Wednesday 11 June, 2014, 09:00 - Spectrum Management
Posted by Administrator
You would have thought that those designing systems that use the radio spectrum would check that the frequencies they planned to use would not cause interference to other systems and equally importantly that they would not suffer interference from other users. Such basic compatibility checks are critical to ensure that different communication systems can inter-operate successfully. So it is a bit of a surprise to find that the designers of the Eurobalise, a technology that forms part of the European Rail Traffic Management System and whose purpose is to assist in the control of train movements (to control their movement and help them know where they are) has chosen a frequency which fails these simple safeguards.Posted by Administrator
![ertms eurobalise](images/ertms-eurobalise.jpg)
Where they have gone wrong is to use a frequency for transferring information between the Eurobalise and the train that is in a European broadcast band!
![yellow eurobalise](images/yellow-eurobalise.jpg)
Do any such transmissions exist? According to short-wave.info, the BBC and Korean broadcaster KBC use a frequency of 3955 kHz on a daily basis, from the BBC's transmitter at Woofferton, Shropshire. If you click on the link (which will take you to Google maps) you will notice that running alongside the village of Woofferton is a grey line - a railway!
But surely fears of interference are unfounded and just another example of scare tactics by spectrum managers bent on safeguarding their highly paid jobs. Sadly not... It appears that the transmissions from Woofferton have been disrupting trains between Leominster and Ludlow! According to the article in the Hereford Times, Network Rail, the organisation responsible for operating the rail infrastructure in the UK, claim:
while the interference does not pose a risk to the safe operation of the railway, it has been stopping trains en-route.
Oops!
![soldiers stop train](images/soldiers-stop-train.jpg)
Maybe, given that there is only one potential location where the choice of frequency, and proximity to a broadcast transmitter, could be a problem, the designers did do their homework after all and decided that it was alright for occasional problems to arise. Maybe. Then again, the other frequencies used by the Eurobalise include a military band and the middle of the 27 MHz Citizens Band!
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![bbc watchdog presenters](images/bbc-watchdog-presenters.jpg)
Train Operator | Connection Available | Time To Download A File | Price |
---|---|---|---|
Cross-Country | 96.7% | 39 seconds | £2 for 1 hour, £8 for 24 hours |
East Coast | 79.4% | 13 seconds | £4.95 for 1 hour, £9.95 for 24 hours |
Virgin Trains | 82.2% | 112 seconds | £4 for 1 hour, £8 for 24 hours |
WiFi services on trains are provided by using antennas on the roof of the trains to connect to mobile networks. These mobile internet connections are then shared amongst all the WiFi users on a train. Companies such as Icomera and Nomad Digital provide boxes that enable multiple mobile internet connections to be combined together to increase the speed of the connection as a single 3G or 4G connection is not going to cut it when shared between multiple WiFi users.
![icomera rail solution](images/icomera-rail-solution.jpg)
In the programme, self-proclaimed IT Guru Adrian Mars then goes on to explain that the problem with the East Coast and Virgin services is that they make use of the signal from just one mobile operator and share that amongst all the WiFi users on the train, whereas Cross-Country use connections from multiple operators. This would explain why the time for which a connection was available on Cross-Country’s WiFi service was so much higher than on the other two, as they can make use of the overlapping coverage provided by multiple operators.
![wifi on board train](images/wifi-on-board-train.jpg)
Where the Watchdog’s IT guru did go astray was to suggest that it might be better for train passengers to rely on the connection to their own mobile phone for internet rather than the on-train WiFi. Why is this wrong? There are two main reasons. Firstly, the antennas used by the on-train WiFi systems are mounted on the train roof, whereas your phone will be lower down, inside the carriage. A previous Wireless Waffle article highlighted the need to get high to improve reception and the signal on the roof of the train will be bigger than that inside by dint of this fact alone.
But there is a much bigger problem… trains are typically constructed of metal. Some, including Virgin Trains’ Pendolino trains, have metallised windows. Passengers are thus enclosed in a Faraday cage which will do a grand job of stopping any signals on the outside of the carriages from making their way into the carriages. According to a paper written by consultants Mott MacDonald for Ofcom:
In modern trains the attenaution[sic] can be up to -30dB.
This means that of the signal presented to the outside of the carriage, only one thousandth of it makes it inside the carriage. Add this immense loss to the difference in height between the roof-mounted antenna and you sat in the carriage and it becomes apparent why using your own phone is highly unlikely to yield a better connection than that available through the on-train WiFi.
