【My Study Note】Introduction to Wireless Networking Technologies
Introduction to Wireless Networking Technologies
The most common specifications for how wireless networking devices should communicate are defined by the IEEE 802.11 standards. This set of specifications, also called the 802.11 family, makes up the set of technologies we call WiFi.
Wireless networking devices communicate with each other through radio waves. Different 802.11 standards generally use the same basic protocol but might operate at different frequency bands.
Frequency Band
A frequency band is a certain section of the radio spectrum that’s been agreed upon to be used for certain communications.
In North America, FM radio transmissions operate between 88 and 108 megahertz. This specific frequency band is called the FM broadcast band.
WiFi networks operate on a few different frequency bands. Most commonly, the 2.4 gigahertz and 5 gigahertz bands.
There are lots of 802.11 specifications including some that exist just experimentally or for testing. The most common specifications you might run into are 802.11b, 802.11a, 802.11g, 802.11n, and 802.11ac.
In terms of our networking model, you should think of 802.11 protocols as defining how we operate at both the physical and the data link layers.
An 802.11 frame has a number of fields.
frame control field.
The first is called the frame control field. This field is 16 bits long and contains a number of sub-fields that are used to describe how the frame itself should be processed.
This includes things like what version of 802.11 was used.
duration field
The next field is called a duration field. It specifies how long the total frame is. So, the receiver knows how long it should expect to have to listen to the transmission.
four address fields
After this, are four address fields. Let’s take a moment to talk about why there are four instead of the normal two.
access points
The most common setup of wireless network architectures includes devices called access points. A wireless access point is a device that bridges the wireless and wired portions of a network.
A single wireless network might have lots of different access points to cover a large area. Devices on a wireless network will associate with a certain access point. This is usually the one they’re physically closest to.
But, it can also be determined by all sorts of other things like general signal strength, and wireless interference.
Associations aren’t just important for the wireless device to talk to a specific access point, it also allows for incoming transmissions to the wireless device to be sent by the right access point.
There are four address fields because there needs to be room to indicate which wireless access point should be processing the frame.
So, we’d have our normal source address field, which would represent the MAC address of the sending device. But, we’d also have the intended destination on the network, along with a receiving address and a transmitter address. The receiver address would be the MAC address of the access point that should receive the frame, and the transmitter address would be the MAC address of whatever has just transmitted the frame.
In lots of situations, the destination and receiver address might be the same. Usually, the source and transmitter addresses are also the same. But, depending on exactly how a specific wireless network has been architected, this won’t always be the case.
Sometimes, wireless access points will relay these frames from one another. Since all addresses in an 802.11 frame are Mac addresses, each of those four fields is 6 bytes long.
sequence control field
In between the third and fourth address fields, you’ll find the sequence control field. The sequence control field is 16 bits long and mainly contains a sequence number used to keep track of ordering the frames.
data payload
After this is the data payload section which has all of the data of the protocols further up the stack.
check sequence field
Finally, we have a frame check sequence field which contains a checksum used for a cyclical redundancy check. Just like how ethernet does it.