I have been steadily going through the physical layer descriptions, if you haven’t yet studied this material its quite fascinating. The basic idea is that each station has to sit and listen to the channel (if its not sending) to detect the beginning of any signal it can receive. A transmitting station will first of all send a preamble. The preamble consists of a stream of 0′s or 1′s that the receiving station can synchronize with, then after this the transmitting station sends the Start Frame Delimiter (SFD) which is a set 16 bits that indicates the PLCP Header is next. This process is all pretty straight forward to understand, however the problem is it just describes how DSSS works and not OFDM.

In the OFDM world the PLCP preamble is also called a training structure, which consists of 10 short symbols and 2 long symbols. These are described as the 10 short symbols are used for AGC (automatic gain control), diversity selection and coarse frequency offset estimation of the carrier signal. The 2 long symbols are used for channel and fine frequency offset. We also get additional information, that the total training length is 16 microseconds and a short training symbol has 12 subcarriers and a long training symbol has 52 subcarriers. Leaving aside for a second that some of this (such as frequency offset estimation) may not immediately make sense one thing that struck me in reading all this was that I wasn’t sure I understood what the word ‘symbol’ meant in this context. It seemed to me that one of the core things to understand about how OFDM works is to understand what an OFDM symbol actually is.

I actually have both editions of the CWAP study guide. I find while studying that it helps sometimes to get two different views on a certain section in order to gain understanding. My first reaction was to look in the index, glossary and list of terms to see if it was more defined. Nope, its assumed to be something known. Maybe it was in CWNA and I just missed it, so I go back to my CWNA guide and find that there is an index entry for symbols. It turns out to be pretty thin, however, and part of a discussion on guard intervals in 802.11n in that book. Not really enough for my purposes. Now this becomes a mission for me, I simply must understand what symbol means in this context.

I first of all hit Google and discover that Wikipedia has Symbol rate as an entry. It begins that a symbol can be described as either a pulse (in digital baseband transmission) or a “tone” (in passband transmission using modems) representing an integer number of bits. It then goes on to give some examples of the relationship between baud rate and bit rate. This is useful but for me I still cannot quite grasp from this explanation how it relates to the usage in OFDM. I post to twitter as well to get input from other WLAN pro’s on this question and get some great answers (as much as you can in 140 characters). Finally my friend Matthew Gast comes out with ‘A symbol is a change in amplitude & phase that carries a defined number of bits’. ‘Data bits get mapped into a constellation by amplitude and phase change; constellation has 2^(# of data bits)’. This to me seems to be getting closer to something I can understand. Matthew adds ‘All PHYs use symbols, CCK, DSSS and FHSS are all simpler than OFDM though’. That to me points out even more its important to understand what symbols are.

I continued to search and found a page on modulation in Wikipedia, which is also very useful in adding to my knowledge as it describes the process of modulating a signal. The analogy to music seems to be common in describing how radio signals work, which is not surprising in that radio signals quite commonly as shown to be waveforms. In the section of this article that talks about digital modulation methods it describes how telephones lines were designed for transferring audible sounds and I’m sure we are all used to the noise of a modem as it syncs with its partner at the other end. The process that is going on here is that the digital computer is using an analog sound, a set of “tones”, to represent data that is being sent. Each tone can be thought of as a symbol of the data it represents. As long as both ends understand what the tone or symbol means, then they can map this to a piece of data.

Now that I have isolated that a symbol is a represenatation of some piece of data that is being sent, I want to dig a bit further and figure out how it works in particular for wireless. Now in case you need a refresher, the last stage before a piece of data is sent over the air is handled by the PCLP (Physical Layer Convergence Protocol). Its function is to take the PSDU (PLCP service data units) and add a PLCP header and preamble to them to form the PPDU. The PPDU is what is actually sent by the PMD over the air. In my mind the PLCP is preparing everything and then instructing the PMD to modulate the data and send it over the air. This means that the PLCP also is responsible for generating the symbols that represent the data that is being sent.

