A quick guide to 5GHz in the UK - Part 2
A More In Depth Look At The Available 5GHz Channels
Below is a list of all the available channels in the 5GHz spectrum along with the central frequencies for each channel and the UK usage rules set by Ofcom. If it states "Indoors" then it means that access point hardware may operate using the channel in question as long as it remains inside a building. DFS means that the unit must be operating with Dynamic frequency Selection enabled and TPC means it must also be operating under Transmit Power Control parameters.
Why Do the Usable 5GHz Channel Numbers Increase By 4 Each Time?
In the 5GHz spectrum each channel is 5MHz wide, however only every 4th channel is usable. The table above lists these channels along with the centre frequency for each one. The reason for the gaps is to provide adequate spacing in order to prevent any channel overlap. For example the centre frequency of Channel 40 in the 5GHz spectrum is 5200MHz (or 5.2GHz). Now to make this a 20MHz channel you would make this frequency the central point of the channel and extend 10MHz in either direction above and below this. This gives you the range of 5190MHz-5210MHz for the 20Mhz wide channel. Now as you can see by extending the channel 10MHz in each direction you have used up 2 x 5MHz wide channels in each direction (what would have been Channels 38, 39, 41, 42). This is why these channels are not considered usable and typically aren't selectable on most hardware. If you were to use them, you could end up with co-channel interference.
Band A (Channels 36 – 64) - More Info You Need To Know
As we covered in my last blog, Band A is 200MHz wide and offer 8 channels at 20MHz or 4 channels at 40MHz. However, Band A breaks down further into 2 bands. These are UNII-1 Lower and UNII-2 Middle.
Band A UNII-1 Lower (Channels 36 – 48)
This set of 4 channels allows for 4 non overlapping 20MHz channels or 2 non overlapping 40MHz channels. As long as you keep to the 200mW output limit all is groovy.
Band A UNII-2 Middle (Channels 52 – 64)
This set of 4 channels also allows for 4 non overlapping 20MHz channels or 2 non overlapping 40MHz channels however DFS and TPC need to be switched on in order to remain compliant with Ofcom regulations. Infact many hardware vendors will force you to have these settings switched on within the firmware when using these channels
Dynamic Frequency Selection (DFS)
Dynamic Frequency Selection allows 5GHz hardware to operate within the same frequency space used by radar such as military and weather. If DFS is enabled on an access point (or bridge unit in access point mode) then the device when it first selects a channel will initially listen for a short period of time to ensure that it is not picking up any radar signals. If radar is detected then a different channel will be used. If the airwaves look clear on that channel then it will remain on the channel. However if radar is later detected then the device will shift to another channel. The access point will use protocols specified in 802.11h to broadcast to the attached stations that a channel switch is taking place in order to allow them to react gracefully. Once a device has moved away from a channel which radar has been detected on then it will stay clear of that channel for a defined amount of time.
40MHz Wide Channels
40MHz wide channels are created by bonding together 2 x 20MHz channels. This is due to some devices only supporting a 20MHz channel. The 2 channels are known as the Primary and Secondary channels and they must be adjacent channels. The secondary channel may be above or below the primary. If a device only supports 20MHz wide channels then it will connect to the primary channel. This primary channel is what you will select within your hardware. The secondary channel can be above or below the primary (as long as the primary isnt on the edge of the usable spectrum). This is sometimes specified as "Channel 40 +1" where the +1 is substituted for a -1 if the secondary channel is below the primary.
High Density Environments - 5GHz Is Your Friend!
As you probably already knows, 2.4GHz only allows for 3 channels without an overlap. These are 1, 6 and 11. This can cause a problem in high density environments as the moment you have more than 3 access points transmitting 2.4GHz signals in a confined area, you will most likely end up with some level of co-channel interference. Co-channel interference occurs when 2 devices attempt to transmit at the same time as each other. Wireless is a shared medium and thus only 1 device may reliably use the channel space at any one time. If any devices on the same channel attempt to transmit at the same time then it can result in a collision and they then both devices have to back off and try again after a randomized amount of time.
To demonstrate why 5GHz is pretty awesome; imagine 500 people in a single room together all using wireless devices. Now lets take an enterprise level access point capable of sensibly handling 50 clients on its 2.4GHz radio. With 3 of these in a single room (channels 1, 6 and 11) you have no channel overlap and the capacity for 150 clients. But what about the other 350 you ask? Well no problem, lets change these 3 access points for dual radio 2.4/5GHz access points. Now each 5GHz radio can take on 50 clients too... that results in 300 clients now being looked after by the network. But wait, there are still 200 clients not being looked after... The problem is we have used the 3 x non-overlapping 2.4GHz channels so can't really use them again as it is a single room with no walls to attenuate the signal. Have no fear though! This is where the larger number of usable 5GHz channels comes in handy. By adding another 4 access points which only have their 5GHz radios switched on you can now handle all 500 clients and haven\'t reused any channels anywhere in the room (3 access point radios on 2.4GHz and 7 radios on 5GHz). Hurrah!
Now the above is just a simple example, and assumes that all the devices being used are dual band devices that support both 2.4GHz and 5GHz. Also, in reality with some clever design incorporating a mixture of cleverly placed directional access points, the right power levels and various other tricks of the trade you may be able to re-use some of your 2.4GHz channels without it being too detrimental but hopefully you get the idea.
Outdoor Point To Point Wireless - 5GHz Is Your Friend Yet Again!
Another situation where 5GHz is your friend is for outdoor point to point links. Due to the high output power allowed (up to 4W), 5GHz is an excellent choice for outdoor point to point links. To demonstrate this take a look at the table below which I shamelessly stole from somewhere (I forget where now as its been sat in my evernote for a year or 2).
The above are just rough figures and assume a 100mW radio is being used. The "effective gain" is the gain of the antenna once any signal loss from cables and connectors has been taken into account. As we know that 5GHz can be legally used to transmit up to 10db more than 2.4GHz in Band A or B and 16db more in band C you can see that in Band A and B the 5GHz signal is able to travel around 15x further than 2.4GHz and in Band C a massive amount further.
Another benefit of using 5GHz for wireless bridges is that it is also less likely to suffer from interference than 2.4GHz as most household networks are 2.4GHz and are spewing out these signals all over the place where as 5GHz in the home is less common and if it is present is going to be on the lower transmit power offered by band A or B leaving band C free to use. Another benefit of 5GHz is that it scatters and reflects better than 2.4GHz which can be useful for near lines of sight situations.
So as you can see 5GHz is a useful tool to have in your wireless arsenal :)
So that concludes Part 2 of my Quick Guide To 5GHz, thanks for reading!