Development of 802.11 protocol

802.11 protocol naming

The naming of the 802.11 protocol was proposed by the IEEE (Institute of Electronics and Electrical Engineers)

802.11 protocol related standards

The 802.11 physical layer standard defines the wireless working frequency band, modulation and coding method, and the highest speed support

The 802.11 MAC layer standard defines how to optimize the transmission between 802.11 base stations; control and maintain the communication between 802.11 base stations on the shared channel

802.11 Physical Layer Protocol and WiFi Generation

802.11 protocol physical layer key technologies

Basic concepts about channels

Carrier: the transmission channel (channel)

Subcarrier: It is a subchannel (dividing the carrier into multiple subcarriers)

Basic concept of spatial flow

The radio sends multiple signals at the same time, each signal is a spatial stream; different spatial streams mean that each antenna transmits different data

If two antennas transmit different data, it is called 2 spatial stream; if two antennas transmit the same data, it cannot be called 2 spatial stream

That is: 3 spatial streams means that 3 antennas are necessary, but 3 antennas are not necessarily 3 spatial streams

Precautions

The 2.4GHz frequency band only supports up to 4 spatial streams

The 5GHz frequency band only supports up to 8 spatial streams

That is: even if there are 12 antennas in the 5GHz frequency band, there can only be 8 spatial streams

Key Technologies of 802.11 Protocol Physical Layer

Channel bonding, OFDM/OFDMA, Short-Gi, MIMO/MU-MIMO, QAM, BSS and other technologies

Short-Gi and channel bonding can be modified; the number of spatial streams is a hardware parameter and cannot be changed after the device leaves the factory

Channel Bonding Technology

A standard channel is 20MHz and contains 52 subcarriers

Two adjacent channels can be bundled together, doubling the channel, doubling the subcarriers, and doubling the rate

Which protocols support channel bonding

802.11n/ac/ax support channel bonding

For 802.11n and 802.11ac Wave 1 protocols, only support bonding to 40MHz

For 802.11ax and 802.11ac Wave 2 protocols, support bonding up to 160MHz

Bond two adjacent 20MHz

OFDM/OFDMA technology

OFDM Orthogonal Frequency Division Multiplexing Technology

Before the emergence of OFDMA, FH (FHSS) and DS (DSSS) technologies were used, and these two technologies are no longer used

The role of OFDM

OFDM divides the channel into several orthogonal sub-channels, each sub-channel uses a sub-carrier for modulation, and transmits data in parallel; improves spectrum utilization

Which protocols use OFDM

802.11a/g/n/ac

OFDMA Orthogonal Frequency Division Multiple Access Technology

The difference between OFDM and OFDMA

Compared with OFDM, there is only one STA and AP communicating at the same time (one user completely occupies all subcarriers in each time segment (this time period is very small, and there is basically no delay when there are few users))

OFDMA supports communication between multiple STAs and APs at the same time (for each time segment, subcarriers are allocated according to user needs)

Precautions

A maximum of 74 concurrent users are supported (when the bandwidth is 160MHz, 160MHz is obtained through 20MHz bandwidth binding)

Which protocols use OFDMA

Currently only 802.11ax (WiFi6) uses the OFDMA protocol

Short-Gi Short Guard Interval Technology

The guard interval is necessary for OFDM technology, there must be GI between data blocks to protect data reliability and avoid collisions with each other

In 802.11a/g, normal GI is used, and the guard interval is 800ns

802.11n has improved it, shortening the guard interval from 800nx to 400ns, increasing the transmission rate while avoiding interference

Which protocols use Short-Gi

802.11n, 802.11ac use Short-Gi

The normal GI used by 802.11a/g

802.11ax has a smaller GI interval

MIMO/MU-MIMO technology

SISO/MISO/SIMO/MIMO - multiple input multiple output

MIMO technology evolved from SISO, MISO, and SIMO

MIMO technology allows only one user to use multiple antennas for multiple data input and multiple output at the same time, doubling the transmission rate

MIMO technology can also be called SU-MIMO technology (single user multiple input multiple output technology)

Which protocols use MIMO and SU-MIMO technology

802.11n, 802.11ac Wave1, 802.11ac Wave2 uplink

The difference between uplink and downlink

Uplink: data sent by the terminal to the AP

Downlink: AP sends data to the terminal

MU-MIMO multi-user multiple input multiple output technology

Allow multiple users to use multiple antennas for multiple input and multiple output communication at the same time

It also supports multiple input and multiple output for a single user at the same time using multiple antennas

Which protocols use MU-MIMO technology

802.11ac Wave2 downlink, 802.11ax uplink and downlink

MIMO - Beamforming

MIMO technology and MU-MIMO technology can realize beamforming technology and improve anti-interference ability through phase difference

What is beamforming

When there are multiple transmitting antennas at the transmitting end, the technology of adjusting the signals sent from each antenna to significantly improve the signal strength at the receiving end (achieved by phase; peak and peak, trough to trough makes the signal strength higher; peak to trough makes signal weakening)

Which protocols use beamforming

Beamforming originated from the 802.11n protocol and continues to be used with the 802.11ac and ax protocols

QAM technology

QAM technology is used to transmit digital signals to the carrier through modulation, which can change the amount of data carried by each subcarrier, change the amount of data transmitted, and change the transmission rate

For example: 256-QAM, 256 is 2 to the 6th power, so each subcarrier can carry 6bit data

Disadvantages of QAM

When the QAM is larger, its transmission range is smaller (for example: the more data a single space carries, the denser the data, the farther away it is, the harder it is to see)

Quick identification of BSS Color

BSS is a technology to distinguish the same channel; currently only 802.11ax supports BSS technology

Add the BSS color field in the Phy header to "color" the data from different BSS

Each channel is assigned a color that identifies a set of Basic Service Sets (BSS) that should not interfere

802.11 MAC Layer Key Technology CSMA/CD Mechanism

Why Wireless Proposed the CSMA/CD Mechanism

CSMA/CA Mechanisms Used by Wired Networks

Listen before sending, listen while sending, stop sending due to conflict, resend after random delay

That is: before sending data, check whether the link is in use; when sending data, also check whether there is other data to send on this link, and stop sending if there is a conflict; wait for a certain random time after stopping sending, and then check the link after the time passes Whether to use it; send it if it is idle, and wait for a certain random time if it is not idle

Can the wireless directly use the wired CSMA/CA mechanism?

