OTU4 (Optical Transport Unit level 4) is the 100G optical transport encapsulation standard defined by ITU-T G.709 for Optical Transport Networks (OTN). Unlike standard 100GbE optical transceivers, OTU4 modules support both Ethernet services and carrier-grade transport applications while providing enhanced reliability, monitoring, and network management.
Typical deployment scenarios include:
OTN carrier transport networks
SDH/SONET transport
5G fronthaul, midhaul, and backhaul
Data Center Interconnect (DCI)
Metro aggregation networks
Backbone optical transmission
Most OTU4 QSFP28 optical transceivers support Dual Rate operation, allowing seamless switching between 100GbE (103.125 Gb/s) and OTU4 line rate (111.809 Gb/s).
| Specification | OTU4 ER4 | OTU4 ZR4 |
|---|---|---|
| Form Factor | QSFP28 | QSFP28 |
| Protocol | OTU4 + 100GbE | OTU4 + 100GbE |
| Standard Reach | 40 km | 80–90 km |
| Optical Interface | Duplex LC | Duplex LC |
| Wavelength | 4 × LAN-WDM | 4 × LAN-WDM |
| Fiber Type | G.652 SMF | G.652 SMF |
| Typical Applications | Metro Networks, 5G Transport | Long-Haul DCI, Backbone Networks |
ER4 is designed for medium-distance transmission, while ZR4 provides significantly longer reach through higher transmit power, greater receiver sensitivity, and a larger optical link budget.
| Parameter | CLOTU4QSFPER4 | CLOTU4QSFPZR4 |
|---|---|---|
| Ethernet Rate | 103.125 Gb/s | 103.125 Gb/s |
| OTU4 Line Rate | 111.809 Gb/s | 111.809 Gb/s |
| OTU4 Lane Rate | ≈28.05 Gb/s ×4 | ≈28.05 Gb/s ×4 |
| Optical Lanes | 4 | 4 |
| Modulation | NRZ | NRZ |
| Wavelength | 1295–1310 nm | 1295–1310 nm |
| Transmitter | EML | EML |
| Receiver | APD | SOA + PIN |
| Connector | Duplex LC | Duplex LC |
| Power Consumption | 4.5–5 W | 4.5–5 W |
| Operating Temperature | Commercial or Industrial | Commercial or Industrial |
The OTU4 line rate is higher than standard Ethernet because G.709 adds OTN overhead bytes and Forward Error Correction (FEC), significantly improving transmission reliability.
| Optical Specification | CLOTU4QSFPER4 | CLOTU4QSFPZR4 |
|---|---|---|
| Maximum Reach | 40 km | 80–90 km |
| Transmit Power | 0 to +5 dBm | +2 to +6 dBm |
| Receiver Sensitivity | -20 dBm @ BER 5E-5 | -28 dBm @ BER 5E-5 |
| Optical Link Budget | ~20 dB | ~30 dB |
| Fiber Loss Tolerance | Medium | High |
| Dispersion Tolerance | Medium | Medium |
With a larger link budget and higher receiver sensitivity, ZR4 supports longer metro and regional optical links than ER4.
Bit Error Rate (BER) is one of the most critical performance indicators in carrier optical networks.
| BER Performance | CLOTU4QSFPER4 | CLOTU4QSFPZR4 |
|---|---|---|
| Pre-FEC BER @28.05Gb/s/Lane | ≤1×10⁻¹² @30 km | ≤1×10⁻¹⁰ @80 km |
| Post-FEC BER @28.05Gb/s/Lane | ≤1×10⁻¹⁵ @40 km | ≤1×10⁻¹⁵ @80 km |
| Pre-FEC BER @25.78Gb/s/Lane | ≤1×10⁻¹² @40 km | ≤1×10⁻¹² @80 km |
| Post-FEC BER @25.78Gb/s/Lane | ≤1×10⁻¹⁵ @40 km | ≤1×10⁻¹⁵ @80 km |
| Host FEC | Optional | Required |
| Zero BER | 30 km @28.05 Gb/s; 40 km @25.78 Gb/s | 80 km after FEC @28.05 Gb/s; 80 km @25.78 Gb/s |
ER4 typically achieves excellent BER performance within its rated distance. To support 80–90 km transmission, ZR4 relies on higher receiver sensitivity and host-side KP4 FEC to correct the higher raw BER and deliver carrier-grade link stability.
