Peek Inside 802.11ac Access Point Hardware Designs
There is large and ever increasing assortment of enterprise access points offered by wireless vendors today. APs have different number of radios, number of streams, 11n/11ac, POE compatibility, peripherals, price, etc. While this diversity is overwhelming, have you wondered what lies in the hardware guts of these APs? What are the hardware design concepts that are responsible for rendering feature personality to the AP? How does the hardware ecosystem work among chip vendors, ODMs and AP vendors? What are state of the art hardware architectures for the 802.11ac APs? This blog post discusses key hardware concepts, such as SoC, dedicated CPU and offload architectures that are commonly found inside the APs, along with the ODM sourcing model for the Wi-Fi APs and its implications for product offerings.
At a high level, the AP hardware has to perform three types of processing: RF, baseband, MAC/L2/packet. The first two are mainly concerned with the signal modulation/demodulation related tasks; while the third takes care of the 802.11 MAC and L2 protocol implementation and provides the hooks into the packet processing to build various features like QoS, firewalls, bandwidth limiting, etc. The RF and baseband processing are done by the “radio module”. The radio module also performs certain time sensitive MAC/L2 functions such as ACKs, RTS/CTS, time stamping, etc. The bulk of MAC/L2/packet processing is performed by the “host CPU module”.
SoC (System on Chip) Architecture
In this architecture, there is a single chip which can do the job of the host CPU module and also one radio module. A dual radio AP can be implemented with the SoC architecture with two chips: The main chip includes the host CPU (usually MIPS or ARM core) and one radio module (a/b/g/n), and the second separate chip is for the second radio module (a/b/g/n or ac/b/g/n). The second chip is fixed to the board or provided on a PCI Express (PCIe) card that plugs into the PCI slot on the board that interfaces with the CPU chip. The PCIe form factor for the radio module allows for changing the radio module to more easily launch the updated hardware versions of the AP at a later point in time. In the SoC designs, both the chips usually come from the Wi-Fi chip manufacturers such as Qualcomm (Atheros), Broadcom etc. The radio modules are usually dual band tunable, but locked to one band – usually the SoC radio is locked to 2.4 GHz and the separate radio is locked to 5 GHz.
SoC designs are cost efficient because of the reduced BoM of the board. They also draw less power making them attractive for designing APs meant to operate within the 802.3af power budget.
Dedicated CPU Chip Architecture
Unlike the SoC design, in this architecture, there is a separate host CPU chip. So, a dual radio AP in this architecture requires three chips – one chip for the host CPU and two additional chips (fixed to the board or provided on the PCIe cards) for the two radio modules which can be 11n or 11ac. The CPU chip typically comes from the embedded microprocessor vendor such as Freescale, Cavium etc., while the radio chips come from the Wi-Fi chip vendor. However, this year we should see designs with the host CPU chip also coming from the Wi-Fi chip vendor since some of them possess powerful embedded microprocessor technology from their other product lines.
Due to the three-chip design, these APs usually cost more and can have difficulty operating at full function within the 802.3af power budget. On the flip side, dedicated CPU provides more processing capacity and may also provide hardware assist features for the IPSec encryption, DPI (deep packet inspection), etc.
Offload Architecture for 802.11ac APs (Second Microprocessor to Assist the Host CPU)
Until and including 11n, the de facto way of implementing bulk of MAC/L2/packet processing was entirely within the host CPU. With increasing speeds in 11ac, new concept of “offload processing” has emerged. In the offload processing concept, there is a second microprocessor (in addition to the host CPU) that is embedded in the radio chip (that is separate from the CPU chip). This second microprocessor handles close to 75% MAC/L2/packet processing tasks on the radio chip itself, leaving only about remaining 25% to be done inside the host CPU.
The offload architecture significantly reduces the load on the host CPU at high speeds as in 802.11ac and thus makes it possible to build full function 11ac AP using the SoC design. With the offload concept and SoC design, a dual radio 11ac AP can use one chip for the host CPU and the b/g/n radio module, and a second chip that includes both the ac radio module and the second microprocessor that handles the offload processing.
The flip side of the offload architecture is higher sophistication required for the software running on the host CPU to perform some special packet handling functions such as raw packet injection. This is because, in the offload design, lot of MAC/L2/packet processing happens in the radio chip itself. So, the host CPU needs to interact with the second microprocessor on the radio chip via API and communication calls to implement special processing tasks.
Original Design Manufacturer (ODM)
ODM vendors (which operate from the manufacturing hubs in East Asia) design AP hardware from the reference designs provided by the chip vendors based on the architectures described above and then offer these hardware platforms to the AP vendors. ODMs often also do some modifications of their own to the reference designs to improve their characteristics, such as fitting better antenna on the board.
The AP vendors choose hardware platforms from the ODM offerings for different products as appropriate for specific market verticals. AP vendors can choose enclosures designed by the ODMs and have then branded with their company logos. That is why, we sometimes see similar looking APs offered by different vendors. AP vendors can also have ODMs design special enclosures for them. In this case, even if the APs may not be similar looking, you can encounter common hardware layouts when you pry them open if they have the same ODM board genesis.
ODMs also assist vendors in platform certifications for the regulatory compliances. Due to easy availability of the validated hardware cores and maturity of the ODM model, AP vendors can now deliver new hardware platforms relatively quickly. Though ODM vendors typically accommodate some platform customization such as changing power amplifier rating or amplifier quality, adding extra Ethernet port(s), providing USB port, adding third radio etc., in general the core hardware differences between the APs will be marginal in the future (barring some highly specialized hardware designs). Couple that with the scenario that Wi-Fi application verticals and deployment use cases expand and become more diverse. Then, the bulk of value needs to come from the software that the AP vendors add on top of these cores in the areas of performance, network services, application enablement, security, manageability and others.
/Images via openwrt wiki
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