![sandwiches or wifi train](images/sandwiches-or-wifi-train.jpg)
![poor cousin](images/poor-cousin.jpg)
As digital switch-over has taken hold, terrestrial broadcasting has had a repreive and is now able to offer true multi-channel television. Where previously it was only possible to broadcast a single television station on a single frequency, that frequency can now hold 10 or more standard definition (SD) channels, or 4 or 5 HD channels. Where there may have been 6 analogue stations on air, there can now be upwards of 60 stations. In many cases, 60 stations is enough for the average viewer and the bouquets of channels offered on cable or satellite may now begin to seem expensive compared to free-to-air DTT. In many countries the draw of cable and satellite TV is no longer the sheer variety of channels available, but the premium content that is on offer. Pay-TV services offering sport and movies continue to be popular, but such premium content is not usually available on DTT. Nonetheless, for many viewers DTT is perfectly sufficient.
![biting at the heels](images/biting-at-the-heels.jpg)
To be able to see the difference that UHD makes compared to standard HD, a very large television set is needed (42 inches or greater) and it could therefore be argued that UHD will always be a niche product. Then again, many broadcasters believed that HD would be a niche, but it is becoming the de facto standard and average television sizes are on the increase.
Technically speaking, UHD requires a bit-rate of around 20 Mbps. Whilst such bit rates are relatively easy for cable and satellite networks to deliver, broadcasting UHD over DTT would require at least half of a DVB-T2 multiplex, and the most advanced video (HEVC) codecs. In practice this means that were a terrestrial frequency currently carries 10 or more SD, or 4 or 5 HD channels, it might, at best, be able to offer 2 UHD channels.
![uhd samsung tv](images/uhd-samsung-tv.jpg)
So where does that leave DTT? Arguably, within a few years, it will once again be unable to compete with the sheer girth of the bandwidth pipe that will be provided by cable and satellite networks. It's probably worth noting at this point that most IP-based video services (with the possible exception of those delivered by fiber-to-the-home) will also be unable to deliver live SHV content. This time there will be no reprieve for DTT as it simply does not have the capacity to deliver these higher definition services.
What is therefore to be done with DTT? Is it necessary to provide continued public service, universal access, free-to-air services that were the drivers for the original terrestrial television networks? Is its role to provide increased local content which might be uneconomic to broadcast over wide areas? Should it be used to deliver broadcast content to mobile devices where it has more than sufficient capacity to provide the resolution needed for smaller screens? Or, should it be turned off completely, and the spectrum it occupies be given over to something or someone else?
In countries where cable and satellite penetration is already high, there is arguably nothing much to lose by switching DTT off. In Germany, for example, RTL have already withdrawn from the DTT platform and there is talk of turning off the service completely. In countries that have not yet made the switch-over, it might be more cost effective to make the digital switch-over one that migrates to satellite (and cable where available) than to invest in soon-to-be-obsolete DTT transmitters.
![tv service gone](images/tv-service-gone.gif)
![esoa dunces](images/esoa-dunces.jpg)
But... even a 10th grade student could complete the sum that is behind the ITU data forecasts and realise that the axis should have read 'PB' all along (and therefore that the internal inconsistencies are not fixed and that the data in the ITU and RealWireless models is still hundreds of times too large). Here, for you to try, are the values - taken from the ITU's 'Speculator' model - and the maths you need to apply. The values are for 'SC12 SE2' which represents people using 'high multimedia' services in urban offices and is with the ITU model in its 'low market' market setting (it has a higher one too).
User density: | 120,975 users per km² |
Session arrival rate per user: | 3.3 arrivals per hour per user |
Mean service bit rate: | 13.83 Mbps |
Average session duration: | 81 seconds per session |
Now for the maths...
- First, multiply the first two numbers to get 'sessions per hour per km²'. (120,975 × 3.3 = 399,217.5)
- Then multiply this by the average session duration to get 'seconds of traffic per hour per km²'. (399,217.5 × 81 = 32,336,617.5)
- Then multiply by the mean bit rate to get 'Megabits of traffic per hour per km²'. (32,336,617.5 × 13.83 = 447,215,420)
- To make the numbers more managable, divide by 8 to get from bits to bytes, then by 1,000,000 to get from Megabytes to Terabytes (447,215,420 ÷ 8,000,000 = 55.9)
The number of days in a month is relatively easy to work out, it's 30.4 on average (365.25 ÷ 12). So monthly traffic per square km would be 559 × 30.4 = 16,994 TeraBytes per month per km².
![ofcom maths skills](images/ofcom-maths-skills.jpg)
![RW report page 085](images/RW_report_page_085.jpg)
![whos stupid now](images/whos-stupid-now.jpg)
Surely there are people at Ofcom who own a calculator, have a GCSE in maths, and possess a modicum of professionalism such that they would want to check the facts before blithely allowing their suppliers to fob them off with an 'oops, we mis-labelled an axis' argument. Presumably they thought that it was ESOA who couldn't handle a calculator properly.
Now who looks silly?