Typically, transceivers are classified by key standards and form factors.
To differentiate between optics with similar appearance but vastly different functions, transceivers are identified by:
- Packaging
- Optics
When looking at part numbers, the initial set of characters confirm the packing type and the following characters confirm the data rate. Packaging is key to defining which devices (for example open line systems, routers, network interface cards, switches) work with the transceiver. The optics confirms the reach and the data rate that can be achieved.
Below data speeds of 100G there are numerous types of transceivers. The key form factors for dense wavelength division multiplexing (DWDM) at 100G or above are shown beneath. Quad small form factor pluggables (QSFP), QSPF+, and QSFP28 transceivers are included for historical background to help highlight the development of DWDM pluggable optics. Just the pluggable optics listed below back coherent detection:
- CFP2-ACO
- CFP2-DCO
- QSFP56-DD
Direct Transceivers
QSFP
Under the guidance of the Small Form Factor Committee they are specified by an MSA (Multi Source Agreement). This is the most common transceiver for DWDM at 100G. The Quad indicates that it works with four bidirectional optical channels which improves the bandwidth available on the link. They contain a cage for electromagnetic interference resistance, plug, guiding plug, and a board mounted electrical connector.
QSFP+
They are the same size as the QSFP and were designed to work with 4x10Gb/s lanes of data. Slowly they are replacing the QSFP.
QSFP28
They have the same form factor as the QSFP+ and are used for InfiniBand EDR (Enhanced Data Rate) 100G and 100G Ethernet ports. These transceivers work with four channels which each run at 28G creating a 100G link.
QSFP56
They are used for 200G applications, for InfiniBand (High Data Rate) 200G and 200G Ethernet ports, they have four channels capable of 50G data rate. They also have the same form factor as other QSFP devices.
QSPF-DD
This is the smallest 400G device and allows 36 ports of 400G to be in a single Rack Unit (RU). The DD denotes a double-density QSFP transceiver which works with 200G and 400G Ethernet. It uses eight lanes at 28G NRZ (none-return to zero) modulation for 200G, or eight lanes of 50G PAM4 (pulse amplitude modulation) for 400G data rates. QSFP and QSFP-DD can be used in the same ports. The specification is 10Km link distances. They will use more power in comparison with other devices so this needs to be thought through.
DWDM QSFP28 PAM4
They use the QSFP28 format for direct detection optics instead of the coherent optics and is the outcome of a multi-source agreement. PAM4 uses pulse amplitude modulation for different band rates and distances on DWDM networks. The reach can be up to 80km, although direct-detect devices are susceptible to dispersion related effects and need precise amplification and dispersion compensation equipment to achieve distances normally essential for data center interconnect and additional DWDM optical link applications. The advantages of the PAM4 include longer reach and lower power.
Coherent Transceivers
CFP2-ACO and CFP2-DCO
The first abbreviation is for Analog Coherent Optic (ACO and the second stands for Digital Coherent Optic (DCO). Within the ACO the Digital Signal Processor (DSP) is placed on the host line card, whereas the CFP2-ACO passes an analog signal to the DSP located in the host line card, this means it can only be used in a network that has the matching DSP on the host line card. With a CFP2-DCO the DSP is located within the optical transceiver. This provides more options as the module can be used with any line card.
DSP allows coherent transceivers to adjust the modulation and / or baud rate to the application, data rate or distance. They are managed by the software control, however there are no standards for the software interface to a DSP. The DSP suppliers design their own software components, there is no interoperability between DSPs. There is no interoperability between CFP2 transceivers from different suppliers, so coherent pluggable transceivers from the same supplier must be used on each end of the link.
QSFP56-DD (ZR or ZR+) 400G, or 100 to 400G
The MSA groups in combination with the Optical Interworking Forum issued the Implementation Agreement for ZR in April 2029. The ZR optics will work in DWDM networks at 400G Ethernet for single-span line and include multi-supplier interoperability. The line rate (60Gbaud) and modulation format (16-QAM) are defined to help confirm the interoperability. To ensure interoperability among suppliers, the baud rate and modulation format will be specified and fixed. This will be the first coherent optic with interoperability for higher order modulation formats. Transmission distance of < 120km are defined and the optical links should provide OSNR of around 30dB.The FEC (Forward Error Correction) and OTN (Optical Transport Node) framing parameters used for ZR will also be standardized for interoperability of these transceivers.
When writing this article, the ZR+ was still at the development stage and progressing as an option for next-generation, technology for higher than 200G. The two groups leading with this are ITU-T Study Group 15 which are looking at use cases of up to 450km and the open reconfigurable add drop multiplexer (ROADM) MSA for ROADM applications
Comparable to the QSFP transceivers mentioned above the DD form aids to decrease the size of the device in which they function whilst noticeably growing the existing bandwidth. This also grows the quantity of transceivers that can be positioned into the faceplate of a router, switch, or optical platform.
CableLabs Distributed Access Architecture (DAA)
DWDM and coherent optics are a key part of the Distributed Access Architecture (DAA) work at CableLabs. The DAA has defined 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. This defines coherent optics for DWDM which is used to deliver increased bandwidth to the access portion of HFC networks.
It specifies the physical layer requirements for 100G optical links for up to 40km with future distances extending to 120km in cases. There is also a focus on single strand, single wavelength bidirectional implementation for this coherent optical standard.
The P2P specification includes input from the ITU (International Telecommunications Union) Symbol mapping, Modulation, OTU (Optical Transport Unit) framing, DWDM frequency grid and FEC as well as input from the IEEE 802.3 working groups. DWDM coherent pluggable optics designed to CableLabs specifications could be housed in the standard packaging described above that meet the space and power needs of the supplier
What is on the horizon for fiber networking?
The need for improved link distance and higher bandwidth is encouraging MSA groups and standards organization to look at new developments to optical connectivity. Coherent transceivers will be the principal DWDM technology for high speed transceivers of 100G and above. AddOn is already striving to deliver 3rd party transceivers in the CFP2-DCO form factor.
DWDM ZR transceivers will be the first commercially available, interoperable, coherent optics on the market. Continuing 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.
AddOn is keeping an eye on the changes with the ZR standards, for electrical design, packaging options, management interfaces and other areas to deliver to our customers the broadest variety of flexible, cost effective solutions.
For more information, the links below cover our series of coherent articles:
- Coherent Optics: The Start of a Universal Approach
- Optic Impairments & Coherent Technology: What You Need to Know
- Coherent Transceivers: All You Need to Know
- Industry Standards Explained & Why They Are Essential