Friday, 16 August 2013

How Much Air-Time Do Beacons Actually Burn?

It’s a well known rule of thumb when designing WiFi networks that you need to try to keep the number of SSIDs broadcast by your wireless network  down to a ‘reasonable’ number. In this article, I take a look at how much of an issue SSIDs (and their beacons) are in consuming valuable wireless air-time.

Generally, it’s recommended to keep the number of SSIDs below around 5 (ish).

The reason for keeping the number of SSIDs to a minimum is that each SSID is advertised using a type of management frame called a ‘beacon’.  Beacons are generally sent 10 times per second for each SSID on the wireless network. Therefore, if you have 10 SSIDs, they will each be advertised 10 times per second, giving us 100 beacons per second.

Air-time is a finite resource – there is only so much data that can be transferred across the air over a period of one second. If a large chunk of air-time is being consumed by SSID beacons, then that doesn’t leave a whole lot of time remaining for actual user data to travel over the air (which is the whole point of having a wireless network!).

I have previously heard statements from various wireless engineers along the lines of up to 50% of available air-time being consumed by beacons once you have 6 or 7 SSIDs being broadcast by a network. I’ve taken this information on face-value and never really thought too much about it.

However, this evening I found myself in a hotel room with some time on my hands, a Cisco WLC, a Cisco AP and a copy of Metageek Eye PA. I thought it was time to test the ‘conventional wisdom’.

My approach was simple: I would set up my AP on channel 11 (2.4GHz) and capture all frames using Eye PA. I would vary the number of SSIDs being broadcast and monitor the results.

I would also vary the lowest mandatory speed supported by the 2.4GHz network between 1Mbps, 11Mbps and 54Mbps. Beacons are sent at the lowest mandatory speed that is configured for a wireless network. Therefore, if 1Mbps is the lowest mandatory speed, beacons are sent at 1Mbps (and hence are a lot slower and consume more air-time)

To determine how much actual air-time is being consumed by beacons, I would use Eye PA’s filtering capabilities to remove all frames except beacon frames, and remove any other local interfering SSID traffic (i.e. the pesky hotel WiFi on the same channel!). This would leave me with just the beacon frames from my AP:

Eye PA Filtering Beacon Frame

Eye PA allows you to select a period of one second of the filtered traffic that you have captured, and also shows the amount of air-time those frames consumed in that period:

I just then applied some simple maths to work out how much time the beacons frames consumed over a period of one second.
I then tabulated the results:

Number SSIDs Broadcast
Lowest Mandatory Speed
Beacons AirTime Over 1 Sec (mS)
Percentage AirTime Used by Beacons

The results show pretty much what I expected, but I was surprised by how little time the beacons consumed, particularly once the lowest mandatory speed is ramped up to 54Mbps. They certainly don’t support the information that had been imparted to me regarding 7 SSIDs consuming 50% of all air-time.

You can clearly see the effect of adding more SSIDs (and consequently more beacons). As more SSIDs are added, more air-time is devoted to beacon traffic. This is a bad thing, if it becomes a significant chunk of your air-time.

You can also clearly see the effect of increasing the lowest mandatory speed supported by the wireless network. Once you increase it to 54Mbps, even with 15 SSIDs, you are only consuming 2% of the available air-space.

I suspect that the conventional wisdom of keeping your SSID numbers down to below 5 is founded on the assumption that many wireless networks are going to be installed using default settings. Often, default settings will configure the lowest mandatory speed to one of the lower 802.11b speeds, which could then make significant numbers of SSIDs an issue.

For me there are several lessons to take away:

  • Verify what the defaults of a system are – what is the lowest mandatory speed configured on your system out of the box?
  • Increasing the lowest mandatory speed on a wireless network is going to increase the efficiency (and hence throughput) of your wireless network significantly – less time will be given over to beacon traffic
  • The ‘less than 5 SSIDs’ rule may be a good starting point, but on a well engineered network, it may not be as relevant as it used to do, especially in the presence of modern wireless clients which do not need to support the lower, legacy speeds of 802.11b/g.

A word of caution though before making any wholesale changes to your network. Make sure you do not have any older wireless clients that need to be able to connect to the network at the slower/legacy speeds. Clients need to be able to initially associate to a wireless network at the lowest mandatory speed supported by a wireless network. If you have older devices that are not in areas that have good coverage, they may not be able to associate at a higher speed and will not be able to join the wireless network in those areas. It is probably worth testing the effect of any changes you make carefully.

