The jump from 80KM to 120KM is not a firmware label change. Three physical parameters shift in ways that matter operationally.
A typical 80KM DWDM SFP launches at around +3 to +5 dBm. At 120KM you need more headroom to survive the additional fiber attenuation, so 120KM modules commonly launch at +7 to +10 dBm. That higher output demands a better laser driver circuit and tighter thermal management inside the module. It also raises the risk of nonlinear effects in the fiber when running a dense DWDM comb, which is why per-channel power discipline becomes important at this reach.
More fiber means more attenuation, which means the signal arriving at the far end is weaker. A 120KM DWDM SFP typically specifies receiver sensitivity of -38 dBm or better, compared to roughly -34 dBm for many 80KM variants. Four decibels sounds modest, but it is often the difference between a link that holds margin through a bad splice and one that does not.
Chromatic dispersion accumulates at roughly 17 ps/(nm·km) in standard SMF-28 fiber. Over 120KM, that gives you approximately 2,040 ps/nm of accumulated dispersion. At 1.25G (SFP), that is manageable without external compensation. At 10G (SFP+), it is not — you will need dispersion compensation modules (DCMs) or built-in electronic dispersion compensation (EDC), depending on the module design. The 120KM tier forces this conversation in a way that 80KM sometimes lets you sidestep.
| Parameter | 80KM DWDM SFP+ | 120KM DWDM SFP+ |
|---|---|---|
| Typical launch power | +3 to +5 dBm | +7 to +10 dBm |
| Receiver sensitivity | -34 dBm | -38 dBm or better |
| Dispersion at 10G (SMF) | ~1,360 ps/nm | ~2,040 ps/nm |
| DCM typically required at 10G | Sometimes | Yes |
| EDFA typically required | Optional on clean fiber | Usually required |
| Tunable options available | Yes | Yes |
| Typical fiber budget | 22–24 dB | 30–33 dB |
A link budget is a running tally of optical power from transmitter to receiver. Here is a realistic example for a 120KM span on standard SMF-28.
Starting point: transmit power
Assume +8 dBm launch power from a 120KM DWDM SFP+.
Fiber attenuation
SMF-28 attenuates at approximately 0.20 dB/km in the C-band. Over 120KM:
120 × 0.20 = 24.0 dB
Connector and splice losses
A realistic field installation will include:
Call it 3.0 dB for connectors and splices combined on a well-maintained route.
Total span loss (no amplification)
24.0 + 3.0 = 27.0 dB
Received power (no amplification)
+8 dBm − 27.0 dB = -19 dBm
Receiver sensitivity: -38 dBm
System margin: -19 − (-38) = +19 dB
That looks comfortable on paper. In practice, real networks have aging fiber, additional splices from repairs, and DWDM multiplexer/demultiplexer insertion loss — typically 3–5 dB per device. Add a mux/demux pair and that margin shrinks to roughly +9 to +13 dB. Still workable, but no longer comfortable if the fiber plant has any degradation.
On routes with higher-attenuation fiber, more connectors, or passive ROADM stages in the path, EDFA shifts from a nice-to-have to a requirement.
An EDFA (Erbium-Doped Fiber Amplifier) boosts optical power mid-span without converting the signal back to electrical form. At 120KM, whether you need one depends on your actual link loss.
EDFA is optional when:
EDFA becomes required when:
A single inline EDFA with 20–23 dB of gain placed at the midpoint of a 120KM span effectively splits the problem into two 60KM segments, each carrying roughly 12 dB of fiber loss. That restores comfortable margin even on older fiber plants.
One practical note: EDFA introduces noise in the form of amplified spontaneous emission (ASE). At 120KM with a single amplifier, this is manageable. Stack multiple EDFAs for longer spans and optical signal-to-noise ratio (OSNR) becomes the binding constraint rather than raw power.
At 10G line rates, 2,040 ps/nm of accumulated dispersion over 120KM of SMF-28 will cause severe inter-symbol interference without compensation. Your options are:
Dispersion Compensation Modules (DCMs): Passive fiber coils with negative dispersion that cancel the accumulated positive dispersion from the transmission fiber. A DCM rated for 120KM of SMF-28 adds roughly 3–5 dB of insertion loss, which needs to be factored into your link budget.
Electronic Dispersion Compensation (EDC): Some 120KM DWDM SFP+ modules include built-in EDC in the receiver IC, handling dispersion electronically without external hardware. This simplifies deployment but has limits — typically up to 2,000–2,400 ps/nm depending on the design.