Mathew confirms my supposition and additionally tells me exactly where in 802.11-2007 the process is described (its in clause 17.3.5.7 and an example is given in Annex G). So now with this reading my understanding finally has the process down. Lets go over it;

1. OFDM is a signaling technique that breaks down a stream of data into subcarriers. This is roughly analogous to a chord on a guitar, which is made up of individual notes played together. You can read a much better explanation of this here.
2. Each of those subcarriers is modulated by using BPSK, QPSK, 16-QAM or 64-QAM depending on the rate needed. The preamble in OFDM is always sent using the lowest rate.
3. This data is goes through a couple of other processes to prepare it and then is broken down into groups of 1, 2, 4 or 6 bits  and then converted into complex numbers representing BPSK, QPSK, 16-QAM or 64-QAM constellation points.
4. These complex numbers are a mathematical representation of the original data and as such can be used to form a signal to be sent to the receiver.
5. The complex numbers then are our symbols as each symbol is a mathematical representation of a change in the waveform.

As an aside, the reason for using the constellation diagrams as shown in the 802.11-2007 standard is that this incorporates defined bit patterns, so that is one bit has an error the receiver can correct it because it knows that the bit should be a different value. This is a property of Gray codes.

I hope you have enjoyed my small explanation of what symbols are, please do let me know if I have made any mistakes in this (hopefully not) or if some part requires further explanation.

I went cold turkey with the main complaint people seems to have with Lion, the changed way of scrolling. I actually find it more natural to use with trackpad type devices than the mouse wheel on my Logitech mouse and so am probably going to swap to a Magic Mouse soon because of this. The main thing I was worried about before upgrading was how it would work in my Windows 7 vm and I found that the scolling is swapped there as well which was a relief. It would be a pain to have to constantly remember to swap back and forth.

Mission Control is quite a change from Exposé and Spaces. I think for the most part Apple has done a good job integrating them and the Dashboard. I particularly like the integration with gestures, swiping to move to another desktop just feels so much more a natural thing to do than the old Ctrl-arrow way (even though that still works). I never really used Exposé a lot as I found it difficult to fit into how I use my desktop. I tend to put a separate application on each desktop and swap between them, so Spaces really just extended my desktop for me in much the same way a second monitor would. This is one thing I loved about Spaces and the other thing (which I now miss and hope Apple will bring back) was the wraparound. Basically this means when you get to the last Space, the next move to the right brings you back to the first Space again. I got quickly frustrated when using Mission Control this week because its setup by default to automatically rearrange the desktops. I like things to be in a particular place and having them suddenly shift was something I didn’t like. Let me show you what I mean here. This first shot you can see here that in Desktop 6 Word is running and at the end in Desktop 10 is MarsEdit. This is my desktops in order.

The default configuration of Mission Control is not setup like this, however, and moves the desktops around. This next shot you see that Desktop 6 has moved and also Chrome has moved as well. This annoyed me as I like to know where the desktops are in relation to each other so I can quickly swipe to get to them.

This also turned out to be an easy fix. I additionally wanted to not have the Dashboard to the left and to go back to it as an overlay like in Snow Leopard. Go into Mission Control in the System Preferences;

Deselect the first two to turn off both the Dashboard and the automatically rearranging Spaces and the desktops stay in order. I must admit as upgrades go this was very smooth and relatively troublefree. I say relatively as I have a version of Trend Micro that is pushed to my MacBook Pro by the IT department at my company. I discovered that I was able to generate a very nasty panic in the kernel which pointed to a kext that Trend Micro was loading. This happened when the system was loading Steam for me consistently, but I’ve now had reports form others at my company they have had similar panics. Once I disabled that kext from loading the problem disappeared. Lets hope Trend Micro fixes this soon.

The Asus Slate I acquired has the following specs;

• Processor – Intel Dual Core i5-470UM 1.33GHz
• Screen – 12.1″ 1280×800 AFFS+ Capacitive Touchscreen
• Memory – 4GB DDR3
• Storage – 64GB SSD
• OS – Windows 7 Home Premium x64
• Battery – 34W/hr
• Ports – Mini HDMI, 2xUSB 2.0, SD Card, Mic
• Weight – 2.53lbs
• Wireless – Bluetooth 3.0, 1×1:1 2.4GHz 802.11n (Atheros AR9002WB-1NG)
• Other – 2MP Camera, Wacom EM Digitizer, Bluetooth keyboard, Folio case, Wacom EM Pen

To the above I have added several accessories that I needed for site surveys and to make the tablet more useful to me in the field;

