What a submarine or long haul span has in common
Terrestrial long haul and submarine links share one property: the distance is large enough that the optical signal has to be managed rather than simply transported. Both use coherent detection, both plan the link as a chain of spans with amplifiers, and both stand or fall on the loss budget of the buried or submerged plant rather than on the module alone.
The difference is the cost of change. A terrestrial span can be reworked in days, while a submarine segment may not be touched for years, so planning margin and the sparing strategy carry more weight than the price of the optics. This guide covers the module and line card side of those links. The transport layer is described on the OTN and DWDM transport platform page, and the rate families are on optics selection guides.
Why coherent detection is the only option
Direct detect links tolerate dispersion and attenuation and stop working when either grows. Coherent detection reads the phase as well as the amplitude of the light, so the receiver can compensate for dispersion and polarisation effects electrically instead of optically. That is what allows a single carrier to close over a chain of amplified spans, and it is why long routes are planned in coherent modes with strong error correction rather than in grey rates.
It also changes the question. The useful enquiry is not which module reaches furthest, but which mode list closes on the measured chain with margin. Reach on a long route is a computed property of the chain, because span loss, amplifier spacing, channel count and the mode in use all move the number.
Recommended optics and line cards for long spans
| Module or card | Rate | Media / wavelength | Reach class | Where it usually lands |
|---|
| 100G coherent line module | 100G | SMF, C band tunable | Metro to long haul classes | Routes where 100G still fills the fibre |
| 400G coherent line module | 400G | SMF, C band tunable | Regional to long haul classes | Regional transport and long span data center links |
| 800G coherent line module | 800G | SMF, C band tunable | Metro to regional classes | New capacity on an existing line system |
| 400G ZR and ZR+ pluggables | 400G | SMF, C band tunable | Metro to regional classes | Router ports where no shelf is wanted |
| Transponder and muxponder cards | 100G to 800G | Client side per service | Per card and mode | Mapping grey clients into long span wavelengths |
| C band and extended band line systems | Per channel | SMF, C band and beyond | Per span class | Amplified chains with many channels |
Reach is a computed property of the chain rather than a figure on a data sheet. The coherent modules themselves are compared on coherent optics and the card route on 800G DCO transponder cards.
Repeatered and unrepeatered links
A repeatered link has amplifier sites along it, which makes gain, gain flatness and loss per span the working variables. An unrepeatered link has no site in the water or in the duct, so everything is spent at the terminals: higher launch power, remote amplification and a stronger code. Unrepeatered routes are usually shorter and cost more per kilometre at the terminals, and in exchange they avoid the cost and the risk of a submerged site. Settle early where the amplifier sites are, because that single fact decides the mode list and the launch power targets.
Channel plan and capacity growth
Long routes are planned in the C band first because standard amplifiers work there, and above it when the C band is full and the fibre cannot be changed. Adding capacity that way is a terminal and amplifier change rather than a cable change, which is why it is the usual way to grow a route that cannot be re-cabled. Channel plans belong in the request: the grid, the occupied channels, the target channel, and the number of channels the line can carry now and later. Line systems and passive modules for those plans are described on the DWDM line system expansion and DWDM mux and demux module pages.
Protection, sparing and the cost of a site visit
Where access is expensive, sparing changes shape. A pluggable or a card can be replaced by a technician in minutes, while an amplifier in a submerged housing cannot. That is why long routes hold spares at the terminals by link family, and why a tunable part is preferred over a fixed channel module: one tunable spare covers every channel in the band. The terminal record should say which mode and channel each working part was set to, because a spare that needs a plan change is a spare in name only.
Coding and third party terminals
Where a module sits in a router or a switch cage it is coded for the host named on the request, tested on that model, and it is not manufactured by that brand. Parts for the common hosts are collected on the Cisco, Nokia and Juniper pages. Line cards in a transport shelf follow the compatibility list of that shelf instead.
How to choose, in order
- Get the span list with measured losses, amplifier positions and the number of spans.
- Fix the client services, then the line rate and the mode list that has to close.
- Decide repeatered or unrepeatered per route, and set the launch power target.
- Set the grid, the occupied channels and the target channel from the line system.
- Choose the module or card per terminal from the host and the space.
- Hold one tunable spare per link family, and record the mode and channel of every working part.
Frequently asked questions
Is submarine equipment different from terrestrial long haul equipment?
The modules and cards belong to the same families. What differs is the planning margin, the sparing plan and the fact that a submerged amplifier cannot simply be replaced in service.
Can a long route be built without coherent optics?
Not at these distances. Direct detect rates are used at the terminals for short client connections and stop well short of an amplified chain.
Which band should be used first?
The C band, because standard amplifiers work there. Move above it when the C band is full and the fibre itself cannot be changed.
How should spares be planned for a long route?
One tunable part per link family at each terminal, with the same mode list as the working part, and the mode and channel of each unit recorded with the terminal.
What do you need to quote a long haul or submarine project?
The span list with measured losses and amplifier positions, the client service list, the line rate and mode list, the grid and channel plan, the host platform at each terminal, the site conditions and temperature, and the quantity per part with spares on a separate line.
Send the span list with measured losses, the amplifier positions and the client service list, and we come back with the mode that closes the chain, the part per terminal and the spare lines set out separately. Related reading: long haul coherent 400G and 800G, DWDM metro transport and all optics selection guides.
