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CFP2-DCO and CFP2-ACO Transceivers – Basic Definitions

By Ferris Lipscomb, Ph.D. on June 29, 2015  |  Leave a Comment

In previous posts we have discussed the three major optical components that are required to make a coherent transmission system:  Narrow Linewidth Tunable Lasers, Coherent Modulators and Integrated Coherent Receivers.  These three optical elements work with a Digital Signal Processor (DSP) to form a complete Coherent Transceiver.  Just as Shakespeare’s seven stages of man, which start with an infant his nurses arms and proceed through schoolboy, lover, soldier, justice and pantaloon to a “second childhood” of old age, optical transmission systems go through definable stages in a predictable arc.

When a new optical transmission paradigm is introduced it utilizes leading edge technologies to reach the highest performance.  The optical and electrical components are usually soldered directly onto line cards, since each one is specialized and its particular needs must be accommodated.  As the technology matures, some margin appears in performance and the components can be customized to fit together in a separate transceiver, which places additional requirements on the size and power consumption of each element.  Examples of this stage are small form factor (SFF) transceivers or 300 pin transponders.

Next, the separate transceivers are made to be “pluggable” meaning that equipment such as routers and switches are shipped with “sockets” into which standardized transceivers can be plugged as their capacity is needed.  This approach has the great advantage that the transceiver does not have to be bought until its capacity is needed and the performance of the transceiver can exactly match the needs.  If the connection is 300 meters long, a transceiver for 300 meters is plugged in, and not one for 40 km.

There have been far too many pluggable transceiver form factors to name them, but some examples are GBIC, X2, XENPAK, SFP, SFP+, XFP, CFP, CFP2, QSFP and so on ad infinitum.  The last stage in the transceiver life cycle comes when the technology is very mature and the application becomes a commodity.  In this last stage, the extra costs of the transceiver housing and the plug and socket become too expensive and the individual components are once again soldered directly onto line cards, or their equivalent.

Coherent transmission is now entering the stage where pluggable transceivers are being offered.  Fortunately, a series of MSA (multi source agreement) form factors had already been defined for client side (short reach, non-coherent) applications which are also being adapted for use by line-side coherent transceivers.  Recently, several companies have introduced 100G Coherent CFP transceivers.  The “C” is the Roman numeral for “100” and the MSA was originally developed for 100G Ethernet applications.  The CFP MSA provides for a package width of 82 mm and a power consumption of less than 24W.

While challenging, this is sufficient to allow all three optical elements (laser, modulator, ICR) and the DSP to be put in a CFP, which forms a complete Coherent transceiver.  This kind of transceiver is termed a “DCO,” which means “Digital Coherent Optics.”  The CFP-DCO can be plugged into any slot designed for a CFP and communicates through the socket with digital signals.  Thus a switch or router equipped with CFP slots can accommodate short reach client side CFPs, or long reach coherent CFP-DCO transceivers in any of the slots.

As in all things, however, time marches on, and the CFP MSA was inevitably followed up with a CFP2 form factor.  The CFP2 has a nominal width of 41.5 mm and initially allowed for 12 Watts of power consumption.   While client side technology could achieve these limits, it was beyond the capability of current coherent technology, both in the optical components and especially in the DSP power consumption.  Therefore a new type of pluggable coherent receiver was defined, the CFP2-ACO.  ACO stands for “Analog Coherent Optics,” and the DSP is moved outside the CFP2 and onto the circuit board into which the CFP2-ACO is plugged.

The CFP2-ACO communicates with the circuit board by sending analog signals across a special connector.  While this is still challenging for Coherent optics, several companies have recently announced CFP2-ACO modules.  Since the DSP is on the circuit board, CFP2-ACO modules must be plugged into special slots equipped with the DSP.  Switches and Routers must have certain slots designated for line side coherent CFP2-ACOs and certain slots designated for regular client side CFP2s.  This greatly reduces the flexibility of using the same form factor for client and line side transceivers, but it is the only level of integration that is achievable at present. The interoperability of different vendors’ DSP is also not possible in most cases.

Of course, electronics is always improving.  As DSPs continue the march to ever smaller process nodes with corresponding lower electrical power, it is expected that CFP2-DCO transceivers will become possible within a couple of years.  By that time, however, client side optics will have moved on to CFP4 and QSFP28 modules, and the line side will ultimately have to follow with CFP4-DCO.