• Energizer Energi To Go XP18000 (18000mAh)
• Rocketfish USB hub
• Shoulder Strap
• Air Magnet Spectrum XT card
• D-Link DWA160 (for Air Magnet Survey)
• 6′ long power adapter cable (Radio Shack)

As you can see from the above picture, I have the tablet attached to a shoulder strap to make it easier to carry. This takes almost all of the weight off my arms and additionally provides a way of resting my hand on the strap when I am making survey marks on the tablet as I walk around. This avoids a problem I discovered the first day I was using it where when I rested my hand on the screen to make a mark the screen misinterpreted it as a finger touch and marked the location where my hand was on the map. The folio cover does not come ready made to have a strap attached to it, so I went to my local hardware store and got some grommets to make a durable hole in the case to attach the strap I had from another bag. I also needed a way to attach the USB hub. This was very easy as the material on the inside of the folio case worked well with a velcro strip I just attached the hub with that. I carry the spare battery, water, camera and other small supplies to keep me going in the secondary case.

The above picture shows the tablet running Air Magnet Survey. As you can see the large screen provides enough space to show all the main parts of the AM Survey interface without cramping it at all. My previous survey netbook was 1024×768 on a 10″ screen, which almost always resulted in not being able to see the whole of the map and the interface being squeezed down.

Here you can see the Air Magnet Spectrum XT software running. As I mentioned above my previous netbook would squeeze the screen down and I was only able to see the FFT graphs from one frequency at a time on the screen. With this screen I am able to comfortably see all the 2.4GHz and 5GHz frequencies and any interfering devices that are detected. You will notice also that the external battery pack is quite small (about 1lb).

I additionally have Microsoft OneNote installed on the tablet and found that this actually makes for a great way to take notes while I am out in the field doing surveys as well. Basically, as I survey I like to take photos and note things down I observe as I walk around. OneNote is just perfect for this task, even converting later my scrawl to typed words. I went to check up the other day on an install that was being done at another site and found that the PDF software I had could be also used to mark up with the pen and make notes as I walked around checking things. Really it was like having a portable notepad to walk around with.

Pluses to the tablet as a survey tool;

• The tablet interface is really really useful for surveys and note taking as you do the survey
• The performance of the tablet is on a par with recent laptops and can easily double as a normal PC with the bluetooth keyboard and a mouse
• Weight is good, still heavier than other smaller tablets but its the tradeoff for the larger screen
• Nice big screen with good resolution
• Well thought out hardware design with well spaced USB ports and SD card to expand storage

Minuses as a survey tool;

• Can sometimes have problems with your hands making survey marks
• Very weak battery life (2 hours max with the above setup)
• Heavy if you are just holding it for several hours in your hand
• Screen is a bit too reflective in sunlight although still visible for outdoor surveys

I have been looking for some time for a way to make my survey’s both easier to do and more efficient and I believe that a tablet based PC with a digitizer pen makes a huge difference from a standard laptop in reaching those goals. As mentioned above I found ways to get around the major weaknesses of the Asus Slate for this purpose and to me having a fully-fledged PC for surveys (with i5 processor) makes a major difference in the field. There are other, purpose built PC tablets that overcome the weaknesses of the Slate (such as Motion Computing’s line) but these tablets are often significantly more expensive.

My conclusion now having spent some time using a digitizer for input during a site survey is that it is far superior to using a mouse for input of survey marks during the survey. Although I could get by with mouse input, its just far quicker to be able to use a pen to mark on the screen where I have walked and this makes my surveys much more efficient. Add to this away of carrying the tablet that avoids tiring your arms and the only thing you have to worry about is getting tired from walking all day long! The major issue with the Asus Slate is its weak battery life, but with an external battery pack this issue disappears. I would recommend this tablet to anyone looking for a reasonably priced and powerful Windows 7 site survey platform.

Looking first at the 330 & 350 APs, they are both smaller and faster and also able to run on 802.af power. Why would an enterprise want to upgrade to 3×3:3 450 Mbps when most of their current client population are not able to make use of all this bandwidth? To answer that I will talk about a feature of the design of these new APs that sets Aerohive well apart from the rest of the pack.

