In this article we will focus on the various form factors and standards for optical transceivers. They are often identified by a) the packaging and b) the optics. This information is typically used in the naming convention of these devices. The first set of characters identifies the packaging type and the second set identifies the data rate. The packaging is important to determine what devices (such as switches, routers, Network Interface Cards, Open Line Systems) can support the transceiver. The optics indicate the data rate and/or the link reach that can be achieved.
There are many types of optical transceivers at data rates of less than 100G. The important form factors for Dense Wavelength Division Multiplexing (DWDM) at 100G or above are listed below. QSFP (Quad Small Form Factor Pluggable), QSPF+, and QSFP 28 transceivers are included for historical context and some background to help understand the evolution of DWDM pluggable optics. Only the following pluggable optics support coherent detection:
Transceivers Supporting Direct Detection:
This is defined by an MSA (Multi Source Agreement) under the direction of the Small Form Factor Committee. It is the most popular pluggable for DWDM at 100G. All transceivers, including the QSFP group, consist of the plug, a cage for electromagnetic interference resistance, a guiding plug, and a board mounted electrical connector. Quad indicates that it supports 4 bidirectional optical channels. This increases the bandwidth available on the link.
This form factor was developed to support 4 x 10Gb/s data rates. There is no difference in size of the QSFP and the QSFP+. It is gradually replacing the QSFP.
These transceivers support 4 channels with each running at 28G and creating a 100G link. It has the same form factor as the QSFP+ transceivers and is used for InfiniBand EDR (Enhanced Data Rate) 100G and 100G Ethernet ports.
They share the same form factor as other QSFP modules. Each of the 4 channels is capable of 50G data rate and therefore the QSFP56 is used for 200G applications, and for InfiniBand HDR (High Data Rate) 200G and 200G Ethernet ports.
DD in the name indicates adouble-density QSFP transceiver capable of supporting 200G and 400G Ethernet. It uses 8 lanes at 28G NRZ (none-return to zero) modulation for 200G, or 8 lanes of 50G PAM4 (pulse amplitude modulation) for 400G data rates. It is the smallest 400G module and enables 36 ports of 400G to be in a single Rack Unit (RU). The standard specifies 10Km link distances. QSFP and QSFP-DD may be used in the same ports. QSFP-DD will typically draw more power so the overall power envelope must be considered when using these pluggable optics.
DWDM QSFP28 PAM4
PAM4 uses Pulse Amplitude Modulation for various distances and band rates on DWDM networks. It applies the QSFP28 format for direct detection optics, not coherent optics and is the result of a multi-source agreement. These DWDM transceivers are available at up to 100G data rates. The reach is extendable up to 80km. However, direct-detect technology is sensitive to dispersion related effects and always requires accurate amplification and dispersion compensation equipment to reach distances typically required for data center interconnect and other DWDM optical link applications. The benefit of PAM4 is low power and longer reach.
Transceivers Supporting Coherent Detection:
CFP2- ACO and CFP2-DCO
These are the first coherent pluggable transceivers on this list. The abbreviation stands for Analog Coherent Optic (ACO) and a Digital Coherent Optic (DCO). In the ACO module, the Digital Signal Processor (DSP) is placed on the host line card. The CFP2-ACO module passes an analog signal to the DSP which is located on the host line card. Therefore, the CFP2-ACO may only be used in a system that has the matching DSP on the host line card. In a CFP2-DCO the DSP is located within the optical transceiver itself. Having the DSP within the CFP2-DCO transceiver provides more flexibility because the module can be used with any line card.
The DSP gives coherent pluggable transceivers the ability to adapt the modulation scheme and/or baud rate to the application, distance, or data rate. These functions are under the software control. There are no standards for the software interface to a DSP. Since each DSP vendor creates their own software environment, there is no interoperability between DSPs. This means there is no interoperability between CFP2 transceivers from different vendors. This means the coherent pluggable transceivers from the same vendor must be used on each end of the link.
QSFP56-DD (ZR or ZR+) 400G, or 100 to 400G
The Optical Interworking Forum in conjunction with MSA groups published the Implementation Agreement for ZR in April of 2020. ZR optics will operate optics in DWDM networks at 400G Ethernet over single-span links and feature multi-vendor interoperability. To ensure interoperability the line signaling rate (60Gbaud) and modulation format (16-QAM), among other parameters, are specified. This will be the first coherent optic interoperability for higher order modulation formats. The FEC (Forward Error Correction) and OTN (Optical Transport Node) framing parameters used for ZR will also be standardized for interoperability of these transceivers. Likewise, to ensure interoperability among vendors, the baud rate and modulation format will be specified and fixed, it will not be variable. Transmission distance of < 120km are specified and the optical links should provide OSNR of around 30dB.
ZR+ is in the concept stage at this time and evolving as a solution for next-generation, technology at greater than 200G. Two groups initiating discussions on this topic are the Open ROADM (Reconfigurable Add Drop Multiplexer) MSA for ROADM applications and ITU-T Study Group 15 which are looking at use cases of up to 450km.
Similar to the QSFP transceivers described above, the DD version helps to reduce the size of the device in which they operate while significantly increasing the available bandwidth. This increases the number of transceivers that may be placed into the faceplate of a switch, router or optical platform and provides additional throughput.
CableLabs Distributed Access Architecture (DAA)
DWDM and coherent optics are an important component in the Distributed Access Architecture (DAA) work at CableLabs. The DAA describes an overall Hybrid Fiber Coaxial (HFC) network design and the specification known as the Peer-to-peer (P2P) Coherent Optics Physical Layer 1.0 Specification, defines coherent optics for DWDM used to bring higher bandwidth to the access portion of HFC networks. It describes the physical layer requirements for 100G optical links for up to 40km with future distances extending to 120km in some environments. There is also work on single strand, single wavelength bidirectional implementation for this coherent optical standard.
The P2P standard incorporates work from the ITU (International Telecommunications Union) Symbol mapping, Modulation, OTU (Optical Transport Unit) framing, DWDM frequency grid and FEC as well as work from the IEEE 802.3 working groups. DWDM coherent pluggable optics designed to CableLabs specifications could be housed in the standard packaging described above to meet the space and power requirements of the vendor.
The demand for greater bandwidth and increased link distances is motivating standards organizations and MSA groups to focus on new approaches to optical link connectivity. Coherent transceivers will be the underlying DWDM technology for high speeds transceivers of 100G and above. ProLabs is already working to provide 3rd party transceivers in the CFP2-DCO form factor.
DWDM ZR transceivers will be the first commercially available, interoperable, coherent optics on the market. Ongoing work for hardware and software standards to support 200G, 400G and 800G interfaces will make low power, small footprint, coherent DWDM pluggable optics at these data rates available for a wide range of network applications. ProLabs is closely following developments with ZR standards, packaging options, electrical design, management interfaces and other areas to bring our customers the widest variety of flexible, cost effective, solutions with a long reach.
Simply follow the links below to see our full series of coherent articles:
- Coherent Optics: The Beginning of a Universal Approach
- Optic Impairments and Coherent Optics: Explained
- All You Need To Know About Coherent Detection Transceivers
- Everything You Need To Know About Industry Standards
Contact us to find out more.