| Feature | 100GbE | OTU4 |
|---|---|---|
| Standard | IEEE 802.3 | ITU-T G.709 |
| Target Network | Enterprise & Data Center | Carrier OTN |
| Transmission Capability | Ethernet Only | Carrier-Grade Transport |
| Multi-Service Support | Limited | Excellent |
| OAM & Performance Monitoring | Basic | Comprehensive |
Compared with standard Ethernet, OTU4 provides richer overhead bytes, OAM functions, alarms, performance monitoring, and maintenance capabilities required for telecom transport networks.
OTU4 delivers advantages beyond transmission distance by offering superior reliability, manageability, and service flexibility.
OTU4 incorporates G.709 FEC, significantly improving transmission quality over long-distance optical links.
Combined with ER4 or ZR4 optics, OTU4 provides excellent performance for metro networks, regional DCI, backbone infrastructure, and 5G transport networks.
Designed for continuous 24×7 operation, OTU4 meets the stringent availability requirements of carrier backbone networks and mission-critical services.
In addition to 100GbE, OTU4 can encapsulate multiple client services, enabling a unified transport platform across telecom infrastructures.
C-LIGHT provides a complete portfolio of carrier-grade OTU4 optical transceivers, including:
OTU4 100G QSFP28 ER4 (40 km)
OTU4 100G QSFP28 ZR4 (80–90 km)
OTU4 100G QSFP28 XZR4 (100 km)
Dual-rate support for 100GbE and OTU4
Fully compliant with ITU-T G.709
Four-channel LAN-WDM architecture
High-reliability EML laser technology
High-sensitivity receiver design
Low power consumption
Commercial and industrial temperature options
100% factory optical performance testing
Compatibility with major carrier OTN platforms and network equipment
For enterprise data centers, 100GbE ER4 is typically sufficient for links up to 40 km. However, carrier transport networks require higher reliability, comprehensive monitoring, and enhanced transmission capabilities.
OTU4 addresses these requirements through the G.709 standard, integrated OTN FEC, carrier-grade OAM, extensive performance monitoring, and superior long-distance transmission. As a result, OTU4 has become the preferred solution for 5G transport networks, metro networks, backbone infrastructures, and long-distance DCI applications.
When transmission distances exceed 40 km, the CLOTU4QSFPZR4 extends reach to 80–90 km, delivering stable carrier-grade performance when deployed with host-side KP4 FEC.
Answer: OTU4 follows the ITU-T G.709 standard and supports OTN encapsulation, Forward Error Correction (FEC), OAM, and advanced performance monitoring, making it ideal for carrier transport networks. Standard 100GbE follows IEEE 802.3 and is primarily designed for enterprise and data center Ethernet networks.
Answer: ER4 is recommended for optical links up to 40 km. For metro interconnects or regional DCI spanning 40–80/90 km, ZR4 is the better choice, provided sufficient optical link budget and host-side FEC are available.
Answer: Yes. Many carrier-grade QSFP28 OTU4 transceivers support dual-rate operation, allowing users to switch between 100GbE and OTU4 modes. Actual compatibility depends on the switch, transport equipment, and module specifications.
Answer: To achieve transmission distances of up to 80–90 km, 100G ZR4 typically relies on host-side KP4 FEC to improve BER performance and maintain reliable long-distance optical links.
Answer: OTU4 is ideal for OTN transport networks, carrier backbone infrastructure, metro optical networks, regional DCI, 5G transport networks, and other long-distance optical communication systems requiring carrier-grade reliability and performance.
