Technology Trends: Transmission Systems

I attended OFC-NFOEC a couple of weeks ago and listened to a panel talk on the issue of “what next beyond 100 Gbps systems.”  This is a familiar issue from the early 1990s when I was involved in transmission planning for Ameritech.  It was also a critical issue in the late 1990s when I helped plan transmission system products for Alcatel.  In the 1980s and 1990s, there were two parallel universes: Transmission systems and data ports, both evolving aggressively as silicon and optical technology improved.  The transmission rule of thumb was 4x while the data port rule was 10x... that is, the next generation of transmission systems technology needed to be 4x in speed (and usually only about 2x in price) to justify transition of whole networks to the new technology.  OC3 systems were replaced by OC12 systems which were replaced by OC48 (2.4 Gbps) systems which were replaced by 10 Gbps systems.  Meanwhile 10 Mbps Ethernet ports were replaced by 100 Mbps ports were replaced by 1 Gbps ports were replaced by 10 Gbps ports.  Why 4x and why 10x?  Coming from carrier transmission planning, I knew that the 4x rule had its basis in transmission deployment business cases, comparing the cost of upgrade to the higher speed (Up) vs. building additional systems at the same speed (Out.)  Up vs. Out business cases had different results for short haul compared to long haul deployment.  The 4x rule was for long haul, but ushered in improved economics for short haul systems whenever long haul upgrade hit volume deployment.

The data port 10x rule had its own Up vs. Out business cases, however, the ports were much more rapidly commoditized and depreciated than transmission systems and the cost of cabling in the business cases were negligible compared to the cost of laying new fiber routes.  I also think the 10x rule was a kind of self fulfilling prophecy... once people got on the 10x band wagon, it was hard to get off.

Back to OFC-NFOEC: the curious thing about this panel was the absence of any discussion of these underlying business cases, rather, the subjective impressions of customers and technologists were explored and people seemed to be equally divided between continuing the 4x rule (carrier planners and their fellow travelers) and continuing the 10x rule (data networking folks and their allies.)

There is, however, an additional factor that may be more important than either traditional Up vs. Out business cases or subjective impressions of the people with the purchase power... fundamental limitations of optical technology.  Transmission systems are beginning to reach fundamental limitations of glass media and optical coding.  Beyond 100 Gbps, we reach the Shannon limit for single channels and the power limit for glass fiber (before the lasers melt the core of the fiber.)  It may not be possible to achieve either 4x or 10x capacity increases in the future, but rather the more modest 20 to 40% improvements of a mature technology.  The future may well be multi-fiber or require new higher power wave guide technology.

What are your impressions of the future of optical transmission systems?  Are you a 4x or 10x technologist?  How does that color your perspective on the march of optical systems?