WiFi is a trademark term for the IEEE 802.11 family standards. The first 802.11 standard was released in 1997 and it was providing up to 2 Mbps wireless link speed. It was slow, but it was the first technology which has allowed to connect several computers in the local area network without cables. The IEEE (Institute of Electrical and Electronics Engineers) organization does not certify the 802.11 compatible devices, they only work on the standards. The certification process is handled by non-profit organization WiFi Alliance which was formed in 1999. However, at the beginning the name of this organization was the Wireless Ethernet Compatibility Alliance and it was renamed to the WiFi Alliance in 2002.
Nowadays, the WiFi is very popular solution. There is about 6 million of public hotspots and over 4.5 billion WiFi devices which are in use today.
IEEE 802.11a/b/g standards
Both 802.11a and 802.11b standards were released in September 1999. There is a lot of differences between them, because 802.11a operates at 5 GHz frequency band, uses OFDM modulation and provides up to 54 Mbps data rate. The 802.11b is about 5 times slower, i.e. the maximum data rate is 11 Mbps. This difference is caused by the fact that the 802.11b standard operates at 2.4 GHz frequency band and uses DSSS modulation.
The 802.11g standard was released in June 2009 and it offers up to 54 Mbps data rate (as same as the 802.11a) at 2.4 GHz frequency band (the same band is used by the 802.11b). The 802.11g uses also OFDM modulation and this solution was copied from 802.11a standard. However, the 802.11g provides the backward compatibility with the 802.11b.
The final version of the 802.11n standard was released in 2009. This is the extension of the 802.11g. The 802.11n introduces: two times wider bandwidth (40 MHz in comparison with 20 MHz), MIMO (Multiple Input Multiple Output) up to 4 streams, two times shorter guard interval and more efficient coding scheme. It also operates in both frequency bands, i.e. 2.4 GHz and 5 GHz, with the same performance. The 802.11n has also less transmission overhead, so it achieves 58.5 Mbps data rate using 20 MHz channel, 800 ns GI (Guard Interval) and 64 QAM 3/4 modulation scheme, the same parameters are used in the 802.11g standard.
The highest possible data rate of the 802.11n is 150 Mbps per a spatial stream, this result is achievable with 40 MHz channel, 400 ns GI and 64 QAM 5/6 modulation. The maximum number of data streams is 4 (it means that the router and the computer have 4 antennas each) thanks to what the maximum data rate is 600 Mbps.
However, the manufactures did not decide to produce the 802.11n devices with 4 antennas. The most advanced routers and adapters have 3 antennas, so it is possible to achieve up to 450 Mbps wireless link. Of course, the 802.11n standard ensure the backward compatibility with older standards, i.e. 802.11g and 802.11b.
The final version of IEEE 802.11ac standard was approved in January 2014, however the first commercial devices was available in 2013. They were so-called 802.11ac Wave 1 devices, i.e. these routers and adapters were consistent with the 802.11ac standard, but they did not support all features. The 802.11ac Wave 1 devices supports 256 QAM modulation, 80 MHz bandwidth channels and up to 3 spatial streams. Thanks to the wider bandwidth and more efficient modulation and coding scheme, the 802.11ac provides up to 433 Mbps data rate in a single stream.
The 802.11ac Wave 2 devices support 4 spatial streams, 160 MHz channel (or two 80 MHz channels using Carrier Aggregation mode) and MU-MIMO (Multi User MIMO). The simply calculation (433 Mbps x 2 times wider bandwidth x 4 spatial streams) shows that the 802.11ac Wave 2 devices (released in 2015) are able to provide up to 3.39 Gbps data rate.
The fully consistent with the 802.11ac standard device supports up to 8 spatial streams, so it is possible to achieve 6.77 Gbps data rate link. However, the purpose of application of 8 antennas is not to have 6.77 Gbps physical throughput between the router and a single computer. The scenario is that there is an eight-antenna access point and four two-antenna laptops. Thanks to the MU-MIMO technique, it is possible to provide 1.69 Gbps wireless links simultaneously between the router and each client.
The future: IEEE 802.11ad (WiGi)
The 802.11ad is being developed and promoted by the Wireless Gigabit Alliance (WiGig) which is the part of the WiFi Alliance. The idea of the 802.11ag is the application of unlicensed 60 GHz frequency band for almost 7 Gbps data rates (6756.75 Mbps to be more precise). This ultra-fast wireless transmission is possible thanks to the usage of 2.16 GHz bandwidth, 64 QAM 13/16 modulation and a single antenna. However, due to the application of extremely high frequency (i.e. 60 GHz band), the 802.11ad requires the line of sight, i.e. there can not be any obsolete between the transmitter and the receiver.
Therefore, the 802.11ad will be used for short but very fast wireless data transmissions. For an example, the perfect application of the WiGi is high quality video streaming. However, perhaps entirely new class devices will be developed. For an example, the 802.11ad will may be used in the wireless hard drives.
Security in the wireless networks
The WiFi network can be protected against the unauthorized access by several methods. The first technique is called WEP (Wired Equivalent Privacy), but this is not recommended to use. The WEP can be cracked in several minutes using the widely available software. Disadvantages of the WEP were corrected in the WPA (WiFi Protected Access) standard. The WPA uses longer encryption keys (256-bit in the comparison to 128-bit keys used in the WEP) and TKIP (Temporary Key Integrity Protocol). It is much safer than the WEP, but it is not the perfect. The WPA is compatible with the WEP hardware, it means that manufactures had to prepare only the firmware update in order to add the WPA support.
Currently, the best protection method is the WPA2 with the AES (Advanced Encryption Standard). There is known a single vulnerability of this security technique, but the attacker has to know the GTK (Group Temporary Key) which is used to the encryption of broadcast packets. It means that the attacker has already access to the WiFi network and wants to overhear packets addressed to other computers.