Wi-Fi

The Wifi is a technology for wireless network area networking based on the standards of the IEEE 802.11 protocol. Wi-Fi has evolved very fast and it is being improved almost constantly. Now, it is present in the life of millions of people. This protocol defines the MAC layer and the PHY layer. There are a lot of versions of the Wi-Fi. The first one was created in 1999: IEEE 802.11. The data rate was 2 Mb/s on the 2.4 frequency band. This version will be approved a lot of times: 802.11a, 802.11b, 802.11g and 820.11n.

The Wi-Fi n in 2007 offered a drastic improvement in the data rate. This is the first protocol that operated on both 2.4 and 5 GHz.

Then, other version arrived.

The IEEE 802.11ac
The ac version, which is the update of the classic versions, appears in 2013. The data rate is now up to 7 Gbit/s. It’s faster and there are more channels than the Wi-Fi a/b/g/n thanks to the 256-QAM. Here we work in the 80-160 MHz frequency band. It has also the particularity to be retro-compatible.

The IEEE 802.11ad
The ad version is a particular one. Indeed, the frequency band is no more 2.4 GHz and 5 GHz but now 60 GHz (the space is completely free and open). A so high frequency means a very high data rate but has a very small range (one room). This version answers to the need of a enterprise that wants extended bandwidth with very-short range devices.

The IEEE 802.11af
The af one transmits date on the unused television spectrum frequencies (called with space). That means that the data are transferred on the 54 to 790 MHz frequency band. It can be used for very long range devices. However, there is in this band a lot of electromagnetic pollution. The debit rate can

The IEEE 802.11ax
The ax version uses the classical frequency band but hasn’t the same modulation. Now we work with the 1024 QAM that improves the symbol rate. More there is symbol, smaller is the frequency band needed. So with the 1024 QAM, we can transmit more information at the same time on the same frequency band. That improves the spectral efficiency.

The IEEE 802.11ah
The IEEE 802.11ah version isn’t available right now, created to support Machine-To-Machine communications (so also for the internet of things). It is the concurrent with the Low-Power Wide area network technologies. Indeed, it’s a very low energy consumption communication by adopting power saving strategies and cost-effective solution for network device manufacturers. It’s a 900 MHz Wi-Fi, that area of the spectre is less congested and guarantee a long range (up to 1 km in outdoor areas). The network can support up to 8191 devices associated with an access point (AP) through a hierarchical identifier structure and a one-hop network topologies. The transmission is done with short and infrequent data transmission (data packet size approximately 100 bytes and packet inter-arrival time greater than 30 sec.).

IEEE 802.11ah designs new PHY and MAC Layers. PHY layer can be considered a sub-1GHz version of the PHY layer on the IEEE 802.11ac. MAC level, for supporting the special constraints of M2M communications. The MAC Layer is designed to maximize the number of stations supported by the network while ensuring minimum energy consumption.

It defines three different types of stations, each with different procedures and time periods to access the common channel: traffic indication map (TIM) stations, non-TIM stations, and unscheduled stations:

Traffic indication map (TIM) stations: This is the only type of station that needs to listen to AP beacons to send or receive data. Their data transmissions must be performed within a restricted access window period with three differentiated segments (multicast, downlink, and uplink). Stations with a high traffic load should use this procedure to access the channel because it combines periodic data transmission segments with energy efficiency mechanisms.

Non-TIM stations do not need to listen to any beacons to transmit data. During the association process, non-TIM devices directly negotiate with the AP to obtain a transmission time allocated in a periodic restricted access window (PRAW). The following transmissions can be either periodically defined or renegotiated. It is advisable to deploy TIM stations for high-volume data applications

Unscheduled Stations: These stations do not need to listen to any beacons, similar to non-TIM stations. Even inside any restricted access window, they can send a poll frame to the AP asking for immediate access to the channel. The response frame indicates an interval (outside both restricted access windows) during which unscheduled stations can access the channel. This procedure is meant for stations that want to sporadically join the network.

Channels and frequencies
Most of the 802.11 protocols use the 2.4-2.5 GHz spectrum, one of the ISM bands. This band is divided into 14 channels spaced by 5 MHz apart. The first one is centered on the 2.412 GHz. The 802.11 protocol specifies the channel center frequency and a spectral mask defining the permitted power distribution across each channel. There is also another frequency band at 5 GHz. There are two bands: from 5.150 to 5.350 GHz and from 5.470 to 5.850 GHz. Those frequencies are used for the 802.11n and 802.11ac protocols. Some channels can be used for others applications like the military application or weather radars depending on the country. So, there are not allowed. For example, in Europe, 5.720 to 5.825 GHz channels are not accessible.

The architecture
There are two types of network: the ad-hoc network and the infrastructure network. In the first configuration, there isn’t a coordinator. Indeed, everyone sees everybody. The connection is point-to-point. In the second one, the network has a central point, called the access point. Each computer who wants to access at the network has to be connected with the access point. In the 2.4 GHz frequency band, there are 14 different channels. It’s the access which gives the frequency where they communicate.

Wi-Fi transmission.
As the Wi-Fi is a wireless communication, the quality of the communication is not a guarantee as for the Ethernet for example. We need an ACK to be sur that the data are well transmitted. In a wireless communication it is difficult to tell the difference between noise and an eased signal. We can’t do a collision detection. It’s why we use the CSMA/CA.

When data are transmitted from a computer, the last one wait during a ACK timeout, time needed to receive a ACK. Indeed, the receiver has the priority on the channel to send back an ACK during SIFS (the other machine can communicate during this time). The time during the machine have to wait is communicated in the data frame. If the data hasn’t be received, the transmitter hasn’t the priority to send back.

If some stations have to send data in the same time, what happen?

The first station is sending data. When the ACK is send, the other stations are going to wait a DIFS and an additional time (random number * slot time). So the machine will not wait the same time. In this case, it’s the station 3 which has the smaller slot time. It’s the second station which is going to send data. The station 2, to be prior next, is not going to take another random number. Its number will be 5-3 = 2.

If computers don’t see each other, how it works?

When a computer wants to communicate, it is going a RTS frame which is a reservation packet. It’s a very small packet, so the probability of a collision is very small. Then, the access point will send to the whole network a frame which says which station can speak. When the data are received, the access point sends an ACK to the whole network.