DWDM network design: line systems, ROADMs, OTN, pluggables

A DWDM network is layered: coloured light on fibre (the line system), a digital wrapper that carries clients and corrects errors (OTN), and the clients themselves (Ethernet, FC, SDH). Where a transceiver plugs in — a transponder shelf or directly into a router as a 400ZR module — decides who owns each layer. This page maps the layers, the building blocks and the design decisions, so that a coloured pluggable can be placed in its context.

Layers

Layer	Elements	Concerns
Client	Ethernet 10/100/400G, FC, SDH/SONET, video	grey optics between client device and transponder, or none if the client uses a coherent pluggable
OTN (ITU-T G.709)	OTU frames wrapping ODU containers, FEC, tandem monitoring	multiplexing, performance monitoring, protection switching
Optical channel	transponders / muxponders or coherent pluggables producing ITU-grid wavelengths	modulation format, OSNR, dispersion
Line system / OMS	mux/demux, amplifiers, ROADMs, OSC, DCMs	span engineering (Amplification & OSNR)
Fibre / OTS	fibre pairs, connectors, splices	loss, dispersion, PMD, OTDR

Line system building blocks

Block	Function	Notes
Mux/demux (AWG or TFF)	combine/separate 40–96 channels	fixed channel plan; ITU grid
ROADM (reconfigurable OADM)	add/drop or pass any channel under software control; degrees = number of line directions (2 for a ring node, 4–8 for a mesh hub)	WSS (wavelength-selective switch) based; colourless/directionless/contentionless (CDC) variants
Amplifiers	booster, inline, pre-amp	Amplification & OSNR
OSC — optical supervisory channel	out-of-band management at 1510 or 1625 nm	carries DCN, amplifier control, span loss monitoring
DCM	dispersion compensation (direct-detect systems)	absent in all-coherent designs
OCM / OPM	per-channel power monitor	commissioning and alarms
OLP — optical line protection	1+1 splitter/switch across two fibre routes	~50 ms switching

Grid and capacity

Grid	Channels in C-band	Per channel	Total	Notes
100 GHz fixed	40–48	10G / 100G	0.4–4.8 Tb/s	legacy and simple metro
50 GHz fixed	80–96	10G / 100G / 200G	up to 19 Tb/s	mainstream
75 GHz	~64	400G (60 GBd)	25 Tb/s	400ZR/ZR+ channel width
Flexgrid (12.5 GHz slots)	variable	100G–800G+	30–40+ Tb/s	super-channels; requires flexgrid ROADMs
C + L band	×2	—	60–80 Tb/s	L-band amplifiers

Coherent transponders trade capacity for reach per wavelength — 400G 16QAM for metro, 300G 8QAM regional, 200G QPSK long haul on the same hardware — so the design picks a format per path (Coherent & long haul).

OTN in brief

Container	Rate	Typical client
ODU0 / ODU1 / ODU2 / ODU2e	1.25 / 2.5 / 10.04 / 10.4 Gb/s	GbE / STM-16 / STM-64 / 10GbE
ODU3 / ODU4	40 / 104 Gb/s	40GbE / 100GbE
ODUflex	n × 1.25 Gb/s	FC, arbitrary
OTU2 / OTU4	10.7 / 112 Gb/s	line frames with FEC
OTUCn / FlexO	n × 100G	200G–800G coherent line

OTN adds: FEC (GFEC RS(255,239) ≈ 6.2 dB gain; vendor SD-FEC ≈ 10–11 dB in coherent line cards), performance monitoring per section and path (BIP-8, tandem connection monitoring), multiplexing of many clients into one wavelength (muxponders), and protection (SNCP, ~50 ms). Clients see a transparent pipe; the OTN layer sees every bit error.

Where the pluggable sits

Architecture	Who terminates OTN / FEC	Optics	Trade-offs
Transponder / muxponder shelf	the DWDM vendor's line card	grey client optics + integrated or CFP2-DCO coherent line side	best reach and monitoring; two boxes, two vendors' management
Coherent pluggable in the router (400ZR / ZR+ in QSFP-DD/OSFP)	the module (CFEC/oFEC) with the router as host	one module, no client optics	fewer boxes and Watts; reach limited to metro/regional; monitoring via CMIS VDM; the line system must be open
Alien wavelength	any transponder on a third-party line system	as above	requires an open line system that accepts foreign channels and a power-management contract
Passive DWDM with fixed/tunable 10G SFP+	none (client FEC only)	tunable DWDM SFP+ direct into switches, passive mux, maybe one EDFA	cheapest for 10G × 40 over ≤ 80 km; no OTN monitoring (Tunable transceivers)

Protection and resilience

Mechanism	Layer	Switch time	Notes
OLP 1+1	optical	~50 ms	duplicate fibre route; halves capacity
OTN SNCP	OTN	~50 ms	per ODU; needs OTN termination
ROADM restoration	optical control plane	seconds–minutes	re-routes wavelengths after failures
Client-layer (LAG, IP FRR, FC multipath)	client	ms–s	often sufficient with pluggable coherent

Design decisions checklist

Traffic matrix and growth → channel count, grid, C or C+L.
Topology: point-to-point, ring, mesh → ROADM degrees per node.
Per-path OSNR and non-linear budget → format and reach class per wavelength.
Pluggable coherent vs transponders per site, and open line system requirements.
OTN: needed for sub-wavelength multiplexing, hard SLAs, tandem monitoring? Otherwise client-layer protection may suffice.
Monitoring: OSC, OCM, per-channel pre-FEC BER, alarm thresholds (Monitoring).
Commissioning baseline: per-channel power, OSNR, BER; store with module identities.

In CodingBox

Coherent and DWDM pluggables enter this network as modules with an identity, a channel setting and VDM/DDM. CodingBox reads and codes their identity (tunable maps, channel grids), and shows the module-side monitors that the line system cannot see — laser frequency error, Tx/Rx power, pre-FEC BER — on DDM and Check transceiver.