Coherent DSP: At 100G and above, coherent optics handle dispersion natively. At the 10G SFP+ tier, you are working with direct-detect optics and need either DCM or EDC.
At 1.25G (SFP form factor), dispersion tolerance is wide enough that 120KM is achievable on SMF-28 without external compensation. That is one reason 1.25G DWDM SFP modules remain popular for telecom access and metro aggregation links.
Fixed DWDM SFP+ modules are locked to a single ITU-T C-band channel — typically on a 100 GHz or 50 GHz grid. They cost less per unit, are simpler to qualify, and work well on static routes where the wavelength plan does not change.
At 120KM, fixed modules are the right call when you have a defined wavelength plan, a small number of channels, and no expectation of reprovisioning the module to a different route.
Tunable modules cover the full C-band (or a significant portion of it) and can be set to any ITU grid channel via software or hardware control. At 120KM, they offer real operational advantages:
The tradeoff is cost. Tunable DWDM SFP+ modules at 120KM carry a meaningful price premium over fixed-wavelength equivalents. For large deployments with a stable wavelength plan, fixed modules often win on total cost of ownership.
HYTOPTODEVICE offers a DWDM catalog spanning from standard metro reaches up to 160KM, covering both fixed-wavelength and tunable DWDM SFP+ options alongside CWDM variants for shorter-reach applications. For 120KM deployments specifically, the catalog includes modules with the launch power and receiver sensitivity needed to meet the link budget requirements described above.
When evaluating any 120KM DWDM SFP+ module, confirm these specifications before ordering:
Q1: Can a standard 80KM DWDM SFP+ reach 120KM if the fiber is very clean?
A: Unlikely without amplification. Even on excellent fiber at 0.19 dB/km, 120KM produces roughly 22.8 dB of fiber loss before adding connector, splice, and mux/demux losses. An 80KM module with +5 dBm launch and -34 dBm sensitivity has a theoretical fiber budget of about 39 dB, but real-world passive component losses consume much of that margin quickly. A module specified for 120KM is the reliable choice.
Q2: Do I always need an EDFA for a 120KM DWDM link?
A: Not always. On a clean fiber plant with low passive component losses and a 120KM-rated module, the link budget can close without amplification. EDFA becomes necessary when total span loss — including mux/demux and any ROADM stages — exceeds roughly 28–30 dB, or when you need a specific system margin to hit availability targets.
Q3: What dispersion compensation do I need for a 120KM 10G DWDM SFP+ link?
A: Standard SMF-28 accumulates approximately 2,040 ps/nm over 120KM at 10G. You will need either an external DCM rated for that dispersion value or a module with built-in electronic dispersion compensation. Check the module datasheet for its stated dispersion tolerance before assuming EDC alone is sufficient.
Q4: What is the difference between 100 GHz and 50 GHz grid DWDM SFP modules at 120KM?
A: Grid spacing affects how many channels you can fit in the C-band, not reach directly. 50 GHz spacing doubles channel density compared to 100 GHz. At 120KM, 50 GHz is achievable but requires tighter wavelength accuracy from the laser and more careful OSNR management when using EDFA, since channels sit closer together.
Q5: Are tunable DWDM SFP+ modules compatible with all DWDM mux/demux equipment?
A: Tunable modules output on standard ITU C-band wavelengths, so they are compatible with any mux/demux designed for the same grid. The tuning mechanism — typically accessed via the module's management interface or a physical dial — lets you select the channel. Confirm that your mux/demux supports the specific channel plan you intend to use.
Q6: How does a 120KM DWDM SFP fit into a network that may later scale to 160KM?
A: If future reach extension is likely, it is worth evaluating whether to deploy 160KM-capable modules from the start or plan for EDFA placement at a midpoint site. HYTOPTODEVICE offers modules rated to 160KM, which gives you headroom if the route grows or fiber plant quality degrades over time — without requiring a full module swap.
The 120KM DWDM SFP tier rewards careful engineering. Work through your actual link budget using measured fiber loss data, account for every passive component in the path, and settle the EDFA and dispersion compensation questions before you order hardware. Fixed-wavelength modules keep costs down on stable routes; tunable modules earn their premium on dynamic or multi-route deployments.
For module specifications, OEM-compatible options, and bulk pricing across the full DWDM reach range, visit hytoptodevice.com.
