Thursday, 3 November 2011

Cisco 3602i and Cisco 3602e APs

Edit (6th Nov 2011): Cisco has officially announced the 3600 series APs - the data sheet is pretty much identical to that detailed below. In addition a few more details of ClientLink 2.0 are available as is a white paper discussing the 3600 series.

This PDF shows that the antennas to be used on the 3602e are dual-band as speculated below. Ironically the gain on the external antennas for the 3602e are slightly lower than the internals at 5 GHz and equal at 2.4 GHz - still you might require the 3602e for ruggedised deployments or of course, other antennas.

Edit (17th Nov 2011):  Cheers to Marcus Burton for correcting me regarding dual-band antennas.That is, no antenna magic is needed for an antenna to utilise two frequencies simultaneously..

For the last few months there has been murmurs in the WiFi-Nerd... err, underground... yer, let's go with that, regarding the pending release of Cisco's new 3600 series access points. These APs have not been officially announced on the Cisco website as yet but 900-odd Google results for the model numbers, mostly from retailers, must mean they are no longer under NDA. In fact, the 3600 series was listed on Cisco's website however they have pulled the details but you know... this is the Internet's and ahh, Google cache exists. A few months ago, waking up in the middle of the night, thinking about wireless as I often do, I speculated that the soon to be released WLC code 7.2 would likely coincide with the 3600 series being released. Well 7.2 has not been released but the 3600 series data sheet lists it as required. Instead we have the recently released WLC code There was no mention in the release notes of 3600 series support however. Google cache again to the rescue; this time for a "no longer listed on" WLC code release. The release notes mention that this release is a restricted limited lifetime release specifically for the 3600 series. The rest of the details are uneventful and in-line with the more or less. Finally, a snazzy photo exists on this Ingram Micro hosted PDF data sheet.

The Cisco 3602e. Now featuring less antennas!

Initially, I had heard that this AP would support four MIMO streams. I thought this was logical considering Cisco is late to the game with their 'better-than-two-stream' offering. Sure, you're almost 12 months behind the earliest three-stream enterprise offerings, but you'll jump straight to four MIMO streams... kind of like Gillette's five-blade razor. However, since hearing this news I began to doubt that this AP would support four streams. Specifically, speculation from Cisco and Aerohive that enterprise vendors may skip four streams and jump straight to 802.11ac, though this really only makes sense if 802.11ac arrives on-time which is unlikely, given the history. 802.11ac delays aside, another issue with a four stream AP arriving now is the lack of client support; specifically there are none. Lastly, the Wi-Fi Alliance does not have a certification program to test four stream APs or clients.

So, a Cisco four stream in 2011 seemed unlikely... and the data sheet indicates exactly that. The 3600 series supports three streams - 4x4:3 but three streams none-the-less. An "in-no particular-order", "assuming the data sheet is accurate" list of my 3600 musings:

4x4:3: I imagine the reason I had been told four-streams was because of some confusion originating from the 4x4 nature of the AP. 4x4:3 means that the AP has four transmit antennas, four receive antennas but only supports three spatial streams. Keeping in mind, this is per band. More correctly, radio chains instead of antennas but antennas are easier to picture so we'll stick with that! So if you are only transmitting or receiving a maximum of three streams concurrently, what are the fourth antennas for? The extra antennas are used in 802.11n MIMO devices for similar reasons to those used in 802.11a/g devices - diversity and in the case of 802.11n, some particularly advanced diversity techniques. Space Time Block Coding (STBC), Cyclic Shift Diversity (CSD), Transmit Beam-forming (TxBF), Maximal Ratio Combining (MRC) are all 802.11n techniques which help to increase the signal strength or reliability available to the client or AP which can result in increased link reliability and throughput. Both the 3500 and 3600 series of APs support CSD, TxBF and MRC however the 3600 series now supports TxBF for a larger range of clients, detailed below. So 4x4:3 gives us three streams (450 Mbps maximum data rate) but with the benefit of 4x4. For reference, the 3500 series does 2x3:2. 

