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pmoyer

FPGA or ASIC? Pro's & Con's of Each Technology

by pmoyer on ‎03-29-2012 04:27 PM (372 Views)

I’d like to follow Greg’s great blog from last week with a related topic. Like his blog, this blog will be focused on router hardware (unlike my previous blogs which were NetIron software related). The topic at hand is a brief discussion of the differences and the pros/cons of FPGA and ASIC technology. I’ll also briefly touch on the advantages of each of these technologies as they apply to high-end IP routers.

FPGA

FPGAs (Field Programmable Gate Arrays) are specialized chips that are programmed to perform very specific functions in hardware. An FPGA is basically a piece of programmable logic. The first FPGA was invented in 1985, so this technology has been around for quite some time. Rather than executing a function in software, the same function can be implemented in an FPGA and executed in hardware. One can think of an FPGA as “soft-hardware”, since it can be reprogrammed after manufacturing. How many of you remember the bygone days of software-based IP routers? If you do, then you should also remember how poorly the Internet performed at that time! Performance was poor in software-based routers due to the fact that a centralized CPU executed all functions, both the control/management plane functions and the data plane functions of the router. Today, all modern routers execute the data plane functions in hardware; and more frequently, some vendors are moving certain control plane functions into the router hardware as well. The Bi-Directional Forwarding (BFD) protocol is one example of this; where portions of the BFD keep-alive mechanisms are implemented in the line card of the router.

While FPGAs contain vast amounts of programming logic and millions of gates, one thing to note is that there is some programming logic in an FPGA that is not used for the “customer facing” or "mission specific" application or function. In other words, not all the logic in an FPGA is designed to be directly used by the application the FPGA is providing for the customer. There are additional gates needed to connect all the internal logic that is needed to make it programmable; so an FPGA is not fully optimized in terms of “customer facing” logic.

Now, what I find interesting is that some people will still claim that FPGAs cannot scale to the speeds that are required in the today’s Internet. However, Brocade has proven this claim to be quite false and has been shipping line-rate, high-end performance routers using FPGAs for over 10 years. As shown in the line card diagram in Greg’s blog, an FPGA in this context is really a programmable network processor.

One great advantage of an FPGA is its flexibility. By flexibility, I’m referring to the ability to rapidly implement or reprogram the logic in an FPGA for a specific feature or capability that a SP customer requires. When a networking vendor has a new feature that it wants to implement, the vendor may have the choice of deciding whether to put the feature in software or hardware. This is not always the case; for example, OSPF needs to be run in the control plane of the router and cannot be implemented in hardware. The question of whether to implement something in software or hardware basically comes down to a decision of flexibility versus scalability (and cost is always part of that decision process, as one would expect). Implementing something in software usually results in a rapid implementation timeframe, but often at the detriment to performance. As usual, there is always a trade-off to be made. However, if the vendor supports programmable network processors, they can implement the feature in hardware with no detriment to performance. While it takes more time to get the feature into an FPGA rather than implementing it in software, the time-to-market timeframe is still considerably less than doing a similar feature in an ASIC. The real advantage of this becomes evident with deployed systems in a production network. When a customer requires a feature that needs to be implemented in the forwarding plane of a router, once this feature is developed by the vendor the deployed systems in the field can be upgraded to use the new feature. This requires only a software upgrade of the system; no new hardware or line cards would be required.  The routers’ software image contains code for the FPGAs, as well as the code for the control and management plane of the router.

Back to the performance question: Industry has shown that high-end FPGAs are growing in density while handling higher-speed applications and more complex designs. Furthermore, if you look at the evolution of FPGAs over the years, they follow Moore's Law just like CPUs have been doing in terms of the amount of logic that you can implement into them.  Recent history has shown that FPGA development in terms of density is on an exponential growth curve.

FPGAs can also be used for developing a “snapshot” version of a final ASIC design. In this way, FPGAs can be re-programmed as needed until the final specification is done. The ASIC can then be manufactured based on the FPGA design.

