they would have to share a single same-frequency drop tributary.

Without contentionless	With contentionless
Same-wavelength drops from two directions = blocked	Both drop on different ports
Restoration may fail because backup path uses same lambda	Same-lambda restoration always works

Flex-grid

The channel slot is sized in 12.5 GHz increments per ITU-T G.694.1, with central frequency on a 6.25 GHz grid (f = 193.1 + n · 0.00625 THz, slot width = m · 12.5 GHz). A 100G PDM-QPSK signal at 32 Gbaud fits a 50 GHz slot; a 400G PDM-16QAM at 64 Gbaud needs a 75 or 87.5 GHz slot; a 1.2T PCS-64QAM super-channel may take 150 GHz. Fixed-grid 50/100 GHz lines cannot carry these higher-symbol-rate signals.

Without flex-grid	With flex-grid
Slot width = 50 or 100 GHz, fixed	Slot width = m × 12.5 GHz, m = 1..32
400G+ symbol rates do not fit 50 GHz	400G fits a 75 GHz slot, super-channels fit > 100 GHz
Spectrum waste at low rate (10G in 50 GHz)	Spectrum efficiency tracks symbol rate

CDC-F Property Matrix

Property	First-gen ROADM (2005)	Colorless ROADM	CDC ROADM	CDC-F ROADM (modern)
Colorless	No	Yes	Yes	Yes
Directionless	No	No	Yes	Yes
Contentionless	No	No	Yes (with MxN WSS)	Yes
Flex-grid	No	No	No (fixed 50 GHz)	Yes (12.5 GHz slot quantum)
Add/drop port count	Per-degree, per-wavelength	Shared per-degree	Shared across degrees	Shared across degrees, any-to-any
Restoration agility	Truck roll	Software (same direction)	Software (any direction)	Software (any direction, any rate)

Multi-Degree ROADM Topology

A multi-degree ROADM has N line directions plus a contentionless add/drop bank serving all N. Each degree’s egress WSS draws from all other degrees plus the add bank; each degree’s ingress feeds all other degrees plus the drop bank.

flowchart LR
    subgraph CDCF ["3-degree CDC-F ROADM"]
        L1IN["Line 1<br/>in"] --> W1I["WSS 1<br/>ingress"]
        L2IN["Line 2<br/>in"] --> W2I["WSS 2<br/>ingress"]
        L3IN["Line 3<br/>in"] --> W3I["WSS 3<br/>ingress"]

        W1I --> W2E["WSS 2<br/>egress"]
        W1I --> W3E["WSS 3<br/>egress"]
        W1I --> DROP["Drop bank<br/>MxN WSS<br/>(contentionless)"]
        W2I --> W1E["WSS 1<br/>egress"]
        W2I --> W3E
        W2I --> DROP
        W3I --> W1E
        W3I --> W2E
        W3I --> DROP

        ADD["Add bank<br/>NxM WSS"] --> W1E
        ADD --> W2E
        ADD --> W3E

        W1E --> L1OUT["Line 1<br/>out"]
        W2E --> L2OUT["Line 2<br/>out"]
        W3E --> L3OUT["Line 3<br/>out"]

        DROP --> XPDR_DROP["Local<br/>transponders<br/>(any colour,<br/>any direction)"]
        XPDR_ADD["Local<br/>transponders"] --> ADD
    end
    style W1I fill:#1D9E75,stroke:#0F6E56,color:#fff
    style W2I fill:#1D9E75,stroke:#0F6E56,color:#fff
    style W3I fill:#1D9E75,stroke:#0F6E56,color:#fff
    style W1E fill:#1D9E75,stroke:#0F6E56,color:#fff
    style W2E fill:#1D9E75,stroke:#0F6E56,color:#fff
    style W3E fill:#1D9E75,stroke:#0F6E56,color:#fff
    style DROP fill:#7F77DD,stroke:#534AB7,color:#fff
    style ADD fill:#7F77DD,stroke:#534AB7,color:#fff
    style XPDR_ADD fill:#D85A30,stroke:#993C1D,color:#fff
    style XPDR_DROP fill:#D85A30,stroke:#993C1D,color:#fff

A 3-degree CDC-F ROADM. Each line direction has an ingress and egress WSS (route-and-select). All ingress WSSes feed all egress WSSes for any-to-any optical routing, and a contentionless MxN drop bank serves the local transponders.

The MxN add/drop bank is the contentionless element: M groups of N ports, with internal WSSes that allow any wavelength on any input to reach any port — so two same-frequency drops from different directions never collide.

Cascade-Narrowing Penalty

Each WSS pass through a ROADM filters the signal with a passband shape that is not perfectly rectangular. The 3 dB filter width is typically 5-8 GHz narrower than the slot width on each side, and the filter is gentler at the edges than at the centre. After N WSSes, the cumulative passband is the product of N individual filter responses — narrower and rounder than any single one. Eventually the signal’s spectral content at the slot edges is attenuated enough to hurt receiver OSNR.

xychart-beta
    title "Cascaded WSS passband — narrowing with hop count"
    x-axis "Frequency offset from slot centre (GHz)" [-40, -32, -24, -16, -8, 0, 8, 16, 24, 32, 40]
    y-axis "Transmission (dB)" -25 --> 1
    line "1 WSS" [-15, -3, -0.5, -0.1, 0, 0, 0, -0.1, -0.5, -3, -15]
    line "4 WSS cascade" [-22, -10, -3, -0.6, -0.1, 0, -0.1, -0.6, -3, -10, -22]
    line "8 WSS cascade" [-25, -19, -8, -2.2, -0.4, 0, -0.4, -2.2, -8, -19, -25]

