What cable access asks of an optical link
A cable operator's access network is a hybrid plant. Coaxial cable covers the last stretch to the home, fibre covers the run from the headend or hub out to the node, and the DOCSIS services customers buy ride on the coaxial section. Everything upstream of the node is an optical link, usually only a few kilometres long, and the count of those links grows every time the plant is rebuilt.
This guide covers the optics on those links, from hub to node and the fibre-to-the-home overlay beside them. Long-haul transport is covered on the coherent optics page and in the optics selection guides collection.
Why the optics list is rebuilt with the plant
Two changes drive the parts list. Node splitting is the first: serving fewer homes per node means more nodes, more fibre links and shorter spans, so cost moves from the span to the port, and link count and port price become the numbers to plan against rather than reach.
The second is the move to a distributed access architecture, where the digital content is converted at the node instead of the hub. The hub-to-node link then carries Ethernet rather than an analogue forward and return path, so the access inventory looks like an Ethernet inventory: SFP+ at 10G, SFP28 at 25G where a node serves more capacity, and single-fibre pairs where the duct is tight.
Recommended optics for a cable access network
| Module | Rate | Media / wavelength | Reach class | Where it usually lands |
|---|
| 1.25G or 2.5G SFP | 1.25G / 2.5G | SMF, 1310 nm or 1550 nm | Up to 40 km class | Legacy node and hub uplinks still in service |
| 10G SFP+ LR | 10G | SMF, 1310 nm | 10 km class | Hub to node on remote PHY links |
| 10G SFP+ ER | 10G | SMF, 1550 nm | 40 km class | Longer hub-to-node spans and small hub sites |
| 10G BiDi SFP+ single fibre | 10G | SMF, 1310 / 1550 nm pair | 10 to 40 km class | Where only one strand is spare in the duct |
| CWDM or DWDM SFP+ | 10G per channel | SMF, CWDM or DWDM grid | Span dependent | Several node links multiplexed onto one pair |
| 25G SFP28 | 25G | SMF, 1310 nm | 10 km class | Higher capacity links from hub to node |
| 100G QSFP28 CWDM4 or LR4 | 100G | SMF, CWDM or LAN-WDM | 2 to 10 km class | Hub and headend aggregation toward the core |
| GPON or XGS-PON OLT optics | 1.25G / 10G downstream | SMF, 1490 / 1550 nm | 20 km class | Fibre-to-the-home overlay beside the coax plant |
The reach column states the standard class behind each part, not a promise; the working limit is the loss measured on the route after splitters and patch panels. Rate families are listed on the 10G, 25G SFP28 and 100G QSFP28 pages, and the single-fibre parts on the BiDi transceivers page.
Distance, node splits and the strand budget
Access spans are short, so reach rarely stops a plan. The binding limits are the strand count and the split ratio: a fibre-deep rebuild adds a link for every node split, each needing working strands plus a spare, so the duct fills up long before distance matters. Count strands per node, and check the loss budget with the splitters in place, since a passive PON branch and a point-to-point link have very different budgets.
Where a node is served at 10G over two to five kilometres, an LR class part is usually enough; the ER class is for longer runs and sites far from the hub. Ordering reach the route does not need is an avoidable cost across hundreds of nodes.
Single fibre and wavelength plans when the duct is full
Two options cover a plant that cannot be re-cabled. A single-fibre pair sends one direction on 1310 nm and the other on 1550 nm over the same strand, halving the strand count, with the two ends ordered as a matched pair; single-direction parts will not come up. Where more links are needed than strands exist, CWDM or DWDM units put several wavelengths on one pair.
Wavelengths need a plan rather than a shopping list: write down the channel per node, keep the first channels free for growth, and match the mux grid to the optics ordered. Our WDM transceivers range covers both grids, and the broadband access configuration guide works through a mixed plant.
Temperature, enclosures and field conditions
Access optics sit where transport optics never do: strand-mounted nodes, pedestals, street cabinets and pole hardware. Wide-temperature grades are the norm there, and the module has to be readable over the link so power and temperature can be watched from the hub. Where one part number spans a conditioned hub and an outdoor enclosure, order the wide-temperature version for the whole plant.
Coding, monitoring and mixed hub platforms
Hub and headend equipment is rarely from one brand, so coding is specified per platform. Optics we supply for branded platforms are compatible parts coded for the platform you name, tested against the port, and not manufactured by the brand of the host. Platforms that appear in cable hubs include Cisco and Nokia.
How to choose, in order
- Count the links the rebuilt plant needs, per node and per hub.
- Measure each link type, including splitters and patch panels, and set the reach class from that.
- Count strands available per node and decide where single fibre or a mux is needed.
- Write the wavelength plan for the routes that share a pair.
- Set the temperature grade for every outdoor position.
- Name the platform and port rate for coding, and hold spares per link type.
Frequently asked questions
Do cable access links need coherent optics?
Not inside the access ring, where spans are short and grey or WDM parts cover the distance. Coherent comes in where hub or headend sites are linked over tens of kilometres.
How much reach do we need per node?
Measure it. Most node links fall in the 10 km class, with the 40 km class for the sites furthest from the hub. One class across the plant is usually cheaper than two.
Can we run 10G node links over one strand?
Yes, with a matched BiDi pair, one part per direction. Keep the pairing in the records, because a single-fibre link will not come up if both ends transmit in the same window.
When is a DWDM unit the right answer?
When the number of node links exceeds the strands available and re-cabling is not an option. Write the channel plan first, then order the optics and the mux to that plan.
What do you need to quote these parts?
The link type and rate, the measured distance and loss, the strand count available, the platform at each end and the temperature grade of the enclosure.
Send the link count, the measured routes, the strands available and the platform at each end, and we confirm the rate, the reach class, the wavelength plan and the coding for every link type. Related reading: the 25G and 50G PON selection guide for the fibre-to-the-home overlay, the GPON OLT and FTTH access range, and all optics selection guides.