I’d welcome any feedback on my testing. If there are any flaws in my logic or testing or there are other considerations I may have missed, then please feel free to drop me a note or comment.

Wednesday, 14 August 2013

5GHz Unlicensed WiFi Channels in the UK - White Paper

(Note: this white paper has been superseded with this new updated version)

I put together a few articles a few months ago talking about how the unlicensed 5GHz band is used for WiFi here in the UK.

I thought it might be a good idea to consolidate all of the information that I found in to one place, so that people researching the topic could find and digest it more easily.

Therefore I put together a white paper about how 5GHz is used for WiFi here in the UK. You can download it from here.

There will no doubt be errors, omissions and other facts that folks would like to suggest. So, please feel free to drop me a note and I'll update this document from time to time to improve the quality of information that it contains.


Download the document from the following sources:
  • Scribd
  • Google Docs

Monday, 10 June 2013

802.11ad - Just for Home Cinema...Right?

One of the things I love about Twitter is that once in a while you stumble across something that completely shifts your view of the world. I spotted this little nugget (posted by @wifichef) a couple of days ago, which made me significantly re-assess my view of the application of 802.11ad technology:
"A deeper dive in to High Capacity WLANs:"
Expecting another deep dive in to 802.11n high density WLANs (...small cell sizes, using 5GHz, band steering, disabling lower speeds etc.) I clicked through the link to see if I could find any new information. However, I was completely surprised to find myself looking at a whitepaper discussing the merits of 802.11ad! In fact, it actually highlighted the disadvantages of a traditional 'legacy' WiFi network - this had me hooked :)

I must admit that I had dismissed 802.11ad (which uses the 60GHz band) as a niche technology that I'd probably hardly ever see in the Enterprise environments that I tend to work in. (I must admit to having only a superficial knowledge of the 802.11ad standard though). After all, what use is a wireless technology that can only travel a few feet, particularly when you have a building of maybe hundreds or thousands of people? How could we ever design usable WiFi networks with cell sizes that small!? You might see it on some consumer-grade wireless routers, perhaps for movie streaming in the home. Beyond that...nah, I just didn't see  it taking off.

But, after reviewing this whitepaper from Wilocity, I had to pause and re-assess my view of 802.11ad. It details testing done in high density client environments using 802.11ad-capable latops. They posted some very impressive link speed and SNR results for a 'high density' of 802.11ad stations in close proximity. This was achieved by deploying a number of 802.11ad laptops, each of which had an 802.11ad wireless docking station next to it on the desk. Each desk had an equal number of laptops and docking stations around the edges of the desk (as you might expect to see in a typical office).

My first reaction was: "Well why would you do that? If each docking station is cabled anyhow, why not just pull the network cable out of the docking station and connect it in to the laptop!?". But, after some thought, I started to consider how this technology might advance in the future... 

It looks like we only have per-laptop 802.11ad docking stations at the moment (which obviously doesn't save you much in cabling, assuming each docking station is cabled). But,  perhaps wireless equipment vendors might be able to manufacture per-desk 802.11ad access points in the future, for just the users occupying that desk? If that could be coupled with fast transition to existing 802.11n/ac office-wide networks, then as users roam about the office, they could hop between 802.11n/ac & 802.11ad networks. This would provide high speed 802.11ad at a user's desk, with lower speed 802.11n/ac as they use the traditional office wide network whilst moving between desks and rooms.

These super-small cells could make the planning of high-capacity wireless networks much easier in office-type environments. Just put an AP on each desk, together with a token blanket of traditional (ceiling-mounted) WiFi coverage to provide slower-speed transit connectivity as users move around. That would certainly make HD wireless surveying a lot more straightforward!

I have no idea how much of this will be technically possible, but I could certainly see the attraction of this type of super-small cell. It is much more akin to the provision of desktop hubs or switches that regular wired network engineers could get their head around, making support and planning much easier than traditional WiFi networks. This would obviously require more cable drops around the office, but there may be some environments where the additional cabling is worth the trade-off for the additional capacity and ease of deployment.

It is going to be fascinating to see how 802.11ad evolves and whether it could introduce yet another paradigm shift around WiFi networking.