The above picture shows the custom designed, high-powered radio that Aerohive has used for these new APs. Using a high-powered radio gives the AP the ability to have a lower error rate or lower EVM (Error Vector Magnitude) because it is not reaching the peak of its range. The analogy here would be a 10W amplifier vs a 100W amplifier for playing music. If you play 10W on the 10W amp you would more than likely get distorted sound, for the 100W amp you would get a much clearer and error free sound. The next part of the custom design is highlighted above, the PA and LNA. In most designs these components are handled by the same single chip. Aerohive has separated these to improve the receive sensitivity of the LNA (Low Noise Amp). By focusing on this part of the design, they have been able to improve the upstream performance especially for low powered clients (such as tablets and smartphones) by as much as 5dBm at the same data rates as the older HiveAP 320/340 and even improve 5GHz coverage for laptops. In a varied client environment this helps immensely to improve the overall client experience.

This feature aims particularly at the fact that users just expect the WLAN to work the same on all their devices. When the CIO comes to you with his new tablet and doesn’t understand why his laptop works fine in his office for streaming video and the tablet doesn’t seem to be able to stay connected its pretty difficult to tell him its because the Wi-Fi in his new tablet just isn’t all that good. I have seen admins out of frustration just end up putting an AP in the CIOs office to ‘fix’ the problem, which can of course make it worse and not really solve it for everyone else.

One last important point about these APs is that Aerohive has priced them exactly the same as the current HiveAP 320/340 models rather than at a premium as other vendors are doing. This will mean they will compete strongly even with existing 3×3:2 APs from those other vendors.

Moving on the the HiveAP 170, this is Aerohive’s first dedicated outdoor AP. Earlier it was possible to buy an enclosure kit for the HiveAP 340 to use it as an outdoor AP, but this will not be available once the 170 starts shipping. This makes a lot of sense to me and I am glad to see that Aerohive is starting to develop this part of its portfolio. The 170 is a hardened AP able to operate in temperatures from -40 to +55C and are fully waterproof to the IP68 standard. This AP will be available at the end of Q3.

Aerohive has certainly had a number of recent announcements that show they are continuing to up the bar on both features for 802.11n APs (such as built in spectrum analysis) as well as new hardware that increases the pressure on competitors. These newly announced APs show that its not only raw speed that matters, especially when there is a large amount of variability in clients, but also being able to make the lower powered clients such as the single stream tablets and smartphones so prevalent in todays enterprise environment able to perform well. One benefits of moving to a 3 spatial stream AP is that you are increasing the ability of the WLAN to handle more clients per AP as well as more data. With the average number Wi-Fi devices per person set to shoot up from 2 (laptop and smartphone) to many more this will become increasingly an issue.

My take on these new APs is that they are not just ‘more of the same’ to bring Aerohive on a par with their competitors. Its obvious that a considerable amount of thought was put into adding new capabilities to these APs that meet the challenges in todays enterprise WLAN. As we move more and more towards Wi-Fi as being a default access method instead of an optional extra we will find new challenges to making sure that as wireless engineers we can keep the WLAN performing in an optimum way for all clients. I’m glad that Aerohive as a company recognizes those challenges and adapts their products to meet them and makes my job easier at the same time.

The basics of Aerohive’s architecture is that each AP can operate independently and co-operate with other APs around it to provide the features that are required by enterprises to have a scalable and manageable WLAN without compromising performance or redundancy. Essentially the idea is that in order to fulfill the vision of having ubiquitous wireless access you need to build it in a distributed fashion much as the Internet itself has grown. I believe that the current BYOD to work phenomena that we are seeing will show if the idea of a totally centralized architecture is scalable enough to handle the vast increase in wireless traffic that those devices and other future devices will load onto the WLAN.

Aerohive provides all the basic features needed in an enterprise class access point with secure WLAN access, captive portal firewall and VPN capabilities. They have added to this QoS capabilities, IDS and several other useful features all able to run on the AP itself. The new 4.0 release adds to this excellent base some new features that reflect must have capabilities in the marketplace along with some enhancements to their cloud based solution.

The first new feature is to enhance management of Mobile Internet Devices (MIDs). Aerohive points out that there is an explosion of these devices and that they most often have no wired connection and therefore users and executives are demanding to be able to connect to the enterprise WLAN. Other WLAN companies are of course seeing the same thing and developing features to deal with these devices for their customers. My take on this is that we see a difference between developing features to identify when someone is using say an iPad or iPhone on the corporate WPA2-Enterprise network and to put in place access restrictions around those devices and to be able to fully manage all aspects of those devices, to make sure any corporate data on them is secured, in the way that the Mobile Device Management (MDM) companies are doing. Aerohive quite rightly sees their main function as being just managing access and identifying who or what the device is that is getting onto the WLAN. As side benefit of this approach is that particularly for guest networks we will be able to better identify what users are trying to get around corporate security policies with their own devices and to lock those devices down with the firewall policies on the AP before the device gets onto the corporate network.

