Lower-Powered 802.11n Chipset Makes Necessary Compromises

By Frank Bulk

Powering the new enterprise-grade 802.11n access points continues to be one of the necessary talking points for vendors and an item of concern for enterprises. That's interesting because, before 802.11n, powering APs hasn't really been a significant discussion item. Some dual-radio APs (i.e., Cisco's 1200 series) have pushed the limits of some PoE (power over Ethernet) switches, but the AP always was well within IEEE 802.3af specifications.

With multiple radio chains and enhanced CPU and network processor requirements, power consumption has become a real consideration in 802.11n access point design and, inevitably, marketing. The same holds true in client designs. Rather than 3x3 designs, many clients use 2x3 or 2x2. Fewer transmitting radio chains mean less power consumed. And while chipset manufacturers have been able to apply expertise gained in 802.11a/b/g design to their 802.11n chipsets, current designs won't be nearly as optimal as those even a year from now. With no new mass-market battery technologies for portable devices on the horizon (except for perhaps this), Wi-Fi chipset vendors looking to stuff the latest and greatest into mobile gadgets are taking drastic measures to meet the run time requirements of consumers. The simplest approach is to reduce the number of radio chains, which is exactly what Redpine Signals has done with its RS9110.

Redpine uses a 1x1 (one transmit, one receive) design with a single spatial stream. That means it derives none of the robustness and performance benefits via MIMO, but depends almost wholly on STBC (Space Time Block Codes). That said, it still retains 802.11n higher PHY rates and it can't be blamed for dragging down performance in the same way that legacy 802.11 gear will do in 802.11n networks. It also supports features such as frame concatenation.

Redpine’s RS9110 1x1 design is clearly designed for power-consumption sensitive devices, claiming to extend battery life for beyond what’s capable with 802.11b/g products on the market today. That's a nontrivial claim, because Broadcom, but especially Atheros, continue to make significant advances with their 802.11b/g chipsets. When I pressed an Atheros spokesperson in a briefing, he admitted that he wasn't sure if any single future revision could duplicate or surpass the "70% less power than competitive solutions on the market in active mode while downloading content." It's interesting that Redpine isn't afraid to mix apples with oranges by comparing mature 802.11b/g product against its new 802.11n chipset.

Redpine claims its improvements were obtained through a system approach:

"The Lite-Fi(TM) RS9110 utilizes advanced power management techniques spread over system, algorithm, and circuit levels. "Unlike the conventional implementations which focus on silicon geometry to get lower power, we took system approaches to optimize the total average power," said Venkat Mattela, Redpine's Chief Executive Officer.”

Atheros states something very similar:

"Atheros' expert design of the AR6002 minimizes power consumption through a combination of advanced hardware and software power-saving protocols and techniques."

Redpine appears to be making a jab at those who reduce their package size, but to say that's their competitors' primary approach would be an oversimplification. No Wi-Fi vendor focuses solely on reducing silicon geometry, but tackles those optimization techniques which offer the greatest savings for the engineering dollar. Besides the obvious hardware considerations, software optimizations such as the timing of transmissions and controlling what components of the chipset are on during certain phases in communication can and do offer significant performance benefits.

Redpine won't hold an indefinite exclusive on mobile-targeted 802.11n chipsets -- you can be sure the other market leaders are working hard to bring their own to market.