The penalty depends on signal symbol rate and slot width. A 32 Gbaud signal in a 75 GHz slot has comfortable spectral guard bands and tolerates ~8 cascaded WSSes; a 64 Gbaud signal in the same slot has tight margins and starts hurting at 5-6 hops:

Configuration	Symbol rate	Slot width	Estimated max ROADM hops
100G PDM-QPSK	32 Gbaud	50 GHz	8-10
400G PDM-16QAM	32 Gbaud (super-channel)	75 GHz	7-9
400G PDM-16QAM	64 Gbaud	75 GHz	5-6
800G PCS-64QAM	96 Gbaud	150 GHz	4-5
1.2T PCS super-channel	~140 Gbaud	200 GHz	3-4

Warning

“Unlimited photonic ROADM cascades” is a marketing claim, not an engineering reality. Every WSS passband has a finite shape, and the cumulative narrowing penalty is hard physics. Network designs that route a single wavelength through more than ~5-8 ROADMs without optical-electrical-optical regeneration (“3R” — re-amplify, re-shape, re-time) need careful penalty analysis.

Rule of Thumb

For modern 400G / 64 Gbaud designs, plan at most 5 ROADM hops in the optical domain before regenerating. For 800G+ super-channels at 96-140 Gbaud, plan 3-4 hops. Provision express bypass at every transit ROADM that exceeds these limits.

Super-Channels and Flex-Grid Slot Allocation

A super-channel carries multiple sub-carriers in a single contiguous frequency block, treated by the network as one optical signal. Examples:

Multi-laser super-channel: 4 × 100G PDM-QPSK at 32 Gbaud, packed into a 200 GHz flex-grid slot, total 400G.
Single-laser high-symbol-rate super-channel: 1 × 800G PCS-64QAM at 96 Gbaud in a 150 GHz slot.
Future 1.2T: 1 × 1.2T at ~140 Gbaud in a 200 GHz slot, requiring TFLN modulator and >100 GHz analogue bandwidth.

Flex-grid lets each super-channel claim exactly the spectral width it needs (m × 12.5 GHz), with no wasted spectrum and no fixed-grid reservation. The line system carries a heterogeneous mix of slot widths simultaneously — 50 GHz here for legacy 10G, 75 GHz there for 400G, 150 GHz for an 800G super-channel — all on the same fibre pair.

Open Line System Adjacency

A modern multi-vendor optical network often separates the line system (amplifiers, ROADMs, fibre) from the terminals (transponders). The line system manages amplification, OSNR, and wavelength routing; the terminals manage modulation, FEC, and client mapping. The Open Line System (OLS) concept lets the line system come from one vendor while transponders from any other vendor (or pluggable coherent optics in router faceplates) interoperate via standardised optical specs (e.g. OIF 400ZR). This is the architectural opening that 08-open-disaggregated-optical-and-ipodwdm explores in depth.

Summary

A modern ROADM is a route-and-select multi-degree node built from LCoS WSSes, with a contentionless MxN add/drop bank to serve local transponders. The CDC-F property set (Colorless / Directionless / Contentionless / Flex-grid) is the standard — anything less is legacy. The hard physical limit is the cascade-narrowing penalty: every WSS pass narrows the passband, and the practical optical reach in pure photonics is 3-8 ROADMs depending on symbol rate and slot width. Beyond that, signals must be regenerated electrically. Flex-grid is the foundation on which super-channels and IPoDWDM rest.

References

Standards (ITU-T)

ITU-T G.694.1 — Spectral grids for WDM applications: DWDM frequency grid (10/2020). https://www.itu.int/rec/T-REC-G.694.1
ITU-T G.872 — Architecture of optical transport networks (10/2017). https://www.itu.int/rec/T-REC-G.872
ITU-T G.798 — Characteristics of optical transport network hierarchy equipment functional blocks (12/2017). https://www.itu.int/rec/T-REC-G.798
ITU-T G.680 — Physical transfer functions of optical network elements (07/2007). https://www.itu.int/rec/T-REC-G.680
ITU-T G.698.2 — Amplified multichannel dense wavelength division multiplexing applications with single channel optical interfaces. https://www.itu.int/rec/T-REC-G.698.2

Books

R. Ramaswami, K. N. Sivarajan, G. H. Sasaki, Optical Networks: A Practical Perspective, 3rd ed., Morgan Kaufmann, 2009.
I. P. Kaminow, T. Li, A. E. Willner (Eds.), Optical Fiber Telecommunications VI-A & VI-B, Academic Press, 2013.

Papers

S. Frisken et al., “Wavelength selective switches based on liquid crystal on silicon,” Optical Fiber Communication Conference (OFC), 2008.
O. Gerstel et al., “Elastic Optical Networking: A New Dawn for the Optical Layer?” IEEE Commun. Mag. 50, S12 (2012). [Flex-grid foundations]
K. Roberts et al., “Beyond 100 Gb/s: Capacity, Flexibility, and Network Optimization,” J. Lightwave Technol. 35, 1 (2017).
OIF 400ZR Implementation Agreement (OIF-400ZR-01.0). https://www.oiforum.com/documents/

Transport Network Guide

Explorer

Ch 07 — ROADM Architectures and Photonic Switching

ROADM Architectures and Photonic Switching

Wavelength Selective Switch — The Building Block

Broadcast-and-Select vs Route-and-Select

CDC-F — Modern Add/Drop Property Set

Colorless

Directionless

Contentionless

Flex-grid

CDC-F Property Matrix

Multi-Degree ROADM Topology

Cascade-Narrowing Penalty

Super-Channels and Flex-Grid Slot Allocation

Open Line System Adjacency

Summary

See Also

References

Standards (ITU-T)

Books

Papers

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