The wireless cannot use the CSMA/CA mechanism, and the specific reasons are as follows

1. For wireless, the wireless is in a half-duplex working state, and the wireless client does not have the ability to send and receive data at the same time; that is, the wireless client cannot receive the intercepted data when sending data

2. Since the coverage of the site (AP) is wired, other sites cannot be detected (as shown in the figure below: STA exists in both site A and site B, but the sites cannot detect each other—this is a hidden node)

Working Mechanism of CSMA/CD

Therefore, wireless cannot detect conflicts, and can only avoid conflicts as much as possible, ensure that conflicts do not occur, and ensure that only one client sends data at the same time; this is the mechanism of CSMA/CD

main components

CS: Carrier Sense; Sense before sending data to ensure the line is free and reduce the chance of collision

MA: Multiple access; data sent by each station can be received by multiple stations at the same time

CA: Collision Avoidance conflict avoidance; minimize the probability of collision

How CSMA/CD works

RTS/CTS (Request To Send/Clear To Send, request to send/permission to send)

Through times to reduce the conflict mechanism caused by node problems; nodes mainly include hidden nodes and exposed nodes

RTS frame - broadcast

When the sender wants to send data, it will first send an RTS signal to reserve the right to use the link with the receiver;

Make other devices within the range of the sender not send data to the sender within the specified time after receiving the RTS

It can be AP→STA, or STA→AP; generally speaking, RTS is sent by STA, and CTS is sent by AP

CTS frame - broadcast

After receiving the RTS, the receiver sends a CTS signal to reply to the sender if it agrees;

Make other devices within the range of the receiver not send data to the receiver within a certain period of time after receiving the CTS

The NAV bar graph represents the NAV timer, which is carried by the header of the RTS and CTS frames

Key Technologies of CSMA/CD

The 802.11MAC layer mainly consists of two sublayers

Distributed Coordination Function DCF (Distributed Coordination Function)

Using the CSMA/CA mechanism, each STA obtains the right to send data frames by contending for the channel.

Point Coordination Function PCF (Point Coordination Function)

Using the access algorithm of centralized control, the sending right of the data frame is handed over to each STA in turn in a method similar to inquiry, so as to avoid collisions and conflicts

There must be a DCF mechanism in the 802.11 protocol, and PCF is optional

Interframe Space IFS

SIFS ( Short Interval Between Frames)

Used to separate a session

The interval between AP and terminal sending RTS and CTS, and the interval between CTS and data and ACK

PIFS (PCF Interframe Space)

Used in PCF to send data frames and management frames

DIFS (DFS Interframe Space)

Used in DCF to send data frames and management frames

When a data frame is sent, the waiting time to send the next data frame

EIFS

In the case of an error in the previous frame, the sending node has to delay the EIFS time period before sending the next frame

backoff time

If multiple STAs need to send data and all detect that the channel is busy, a backoff algorithm needs to be executed (if the channel is not detected to be busy, the backoff time also needs to be executed, and the method conflicts)

Each STA randomly backs off for a period of time before sending data (the backoff time is an integer multiple of the time slot)

The STA detects the channel every time it passes through a time slot. If the channel is idle, the backoff time continues to count down

If the channel is busy, freeze the backoff timer; wait for the channel to become idle again and then wait for DIFS, then continue counting down from the remaining time until the backoff timer is reduced to 0, and then the STA starts sending data frames

hidden nodes and exposed nodes

Solve the conflict between hidden nodes and exposed nodes through RTS/CTS

The RTS/CTS of 802.11m cannot solve the hidden node problem, only the RTS/CTS of 802.11ac and ax can solve it (by carrying the bandwidth indication)

Hidden node—both STAs are within the coverage of the same AP, but the two STAs cannot detect each other

(STA1 and STA2 both belong to AP1, but they cannot detect each other; from the perspective of STA1, STA2 is a hidden node)

If there is no RTS/CTS mechanism, STA1 and STA2 cannot perceive whether the other party is sending data, and may conflict

If there is an RTS/CTS mechanism

STA1 sends RTS to reserve the channel

AP broadcasts CTS after receiving RTS

STA1 prepares to send data after receiving CTS; STA2 enters a silent state after receiving CTS

Exposed Node—Two STAs can be detected, but the other STA belongs to a different AP coverage area

(STA1 and STA2 can detect each other, but STA1 belongs to AP1, STA2 belongs to AP2)

AP1 and AP2 belong to the same channel, when PC1 sends data to AP1, PC2 also wants to send data to AP2

If there is no RTS/CTS, when PC2 listens to the channel, it will hear that PC1 is sending data, and mistakenly think that it cannot send data to AP2 at this time, but in fact its sending will not affect the data reception of AP1, which leads to The emergence of the so-called exposed node problem of PC2

With the RTS/CTS mechanism

PC1 sends RTS, after PC2 receives it, PC2 does not send data to PC1

AP1 sends CTS to PC1, but PC2 does not receive the CTS sent by AR1; at this time, PC2 can send data to this channel