The pace of the development of network technology has never stopped, from Fast Ethernet to Gigabit Ethernet, 10G Ethernet, 40G Ethernet and 100G Ethernet. To keep up with the pace, many data centers has deployed 100G network. We know that there are many types of 100G transceivers, such as 100G CFP/CFP2/CFP4, CXP and 100G QSFP28. Recently, new 100G DWDM technology has been the choice for 100G network applications over long distance. There are two types of 100G DWDM transceivers—Coherent 100G DWDM transceiver and PAM-4 (Pulse Amplitude Modulation) 100G DWDM transceiver. How much do you know about them? This article is going to give an introduction to 100G DWDM transceiver.
Coherent 100G DWDM transceiver refers to coherent CFP transceiver. DWDM CFP transceivers can achieve long-distance data transmission in applications, when previously only SR4/LR4 options were available. It is an important feature enhancement to provide DWDM connectivity in a CFP transceiver for transporting 100G traffic over long distances. What’s more, coherent DWDM transceivers cost less than ER4 and ZR4 CFP transceivers. Now on the market, there are two types: CFP DCO (Digital Coherent Optic) and CFP2 ACO (Analog Coherent Optic).
CFP DCO has a high speed DSP (digital signal processing) chip built in which makes the coherent CFP a real workhorse. The DSP provides the electronic dispersion compensation and this eliminates the need of separate DCMs (dispersion compensation modules) of CFP. As a result, coherent CFP is able to support transmission distance of more than 1000km between sites.
The available CFP2 coherent DWDM transceivers are analog. Judging from the size, CFP2 ACO is half the width of CFP. Apart from this, another difference between CFP2 ACO and CFP is that CFP2 ACO does not has a DSP. In order to take the full advantages of the coherent features, CFP2 ACO requires a separate DSP on the host board. Though the DSP increases the power consumption and cost, it means that there is no need to add DSPs to the board.
PAM-4, adopted by the IEEE, is expected to be the most cost-effective and efficient enabler of 100G and beyond in the data center. PAM-4 makes more efficient use of electronic and optical components by packing two bits for every symbol sent over the fiber. Single-lambda 100G PAM-4 offers the simplest architecture, lowest component count, the most streamlined data path, higher reliability and an easy upgrade path to 400G Ethernet and beyond. And it enables the lowest-cost 100G transceiver. The 100G DWDM transceiver utilizes PAM-4 solution in QSFP28 form factor. For those who want to build an embedded DWDM network, they can use this transceiver directly in the switch. It is a very simple and cost-effective solution, but there is one tradeoff: it needs amplification to get out of the blocks and dispersion compensation to go beyond 5-6km. As a result, a separate DWDM multiplexer with an amplification system and dispersion compensation is required to connect data canters together. In addition, there is one thing to note: before the QSFP28 PAM-4 transceiver is added to an existing DWDM network, the network must prepare right DCM (dispersion compensation module) and amplification system in place.
Utilizing DWDM technology in the transceiver offers a simple and cost-effective solution. For coherent 100G DWDM transceiver, it is suitable for applications which needs longer reach; while for PAM-4 100G DWDM transceiver, it meets the needs of data center operators, such as Microsoft, that intend to turn up 4Tb/s of transmission capacity in a point-to-point fashion between data centers in a 70km metro-distributed network. Which one to choose depends on the specific deployment requirements.