The primary advantages of Wi-Fi are it operates in unlicensed spectrum, or unlicensed frequencies, and Wi-Fi does not require extensive infrastructure. You do not need to have a license from a local government to set up and run a Wi-Fi transmitter, so long as the transmitter is lower than a specific power level (normally around 250 milliwatts).
Average users or network operators can set up a Wi-Fi system without permission, providing coverage for devices within a house or building. The owner of the system decides which networks Wi-Fi connected users can reach, including
• Devices attached to the internal network only
• Devices attached to the internal network and the global Internet
• Devices attached to the global Internet only (often called a guest network)
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Operating on unlicensed spectrum is also a disadvantage because Wi-Fi systems must operate with limited transmission power. Some specific problems resulting from the use of unlicensed spectrum include
• Building a Wi-Fi system covering large outdoor spaces, such as a park, is difficult.
• You cannot “roam” between different Wi-Fi networks. Each network is owned and maintained by a different operator, and each operator may have different rules for connecting to the network, what destinations users can reach, etc.
• Many other kinds of devices also operate in these same frequency ranges, such as microwave ovens, IoT devices, and even cellular telephones. Because these frequencies are crowded, other devices are more likely to interfere with the Wi-Fi signal.
• Wi-Fi networks do not limit the amount of data attached devices can send or receive. Network performance will degrade if the network is overloaded.
Sharing the Bandwidth
Another disadvantage of Wi-Fi, when compared to switched wired networks, is all the hosts attached to a single AP through a single channel share the same bandwidth—just like all the hosts being connected to the same physical Ethernet segment. Just like Ethernet, there is a system to share the bandwidth.
Ethernet uses Carrier Sense Multiple Access with Collision Detection (CSMA/CD). Wi-Fi uses Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA). Rather than detecting collisions and backing off before transmitting again, Wi-Fi avoids collisions using Request to Send signaling after hearing another AP send a packet.
Some APs use beam forming to separate transmissions, so each host has separate bandwidth.
Channel Selection
Table 8-3 and Table 8-4 list a lot of channels, each with its frequency. How can you choose which of these frequencies to use?
For large-scale Wi-Fi deployments, engineers use radio receivers to detect the current electromagnetic radiation present in the area and choose a channel with the least amount of existing interference. For smaller installations, operators would try different channels until they find a workable set of frequencies to use.
For small-scale deployments, such as small businesses and residences, most newer APs will survey the electromagnetic radiation and select an optimal set of channels on which to operate. The capability of an AP to automatically select a channel with the least interference is called Dynamic Frequency Selection (DFS).
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