01-02-2014 07:15 AM
Broadcom's 11ac chips are very popular among the large players, Aruba included. I understand there is a 50 encrypted client limitation per radio. Does the 220 series access points have the same limitation ? IMHO the big advantage of 802.11ac is not the amount of bandwidth you can throw at the network, rather the advantage is airtime fairness. Airtime fairness could possibly lead to higher client density in some situations. Does Aruba have this limitation because of the Broadcom chip ?
802.11ac layer 2 protocol analysis is a real challenge. WildPackets commented, using actual 802.11ac access points as sniffers may be the only real solution to capture wave 1 and wave 2 multiple stream transmissions. Curious what does one do to troubleshoot an Aruba 802.11ac network ? Is there a way to put the Aruba 220 series AP into sniffer mode for layer 2 protocol analysis ?
I notice vendor 802.11ac access point offerings are delivered with a higher radio output power (200mW). Double the power over most legacy access points (100mW). Can anyone explain why this is ?
01-02-2014 09:46 AM
Really, really great questions. I will handle the Broadcom 50 client limitation.
While it is true that we use the Broadcom chip, this limitation is centered around the encryption functions of the chip. In our designs, we would recommend that the AP run in "tunneled" mode meaning that the encryption from the client is handled by the controller. Because of this advantage, we do NOT run into the 50 client limitation as that function is handled by our controllers.
In addition, we can overcome the 50 client limit in our Instant version or running in "decrypt-tunnel" mode where the AP performs the encryption functions to/from the client. In this scenario, we can offload this to another chip on the 220 APs.
Hope this helps!
Consulting Systems Engineer - ACCX, ACDX, ACMX
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01-02-2014 11:16 AM
Adding to Seth's reply: even when encryption is handled by the AP when in decrypt-tunnel mode, the crypto hardware of the CPU will handle encryption for clients 51 and up. Some of Aruba's competitors are not able to handle the BRCM chipset limitation in this way, but the 50 client limit does not apply on the Aruba product.
/Onno Harms, Aruba
01-02-2014 11:27 AM
Concerning the transmit power, the new modulation scheme that's added in 802.11ac (256QAM) requires a much higher quality (cleaner) signal than what was required for 64QAM in 802.11n. In order to properly decode these higher rate signals, the error vector magnitude (EVM) of the transmitted signal (which is basically a measure for the amount of noise present in the signal)has to be much lower than for the highest 11n rates (by 6-9dB). This in turn means that most 11ac devices use a higher quality power amplifier than older devices, which typically have more headroom, allowing for higher power levels even at lower rates.
Note that the 200mW number (23dBm) is the aggregate power from the three radio chains (18dBm per chain). It excludes any antenna gain, and applies to lower rates only. The transmit power typically drops as rates go up, again to make sure the increasing EVM requirements are met.
/Onno Harms, Aruba