WiFi Basics Part 1: Frequencies and Channels
In 1947, the International Telecommunication Union (ITU), a UN agency, designated certain bands of the RF spectrum as ISM. The idea was to have an international standard of frequencies that were reserved for Industrial, Scientific, and Medical equipment. Ironically, telecommunications was not the original purpose.
Before we discuss any particulars, I just want to note that this article will be primarily based on the US regulatory domain. Different regulatory domains have specific requirements regarding transmitted frequencies and power settings.
Although there are twelve ISM bands, for now weâ€™ll concentrate on just two of them. Most people refer to them as the 2.4GHz and 5GHz bands.
Letâ€™s start with the 2.4GHz ISM band (2.400GHz-2.500GHz). This small, crowded set of frequencies is by far the most heavily used despite the fact that the majority of laptops, smartphones, and tablets come equipped with dual radios in the last few years.
The reason for this is physics. The general rule of thumb is that the lower the frequency, the longer the propagation and the better the penetration. Here are 2 real-world examples:
- You are sitting in your car at a stop light. Someone pulls up next to you with their radio blaring loudly. If you donâ€™t like their taste in music, you might roll up your window. Some of that sound is reduced, but youâ€™ll probably still hear that bass (low frequency) very well.
- Another example is a very specialized radio system used by the military to communicate with submerged submarines. It is called ELF (Extremely Low Frequency) and has a wavelength so long that the antenna required can be miles long.
Okay, so now we know that lower frequencies offer a larger coverage area and better signal penetration through obstacles. If the 2.4GHz band is so great, do we really need 5GHz? The answer is yes.
2.4 GHz Limitations
Only 3 non-overlapping channels. The 2.4GHz band is divided into 14 channels, most of them 5MHz apart. That 5MHz spacing was no problem in the very early days of wireless use. The technology at the time was limited to around 1Mpbs of throughput. Then in the late 1990s, 802.11b with its spread spectrum technology became the standard.
The advantage is that it could achieve 11Mbps throughput. The downside was that it used 22MHz of the spectrum. So if, for example, you are using channel 6, then at the very least, channels 4,5,7, and 8 are unusable. This limitation still applies today, whether you are using older 802.11g or the newest 802.11n(2.4GHz) WiFi equipment. This is one of the most significant limitations.
It is overcrowded. This goes hand-in-hand with the previous limitation. Although the 802.11a (5GHz) and 802.11b (2.4GHz) standards were released, due to cost differences, only 802.11b saw widespread adoption at the time. The result is that, even today the majority of WiFi traffic uses the 2.4GHz band.
Stand on a downtown street corner of any decent sized city and run WiFi scanning software. In all likelihood, you will see at least 20 signals; Iâ€™ve seen over 40 in downtown Washington DC. Now remember that all 40 of these signals are operating on just 3 channels.
It is an ISM band, and by definition, open to other types of devices. There are many non-WiFi devices that have the potential to cause interference: microwave ovens, cordless phones, Bluetooth devices, baby monitors, video cameras, garage door openers, etc.
These things can severely disrupt your network and are extremely difficult to detect without specialized spectrum analyzer hardware and software.
Next, we will talk about the 5GHz band
In addition to the standard ISM band (5.725-7.825GHz), the FCC has added spectrum from the UNII (Unlicensed National Information Infrastructure) bands to spur the use of wireless technology. Also note that the various regulatory agencies are working through the process of adding 195MHz of spectrum available in the 5GHz range.
As we discussed earlier, higher frequencies do not propagate as far nor do they penetrate obstacles as well. Letâ€™s compare with the 2.4GHz band in real world use.
- In the open air, 5GHz signals will cover about 1/3 to Â½ the distance.
- Useable signal quality will degrade greatly after punching through 1 interior wall, as opposed to about 3 walls for 2.4GHz signals.
This leads to another less obvious disadvantage, cost. The price tag to deploy a 5GHz (well, dual band) WiFi network of any size is at least 2.5 times the cost of an equivalent 2.4 only network. You need a lot more access points, typically 2.5x to 3x. Add in the cabling, licensing, maintenance, etc.
Some channels are shared use. Specific channels are designated as DFS, Dynamic Frequency Selection. These channels, located in the UNII-2 and -2extended bands share the spectrum with some radar systems, mostly in Europe.
Because of this, the WiFi system must be designed to scan for radar pulses before using the particular channels. Of course if radar pulses are detected, it immediately disables those channels affected.
As you can plainly see in the chart above, there are a lot more than 3 channels. Also notice the channel spacing â€“ at least 20MHz. This means that no channels overlap; all are useable simultaneously.
Much less crowded. Unlike 2.4GHz devices, 5GHz devices have only become widely distributed within the last five or six years. Thereâ€™s also the fact that it takes time to upgrade an infrastructure. There are still a significant number of businesses running 2.4 only networks.
Very few interference sources. Except for the aforementioned radar on certain frequencies, the chances of non-WiFi interference is extremely low.
Higher throughput capability. Theoretically both are capable of 600Mbps. However in practice, that is not the case because much of that enhanced capability depends on channel bonding. This is an optional mode where the device uses multiple adjacent channels simultaneously. Remember earlier how we discussed the fact that there are only 3 non-overlapping channels available in the 2.4 band?
This technique would use 2 of those 3. So, not only would you really irritate your neighbors, all the co-channel and adjacent-channel interference would probably make your network perform much worse. To summarize, DO NOT enable channel bonding in the 2.4 band.
There are some newer standards coming out that make use of other frequency ranges. We will discuss them in an upcoming article.
For a pdf of the FCCâ€™s official spectrum map: http://www.ntia.doc.gov/osmhome/allochrt.PDF
I hope you enjoyed this article. I think my next one will be a discussion about the different standards.
If you have a topic suggestion relating to WiFi or general networking, let me know in the comments.Advertisement