TECHNICAL
modules require space and enough power; therefore, they are only available on CFP and CFP2 pluggable form factors. The PAM4 modulation uses multiple levels of pulse-amplitude enabling to carrying 2 bits per symbols, doubling the bitrate:
A CFP2 DCO version has been released in 2017 as technology has been mature enough to include the DSP into a CFP2 form factor. The Coherent pluggable transceivers are only available in CFP and CFP2 form factors. There is no availability in CFP4 and QSFP28 form factors because there is not enough power supplied to the components, neither enough space. CFP2 Form Factor DCO (Digital Coherent Optics) - DSP included in transceiver n CFP2 DCO 100G (up to 300km without amplification) n CFP2 DCO 100G DWDM (up to 800km with amplification). n CFP2 DCO 100G DWDM (up to 2500km with amplification). ACO (Analogue Coherent Optics) – DSP not included in transceiver but on host-board n CFP2 ACO 100G DWDM (reach depends of EDC embed in host- board)
DWDM wavelengths, the main advantage is the compatibility with 100Ghz DWDM single channel enabling direct upgrade from 10G DWDM 100Ghz to 100G DWDM 100GHz.
Coherent vs PAM4
In today’s market, it is possible to obtain 100G optical transceivers with either PAM-4 or Coherent modulation – Figure 3 opposite shows the reach available with each type. The PAM-4 solution covers a portion of the optical reach needed to interconnect data centres. Below 10km, IEEE 802.3ba 100G pluggable optics are readily available with 100GBASE-LR4 supporting 10km reach in a QSFP28 package. The 100GBASE-ER4 specification for 40km reach has been more challenging for optics suppliers to deliver and remains either in larger packages (example, CFP/ CFP2 with SOA or QSFP28 with FEC on host board) or in nonstandard formats, meaning non-interoperable across vendors. So where does the PAM-4 technology fit? In general, its initial fit appears to be in the <40km range as an alternative to existing, pluggable solutions. The solution also plays in the 40–80km range as an alternative to optical DCI/ coherent DWDM solutions for some deployment scenarios. Based solely upon reach, a logical question is how much of the optical DCI/coherent DWDM market is covered by 40–80km? ACG Research recently completed a worldwide survey of data centre service providers, including network service providers, cloud service providers, Internet content providers and Internet eXchange providers. One of the questions asked of the service providers was the proportion of optical reach needed to cover their data centre interconnections today and in 2019. What was found is that service providers on
The main advantage of PAM4 is the easiness to use regular electronics and optical components suitable for small form factor such as QSFP28 form factor. The power consumption is drastically reduced and can be used for data centres interconnect application. The main disadvantage is that PAM4 requires amplification and dispersion compensation system on the optical link for reach longer than 5km at 100Gbits.
The PAM4 technology is today available on QSFP28 pluggable form factors:
QSFP28 Form Factor QSFP28 PAM4 80km DWDM 100Ghz “Super Channel” 4.5W (require ampliier).
PAM4
The QSFP28 PAM4 80km DWDM 100GHz “super-channel” is a combination of two 50GHz carriers also called “dual- carrier”. While it is using 2x 50GHz
The IM-DD technology is based on NRZ encoding with 1 bit per symbol. This technology is today largely used but has some limitations and increasing the output bitrates start to be challenging on fibre mainly due to dispersion (chromatic and polarisation). The Coherent technology has adopted more advances modulations such as QPSK, 8QAM or 16QAM enabling to double, triple or quadruple the bitrates. However, this modulation requires an advanced technology which is coherent detection on receiver side and a DSP to correct the dispersion. While the cost per bit remains interesting, the components costs are high, and the pluggable
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Volume 47 No.1 MARCH 2025
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