What metro DWDM transport has to solve
Metro transport is the layer between access sites and the long haul: a few central offices, a ring or two, and spans running from a few kilometres inside a city to tens of kilometres between towns. Traffic arrives as 10G, 25G, 100G and increasingly 400G ports, and the fibre available is a small fraction of the port count, which is why wavelengths rather than strands carry the load.
Two routes are open. Grey optics with transponders put each client port onto a wavelength in a dedicated transport shelf, and coherent pluggables put the wavelength straight into the router or switch port. The choice is settled by span length, channel growth and how much equipment the site can hold. This guide covers the decision points in order. The columns behind it are OTN and DWDM transport and WDM transceivers, with the full range on optical communication products.
What the span and the site dictate
The first number that matters is the loss on the span, measured through the patch field, not the distance on the drawing. Passive mux pairs add loss, splices and connectors add more, and a footpath route with many joints behaves nothing like a straight line of the same length.
The second is whether the site can host amplification. An unamplified link lives inside a few tens of kilometres; with an amplifier the same fibre carries many channels over much longer spans. The third is the channel count and the growth path, because a 40 channel mux chosen for today's six links is what stops the next four from being a new cable job. Protection is the fourth: a ring that has to survive a single fibre cut needs two directions and a switching rule, and that rule has to be in the plan before the modules are ordered.
Recommended modules for metro DWDM transport
| Module | Rate | Media / wavelength | Reach class | Where it usually lands |
|---|
| 10G SFP+ DWDM, fixed channel | 10G | SMF, 100 GHz or 50 GHz grid | 40 to 80 km class | Legacy client ports onto a mux pair |
| 25G SFP28 DWDM | 25G | SMF, DWDM grid | 10 to 40 km class | Fronthaul and mid-haul onto the same fibre |
| 100G QSFP28 coherent or DWDM | 100G | SMF, fixed channel or tunable | 80 km to metro DCI class | Aggregation links between metro sites |
| 400G QSFP-DD ZR and ZR+ | 400G | SMF, DWDM C-band tunable | OpenZR+ class upward | Router ports carrying wavelengths directly |
| 800G QSFP-DD or OSFP ZR class | 800G | SMF, DWDM C-band tunable | Metro and DCI class | New high rate metro and AI backhaul links |
| DCO transponder and muxponder cards | 100G to 800G | SMF, DWDM C-band tunable | Per card class | Sites that keep client optics and transport separate |
| Passive mux, demux and OADM modules | Per channel | SMF, 40 or 80 channel plans | Site dependent | Adding and dropping wavelengths at a node |
Reach classes are the standard ones for each family and the working limit is the measured channel budget. The transponder and muxponder side is collected on 400G transponder and muxponder cards and 200G and 10G OTN cards, with the coherent pluggable option covered by the 400G ZR DCI selection guide.
Fixed wavelength parts or coherent tunable parts
A fixed channel module is simple: the wavelength is set at purchase and the part can only be moved by moving the channel plan with it. Coherent tunable parts carry their own channel selection, which is the point of them, but they also expect the far end to be the same class of part.
The dividing line is operational. A ring that carries the same wavelengths for years and grows by whole mux units suits fixed parts well. A network that re-plans channels, adds a site or runs out of a specific wavelength on a long span suits tunable parts, because a spare covers every channel instead of one.
Mux, add drop and amplification in the metro node
At a node with no traffic to terminate, an optical add drop module lets a wavelength pass or turn, and a ring stays simple when the channel plan keeps add drop points off the common paths. Where channels are added and dropped often, the node grows an add drop arrangement instead of a plain mux pair.
Launch power per channel belongs in the same planning conversation. Forty channels at the wrong power saturate an amplifier or leave the far receiver short, and per channel monitoring turns that from an argument into a reading from the first wavelength, not the fortieth.
Coherent pluggables against a transport shelf
A coherent pluggable removes the transponder from the path, which saves space, power and a pair of patch cords at every site. It also removes the boundary between the router and the transport layer, so the wavelength is now the router team's problem, and the optical budget has to be visible to them.
A transport shelf keeps that boundary sharp and handles clients that cannot host a pluggable, which is common on older or leased equipment. Mixed estates usually end up with both, and the rule that works is to put the wavelength where the client port is.
How to choose, in order
- Measure the span and record the loss including mux pairs and patch panels.
- Set the channel plan for the growth case, not only for the first links.
- Decide fixed or tunable per route, and count the spare rule from that decision.
- Choose where the client port can host a pluggable and where a transponder card is needed.
- Fix the protection rule and the patch record before the first wavelength is turned up.
Frequently asked questions
Can a coherent pluggable replace a transponder entirely?
Where the client port has a cage at the right rate, yes. Where there is no such cage, or the router and transport layers are kept apart on purpose, a transponder or muxponder card is the route.
How many channels should a metro mux pair carry?
Size it for the growth case. A 40 channel plan half full after two years is normal, and moving to a wider plan later means replacing the passive layer.
Does metro transport need amplification?
Only when span loss and channel count ask for it. Short city spans with a few channels often run unamplified; long spans and full channel loads need amplifiers.
How are wavelengths protected on a ring?
By carrying traffic in both directions and switching on an agreed rule: fibre, channel or equipment protection. The rule decides how many parts each node holds, so it belongs early in the plan.
What do you need to quote a metro DWDM build?
The site list with span losses, the channel plan and growth, the client rates per site, the protection rule, whether amplification exists, and the quantity per part with spares as separate lines.
Send the site list with span losses, the channel plan and the client rates, and we come back with a part per position, the reach class, the passive layer and the spare lines set out separately from the working wavelengths. Related reading: metro optical networks, DWDM network expansion with mux, amplifier and monitoring, and all optics selection guides.