ASIC

ASICs (Application Specific Integrated Circuits) are also specialized chips that perform specific functions, and which also operate at very high performance levels. As technology has improved over the years, the maximum complexity (and hence functionality) possible in an ASIC has grown from 5,000 gates to over 100 million. ASICs are often used for very dense applications; for example, where port density on high-end IP core routers is a critical requirement. ASICs have a higher R&D cost to design and implement, as compared to an FPGA. However, once an ASIC is fabricated, it is not reprogrammable like an FPGA is. The layout of the internal chip constructs are fixed and cannot be modified without a “re-spin” of the ASIC. This makes ASICs much less flexible than FPGAs.  ASICs are very dense chips, which typically translates to high scalability with less real-estate requirement on a line card. In cases where scalability is critical, this is where ASICs really shine. In terms of useful logic, table sizes, and resource availability, ASICs will outperform FPGAs in terms of scalability. While ASICs typically have a higher R&D cost to design, in high volume applications the lower costs of manufacturing ASICs are attractive.

While ASICs have very high density in terms of logic gates on the chip, the result of higher scalability in terms of the same power metric can give ASICs a competitive edge over an FPGA. One thing to note is that an ASIC is designed to be fully optimized in terms of gates and logic. All the internal structures are used for customer facing or mission specific applications or functions. So, while an ASIC may consume more power per unit die size than an FPGA, this power is amortized over a higher density solution; and hence, provides better power efficiency.

Compare/Contrast of FPGA-ASIC

So, FPGAs and ASICs are both specialized chips that perform complex calculations and functions at high levels of performance. FPGAs, however, can be re-programmed after fabrication, allowing the line card's feature set to be upgraded in the field after deployment. Being able to upgrade the data plane of a deployed router extends the useful lifespan of the system; which correlates to extended investment protection. Since an ASIC is not re-programmable, an ASIC-based line card cannot be upgraded in the field. This is a huge differentiator between the two technologies.

One excellent real-world example of this is when Brocade introduced support for 64 ports in a single LAG. This is industry leading scale (64 10GbE ports in a single LAG!) and since this functionality is implemented in the forwarding plane of the line card, it required reprogramming the Brocade network processor. While this type of capability is in the hardware of the router, it was implemented with a system software upgrade and no hardware needed to be replaced.

There are network scenarios or use cases where it makes more sense to have an FPGA-based product and there are use cases when it makes more sense to have an ASIC-based product. For example, a SP may determine that a high density solution is more important than a solution that provides quicker feature velocity and, thus, may choose an ASIC-based product. ASIC-based line cards are often denser in terms of numbers of ports and the cores of SP networks typically do not require high feature velocity. Most of the feature velocity in today’s SP networks is at the edge of the network (ie: at the PE router) or in the data center, where innovation is currently happening at a rapid pace. The general flexibility of an FPGA results in time-to-market advantages for feature implementation and soft-hardware bug fixes.

For smaller applications and/or lower production volumes, FPGAs may be more cost effective than an ASIC. The non-recurring engineering (NRE) cost of an ASIC can run into the millions of dollars. Conversely, in high volume applications the front-end R&D costs of an ASIC are offset by a lower cost to manufacture and produce. For example, in high-end IP core routers, ASIC-based line cards are more economical due to the lower manufacturing cost, combined with the higher port density of the line card that ASICs can provide.

As costs related to ASIC development are increasing, some recent trends may suggest that FPGAs could be a better alternative even for high volume applications that traditionally used ASICs. It is unclear whether this trend is indeed sustaining or a somewhat temporary aberration.

To summarize the primary differences between FPGA and ASIC based line cards; at the highest level it basically comes down to a scalability versus a flexibility question (again, with cost a large contributing factor). ASICs are advantageous when it comes to high port density applications. FPGAs are advantageous when it comes to feature velocity with a shortened time-to-market requirement. In high end core routers, high density ASIC-based line cards can provide higher density at a lower cost than FPGA-based line cards. So, it’s based upon the use case and network application to determine which type of technology would be favored over the other.

As usual, any questions are comments are welcome!

Comments
by (anon) on ‎05-16-2012 01:09 AM

Very good 101 into the topic and comparison.