Three spatial streams: Three streams with 40 MHz channels and a 400ns Guard Interval tops out at 450 Mbps. One of the major advantages of three streams is the ability to move back to 20 MHz channels at 5 GHz whilst still achieving a maximum data rate of 216 Mbps. Not quite the 300 Mbps afforded by using 40 MHz channels but much better than the 144 Mbps from 20 MHz channels. These extra channels are particularly important in Australia where we currently only have access to six 40 MHz channels and potentially four, if radar starts causing you significant grief. Whilst there aren't many three-stream clients available, your APs are going to be in place for several years at which point there will hopefully be a significant portion of three-stream enabled laptops - I doubt we'll ever see a three or four stream smart-phone on the other hand. 450 meg clients, where art thou!

ClinkLink 2.0: Cisco's beefed up transmit beam-forming (TxBF) offering. For reference, the 3500 series (amongst others) offers TxBF in the form of ClinkLink (1.0). A few weeks back, I was pondering why Cisco only offered ClientLink to legacy (802.11a/g) clients. I thought that 802.11n single-stream devices (smart-phones, tablets, etc) could benefit greatly from beam-forming - these are precisely the sorts of devices that could see a benefit from a few extra dB due to their generally weak receive sensitivity. To support two or three stream clients though you need an extra antenna and the 3500 series only does 2x3:3 (two transmit antennas) and therefore does not have a spare transmit antenna free when servicing a two stream client. This is where the 4x4:3 comes in handy - that fourth transmit antenna can be used to provide beam-forming to one, two and three-stream 802.11n clients. So now we have TxBF for legacy, one, two and three stream 802.11n clients. Hopefully ClientLink 2.0 implements some antenna beam-forming action over the ClientLink 1.0 on-chip beam-forming where marginal gains were, at least originally, achieved. Count-down to a Ruckus blog post dissing this TxBF implementation as half-assed ;). To be fair, Ruckus are the bomb when it comes to antenna magic!

200 mW (23 dBm) of transmit power: The 3500 series only offered 200 mW for 802.11b data rates which should be disabled in almost every new deployment anyway. As it is best practice to match the APs maximum transmit power to that of the poorest client, I can't see where this extra power would come in handy. If you were using the 3602e for a bridge link perhaps but a higher gain antenna would be a better bet. I can't help but think that this power may be spread between the antennas when performing one of the supported types of transmit diversity though... hmm.

Antenna gain: Down from 4 dbi on the 3500 series to 2 dbi on the 3600 series at 2.4 GHz. Up from 3 dbi on the 3500 series to 5 dbi on the 3600 series. With only four external antennas on the 3602e my guess is that they are dual-band antennas. Whilst this works fine on a client as it is only associated to one band at a time, pretty much all decent APs must be able to use both bands concurrently. For an AP to transmit or receive on both bands simultaneously I imagine some sort of smart antenna technology is being employed - pure speculation on my part and all based on the photo! We will see.

PoE: Powered by 802.3af like all other two and three stream enterprise APs (the EOL Cisco 1250 not with-standing). Surely it is in the various Cisco switching business units interests to require 802.3at for the AP ;). Of course, customers would not be happy knowing that all other three stream APs run off a mere 12.95W!. At this rate we will have to wait for 802.11ac to see some 30W AP lovin!

Band support: The 3600 series appears to only be available in a dual-band version. About time really, as I find the single-band APs to be a waste of silicon!

Weight: Light! Like the 3500 series, due to the CleanAir capabilities, this AP is light-weight only. Now the question is, which autonomous AP IOS can be used to survey with the 3600 series!

The LED: It's bigger - surely its most impressive feature, by far! ;) 

EOF... almost
Like most vendors with a large market share, Cisco receives their fair share of flack regarding a lack of innovation, resting on ones laurels and so on. As the first enterprise vendor to offer 4x4:3, up to three stream beam-forming and the 'not-really-that-long-ago' CleanAir (AP spectrum analayser integration) I think at least the Cisco Wireless Business Unit still has the ability knock out an innovative product. Now I wait for the official release to get my hands on one and do some testing!

Since the first three-stream enterprise AP was released I kept seeing Cisco down-play the importance of three-streams. I assume that downplay will end right about ... ;)


  1. Nice post, and many valid speculations. I just wanted to comment that dual-band antennas can carry both frequencies at the same time. There is no need for additional antenna intelligence. It is pure RF that allows it. :)


  2. Cheers for the heads up Marcus. :)