Tag: Radio Spectrum

Public wifi – why use more radio spectrum than you need?

Here’s the second of my series of little rants about poor public wifi – this time, why use more spectrum than you need?

Using the Wifi Explorer tool, I was recently checking out a venue with a modern public wifi installation. Here’s what the 5GHz spectrum looked like:

crowded_5Ghz_spectrumI’ve redacted the SSIDs so that we aren’t naming names and we’re hopefully saving face.

You’re probably thinking “They aren’t using much spectrum at all“, right? All their access points are all clustered down on 4 channels – that in itself not being a good idea.

Note that they are using “wide” 40MHz channels – the signal from each access point is occupying two standard 20MHz channels. Networks are usually setup like this to increase the amount of available bandwidth, by using multiple signals on multiple radio channels at once between the base station and the client.

This was also a brand new installation, and the access points were supporting 802.11a, n and ac, and the Wifi Explorer tool reported each AP could support a theoretical speed of 540Mb/sec.

What if I told you the access circuit feeding this public wifi network, and therefore the most bandwidth available to any single client, was 100Mb/sec?

Vanilla 802.11a would give a maximum data rate of 54Mb/sec (probably about 30Mb/sec usable payload) on a single channel, this could be 150 or 300Mb/sec with 802.11n (MIMO). Plenty for getting to that 100Mb.

Thus rather than having as many as 4 overlapping access points sharing the same channels, this could be reduced significantly by only using 20MHz channels. This would result in less radio congestion (fewer clients on the same frequency), and probably wouldn’t negatively effect access speeds for the clients on the network.

There’s also the question of why all 6 access points visible in this sweep are spread across just two 40MHz channels.

The main reason is that DFS (Dynamic Frequency Selection) and TPC (Transmit Power Control) is required for any of the channels highlighted with blue numbers in the chart above – it’s also known as “Radar Detection”, because some radar operates in these channels. An access point running DFS will “listen” first for any radar signals before choosing an unoccupied channel and advertising the network SSID. If it hears any radar transmissions, it will shut down and move channel.

Sure, avoiding the DFS mandatory channels gives more predictability in your channel use, and means you aren’t affected by an access point needing to go off air.

However, an option in designing the network could be to use the DFS mandatory channels to increase available spectrum, but strategically place access points on non-DFS channels spatially in between those using DFS, getting away from the “listen on startup” phase (e.g. if there’s a need to reset an access point), or from the service suddenly going offline because of radar detection.

Also, remember that this is an indoor deployment and well inside a building. The chances of encountering radar interference are relatively low. I don’t recall seeing a problem using DFS when I’ve deployed temporary networks for meetings.

The other thing to note is that this deployment is not using a controller-based architecture. It is made of access points which can signal control messages between each other, but each access point maintains effectively it’s own view of the world. (Those of you in the Wifi space can now probably work out who I’m talking about.)

Is the above configuration using so few channels, and using them unwisely considering the target bandwidth actually available to the wifi clients, just asking for trouble once a few hundred users show up?

 

UK 4G LTE Launch and the scramble for spectrum

So, the next path on the road to fast mobile data, 4G LTE finally launches in the UK, after much barracking from competitors, on the “EE” network (the combined Orange and T-Mobile brand).

It’s only available in a handful of markets at the moment, and the BBC’s tech correspondent, Rory Cellan-Jones, did many articles for TV and Radio yesterday, while conducting countless speedtests, which he has extensively blogged about.

Some of the comments have been that it’s no better in terms of speed than a good 3G service in some circumstances, while others complain about the monthly cost of the contracts.

Get locked-in to 4G(EE)?

The initial cost for the early adopters was always going to attract a premium, and those who want it will be prepared to pay for it. It’s also worth noting that there are no “all you can eat” data plans offered on EE’s 4G service. Everything comes with an allowance, and anything above that has to be bought as “extra”.

The most concerning thing as far as the commercial package goes are the minimum contract terms.

12 months appears to be the absolute minimum (SIM only), while 18 months seems to be the offering if you need a device (be it a phone, dongle or MiFi), and 24 month contracts are also being offered.

Pay As You Go is not being offered on EE’s 4G service (as yet), probably because they’ve no incentive to, because there’s no competition.

Are EE trying to make the most of the headstart they have over competitors 3, O2 and Voda and capture those early adopters?

Penetrating matters?

Rory Cellan-Jones referred in his blog about problems with reduced performance when in buildings.

A number of factors affect the propagation of radio waves and how well they penetrate buildings and other obstacles, such as the nature of the building’s construction (for instance a building which exhibits the properties of a Faraday Cage would block radio signals, or attenuate them to the point of being useless), but also the frequency of the radio signal.

Longer wavelengths (lower frequencies) can travel further and are less impacted by having to pass through walls. I’m sure there’s an xkcd on this somewhere, but the best I can find is this….

Electromagnetic Spectrum according to xkcd

The reason EE were able to get a steal on the other mobile phone companies was because OFCOM (the UK regulator, who handle radio spectrum licensing for the Nation) allowed EE to “refarm” (repurpose) some of their existing allocated frequency, previously used for 2G (GSM), and convert it to support 4G. The 2G spectrum available to EE was in the 1800 Mhz range, as that was the 2G spectrum allocated to EE’s constituent companies, Orange and T-Mobile.

Now, 1800 does penetrate buildings, but not as well as the 900 Mhz which are the 2G spectrum allocated to Voda and O2.

Voda are apparently applying to OFCOM for authority to refarm their 900 Mhz spectrum for 4G LTE. Now, this would give a 4G service which had good propagation properties (i.e. travel further from the mast) and better building penetration. Glossing over (non-)availability of devices which talk LTE in the 900 Mhz spectrum, could actually be good for extra-urban/semi-rural areas which are broadband not-spots?

Well, yes, but it might cause problems in dense urban areas where the device density is so high it’s necessary to have a large number of small cells, in order to limit the number of devices associated with a single cell to a manageable amount – each cell can only deal with a finite number of client devices. This is already the case in places suce as city centres, music venues and the like.

Ideally, a single network would have a situation whereby you have a high density of smaller cells (micro- and femto-cells) running on the higher frequency range to intentially limit  (and therefore number of connected devices) it’s reach in very dense urban areas such as city centres, and a lower density of large cells (known as macro-cells) running on lower frequencies to cover less built-up areas and possibly better manage building penetration.

But, that doesn’t fit with our current model of how spectrum is licensed in the UK (and much of the rest of the world).

Could the system of spectrum allocation and use be changed?

One option could be for the mobile operators to all get together and agree to co-operate, effectively exchanging bits of spectrum so that they have the most appropriate bit of spectrum allocated to each base station. But this would involve fierce competitors to get to together and agree, so there would have to be something in it for them, the best incentive being cost savings. This is happening to a limited extent now.

The more drastic approach could be for OFCOM to decouple the operation of base stations (aka cell towers) from the provision of service – effectively moving the radio part of the service to a wholesale model. Right now, providing the consumer service is tightly coupled to building and operating the radio infrastructure, the notable exception being the MVNOs such as Virgin (among others), who don’t own any radio infrastructure, but sell a service provided over one of the main four.

It wouldn’t affect who the man in the street buys his phone service from – it could even increase consumer choice by allowing further new entrants into the market, beyond the MVNO model – but it could result in better use of spectrum which is, after all, a finite resource.

Either model could ensure that regardless of who is providing the consumer with service, the most appropriate bit of radio spectrum is used to service them, depending on where they are and which base stations their device can associate with.