120km vs 80km Gigabit SFP: Why TX Power Looks the Same
Why does a 120km Gigabit SFP ship with the same TX power as the 80km variant if it has to push the signal further? Reach is not a transmitter-only property. It is set by the optical link budget (TX power minus receiver sensitivity, minus fiber and connector losses), and the 120km module gains its extra range on the receiver side.
Many 120km modules use an APD (avalanche photodiode) receiver instead of the PIN photodiode common in 80km modules. The APD's internal gain detects signals 5 to 10 dB weaker than a PIN, adding ~8 dB to the usable link budget.
How optical link budget actually works
The optical link budget is the dB headroom between TX power and receiver sensitivity. With +5 dBm TX and -24 dBm sensitivity the raw budget is 29 dB. With the same +5 dBm TX and a more sensitive -32 dBm receiver, the budget is 37 dB. The APD buys that extra 8 dB.
Subtract fiber attenuation (roughly 0.20 to 0.25 dB/km at 1550 nm on ITU-T G.652 SMF) plus connector and splice losses. A 29 dB budget at 0.25 dB/km supports about 80 km after overhead. A 37 dB budget supports about 120 km, where EZX-class modules land.
Why APD photodiodes give 120km modules their range
A PIN photodiode converts each absorbed photon to one electron-hole pair (no internal gain). An APD applies a high reverse bias so each photon triggers an avalanche of secondary carriers, multiplying the signal inside the device. That gain pulls noise-equivalent input power down by 5 to 10 dB versus a PIN (per IEEE Xplore, 'Recent Advances in Telecommunications Avalanche Photodiodes'). Many 1000BASE-EZX modules pair an APD with a TIA preamplifier, which is why they detect near -32 dBm while an 80km PIN receiver tops out near -24 dBm.
The trade-off sits on the high-power end. APDs have a much lower maximum input power than PIN receivers, so a 120km module on a short span can saturate the APD receiver, causing link errors or loss of signal. The Cisco GLC-ZX-SMD (80km) calls for a 10 dB inline attenuator when fiber span loss is below 8 dB; the 120km EZX-class recommendation is at least 15 dB on short links.
Standards: what is and isn't IEEE 802.3
IEEE 802.3z (1998) defines 1000BASE-SX (850 nm MMF), 1000BASE-LX (1310 nm SMF up to 10 km), and 1000BASE-CX (copper). It does not define 1000BASE-ZX or 1000BASE-EZX. Both ZX (70 to 80 km, 1550 nm) and EZX (around 120 km, 1550 nm) are SFP MSA vendor extensions governed by the SFP MSA and SFF-8472, not by IEEE 802.3. Specs in this article come from vendor datasheets.
Spec example: Cisco GLC-ZX-SMD vs GLC-EZX-SMD
Cisco GLC-ZX-SMD (80km): TX power up to +5 dBm, receiver sensitivity less than -24 dBm, optical budget about 21 dB at minimum TX (per GLC-ZX-SMD specifications).
Cisco GLC-EZX-SMD (120km): TX power up to +5 dBm, receiver sensitivity less than -32 dBm, optical budget about 32 dB at minimum TX. TX power matches the 80km; the ~8 dB extra budget comes from the APD receiver.
Exact figures vary between vendors. Nokia's 1000BASE-ZX 80km specification, for example, lists receiver sensitivity at -22.5 dBm. Cross-reference the specific module's datasheet, and reserve 1 to 3 dB of end-of-life margin.
For deployment, EdgeOptic stocks 120km Gigabit SFP modules in the EZX class. To match a module to a specific span loss, attenuator requirement, or vendor compatibility table, contact our sales team.
Long-Reach Gigabit SFP FAQs
Why does a 120km Gigabit SFP have the same TX power as the 80km version if it needs to reach farther?
Reach is set by the optical link budget, which is transmit power minus receiver sensitivity. Both 80km and 120km 1G SFP modules typically share the same TX power range (roughly 0 to +5 dBm). The 120km variant gains its extra reach on the receiver side: many 120km modules use an APD (avalanche photodiode) receiver that detects optical signals around 8 dB weaker than the PIN photodiode common in 80km modules, which adds the same ~8 dB to the usable link budget without changing TX power.
What is the difference between APD and PIN photodiodes in an SFP receiver?
A PIN photodiode converts incoming photons to electrons one-for-one. An APD (avalanche photodiode) applies a high reverse bias so each absorbed photon triggers a short electron avalanche, multiplying the signal internally before it reaches the trans-impedance amplifier. That internal gain gives APD receivers roughly 5 to 10 dB better sensitivity than PIN receivers in fiber-optic systems (per IEEE Xplore, 'Recent Advances in Telecommunications Avalanche Photodiodes'). The trade-off: APDs cost more, run at higher bias voltages, and have a much lower maximum input power, which is why a 120km module on a short link needs an inline attenuator.
Will a 120km SFP work over a short link, or do I need an attenuator?
An inline attenuator is required when the fiber span loss is below the receiver overload threshold. The Cisco GLC-ZX-SMD (80km) datasheet specifies a 10 dB attenuator when fiber span loss is less than 8 dB. For the 120km GLC-EZX-SMD class, vendor product notes recommend at least a 15 dB attenuator on short links to protect the APD receiver from overload. The exact maximum input power varies by module, so confirm against the specific transceiver datasheet before deploying a long-reach module on a short span.
Is 1000BASE-ZX an IEEE 802.3 standard?
No. IEEE 802.3z (1998) formally defines 1000BASE-SX (850 nm multimode), 1000BASE-LX (1310 nm single-mode, up to 10 km), and 1000BASE-CX (copper). Both 1000BASE-ZX (70 to 80 km, 1550 nm) and 1000BASE-EZX (around 120 km, 1550 nm) are SFP MSA-compliant vendor-defined extensions, not IEEE standards. Optical specifications such as TX power, receiver sensitivity, and link budget come from individual vendor datasheets (for example, the Cisco GLC-ZX-SMD and GLC-EZX-SMD specifications), not from IEEE 802.3.