Below you will find a summary of the differences for 100G ER4 transceiver types. There are 3 different MSAs, or standards, targeting transmission distances of up to 40 km:
100G ER4 supports both 100GE and OTU4. In this approach the extended transmission distance is achieved by the use of a Semiconductor Optical Amplifier (SOA) amplifying the light signals before entering the PIN photo detector. The below diagram shows the key building blocks. The line rate on the transmit side either 25.78 Gbps (100GE) or 27.95 Gbps (OTU4). The optical transmitters are operating at the so called Lan WDM wavelengths (1295, 1300, 1305 and 1310nm). A Multiplexer (MUX) is used to combine the wavelength signals onto a single fiber. On the receiver side, the SOA amplifies the signals before a demultiplexer (DEMUX) splits the signals into individual channels. The PIN receivers are used to convert the channels back to electrical signals.
As the power consumption and cooling requirement of this approach is relatively high the only form factors supporting it are CFP and CFP2.
In 100GE operation there is no Forward Error Correction used, but in OTU4 operation FEC can be present. The FEC is implemented in the host equipment. The ER4 transceiver removed the need for external SOA amplifiers in 100G LR4 transmission links.
100G ER4 Lite
In the 100G ER Lite approach both 100GE and OTU4 are supported. Instead of the SOA, 25G APD receivers are used to extend the transmission distance. The APD receivers have a saturation and damage threshold and therefore 100% compliance to the ER4 standard cannot be achieved. However, the lower power consumption enables the QSFP28 form factors. Also the 100G ER4 Lite approach is defining 100GE performance without FEC and OTU4 performance with FEC. The illustration below shows the basic components
As switch ASICs improve KR4 FEC was introduced. KR4 FEC was initially used for copper and multimode interfaces, but also gained quick acceptance in singlemode applications.
The 4WDM MSA group adopted the advantages from the KR4 FEC to reduce the optical requirements on the lasers and receivers. The 4WDM-40 MSA defines only applications with 100G KR4 FEC implementation. The basic architecture of the transceivers is identical to the 100G ER4 Lite approach.
Summary
The below table shows typically supported form factors and applications for the different approaches.
The above described approaches offer now suitable solutions for covering distances up to 40km. However, the different implementations of 100G Ethernet (with or with FEC) needs to be considered when designing networks with different form-factors.