The next major feature is spectrum analysis at the AP. I have discussed before this feature in my Cisco’s CleanAir vs the Atheros vendors article. Aerohive is also using an Atheros chipset the same as Meraki and Aruba. Like Aruba and Meraki they are able to get the same resolution bandwidth from the Atheros chipset of 357kHz. In their current imp Atheros lementation this works with the Atheros 9200 chipset model APs (the Aerohive 110/120), have a variable sweep time down to 1 second and come with signatures for microwaves, bluetooth, DECT phones, baby monitors and video cameras. Its also useful to note the AP doesn’t have to go offline from serving clients to do this, it permits simultaneous data forwarding and spectrum analysis. The spectrum sweep covers the entire 2.4GHz and 5GHz bands, so as you may have guessed by now the implementation is using a time slicing technique to do this. Aerohive tells me the 320 & 240 APs with the older 9300 chips will be supported in the next feature release sometime this summer.

The major difference here is that Aerohive’s spectrum analysis feature is nada, free, zip cost to existing customers. You get the capability of being able to identify interference sources on a map using common signatures to identify what kind of device it is plus all the things that I love to see as a wireless engineer such as FFT, duty cycle and swept spectrogram graphs. Oh and by the way, because this is all HTML5 based there is no requirement for flash or a PC client so you can quite happily view all this on your iPad as you go to investigate the interference problem. How cool is that? I love it when I get something like this for free and I’m sure that Aerohive’s existing customers will be glad of it too.

Another side note here, Cisco made a big deal of telling us at Wireless Tech Field Day how their sage chip on the AP provided an ability to locate and identify interferers and use this to change channels to avoid the interference. I discussed this with the Aerohive folks who said that their Aerohive Channel Selection Protocol (ACSP) already was factoring in both Wi-Fi and non-Wi-Fi interference sources even before this feature was announced so there isn’t really a big plus for Cisco’s CleanAir in this feature.

The final new feature is primarily of interest to Aerohive resellers as it expands on the cloud platform that Aerohive is concentrating on to provide a reseller the ability to offer a managed service to companies that may lack the resources to deal with WLAN issues. This makes a lot of sense for Aerohive in that they can get a deeper penetration into the small and medium size business market while still being able to offer larger companies with a dedicated wireless team a way of managing a large distributed deployment of APs. This is really one of Aerohive’s strengths in that their architecture fits in very well into the distributed enterprise space which is a challenge at best for the centralized controller companies to handle well.

Aerohive participated in the Wireless Tech Field Day and has followed up the contact made then with giving me access to information about their 4.0 release. At the time of the field day, I felt I had nothing more to add to the already excellent coverage about Aerohive given by other folks. Now with the information about their new release above I am able to write a post that covers some important developments and how Aerohive is constantly innovating and pushing ahead with their wireless vision. This release responds to changes in the marketplace, particularly the large influx of BYOD devices appearing in the enterprise and the challenges involved in providing access to wireless only android devices, iPhones and iPads. Additionally spectrum analysis on the AP is fast becoming a basic feature and Aerohive has smartly added this in without requiring an additional fee or license.

My take on Aerohive is that they are definitely a company with a vision and a belief in the advantages of their architecture over the prevailing centralized controller architecture. I’m glad to work in an IT field that is still new enough to be having game changing companies like Aerohive come in and present a challenge to incumbents that forces them to respond. Take for example the recent announcements from Cisco, Aruba and Motorola. All of them in one form or another announced some kind of more distributed architecture that moves them closer to Aerohive’s vision than the fully centralized play they have been focused on until now. This points to me at least that Aerohive is a company to watch and could easily grow their vision to overtake the smaller WLAN companies and even challenge directly the market share of the biggest players. All in all its an exciting thing to be part of and to be able to talk about. I recommend anyone who is investigating either a WLAN refresh or new installation put Aerohive on their shortlist of companies to evaluate.