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WLAN Standards - Section 3 [From Beginner to Expert - WLAN Fundamentals]

Latest reply: Jul 5, 2019 07:39:25 6327 3 3 0 2

Hello, everyone!

This post is about WLAN Standards, as part of the From Beginner to Expert - WLAN Fundamentals section on our Forum. Please see more information as you read further down.

IEEE802.11

Throughout the history of WLAN, many technologies and standards have been developed, such as IrDA, Bluetooth, and HyperLAN2. But today, only Institute of Electric and Electronic Engineers (IEEE) 802.11 standards see widespread commercial use on a large scale in the WLAN field. WLAN is usually defined as a wireless LAN built on the IEEE 802.11 standards. Since 802.11 always appears on WLAN products, you may be no stranger to it, so we won't list the basic concepts here. The remainder of this post will instead describe five landmark 802.11 standards: 802.11a, 802.11b, 802.11g, 802.11n, and 802.11ac. Standards may be boring, but they are what helps us deploy WLANs. So, let's explore these standards.

Wireless access is what distinguishes WLANs from wired LANs. During the previous section, we learned that the communication media in WLANs are radio frequencies whose physical characteristics are totally dissimilar to those of the media (electric or optical cables) in wired LANs. Consequently, the physical layer and Media Access Control (MAC) layer in WLANs differ from those in wired LANs. The 802.11 standards mainly deal with these two layers in WLANs.

In the early 1990s, to meet the growing demand for WLANs, IEEE set up the 802.11 working group to work on WLAN standards. In June 1997, the first WLAN standard IEEE 802.11-1997 was issued. In this standard, the physical layer operates in the 2.4 GHz industrial, scientific and medical (ISM) radio band. The maximum theoretical data rate was 2 Mbps. The data rate and transmission distance in this standard cannot meet the demand in the modern WLAN field. Therefore, this standard is not widely used.

In 1999, IEEE introduced 802.11a and 802.11b. 802.11a was used in the 5 GHz ISM band. It offered a data rate of 54 Mbps through the use of Orthogonal Frequency Division Multiplexing (OFDM), which mitigates the adverse effects of multipath fading and improve the spectrum utilization. 802.11b still used the 2.4 GHz ISM band, but improved on technologies found in 802.11-1997 so that it offered a data rate of up to 11 Mbps.

OFDM is a form of multicarrier modulation. Multiple sub-carriers within the same single channel are modulated independently and transmitted in parallel, improving the spectrum utilization.

Compared with 802.11a, 802.11b offered a lower data rate. But the R&D of 5 GHz chips lagged behind 2.4 GHz chips. When 5 GHz chips were launched, 802.11b was already widely used. 802.11a was not popularized because 802.11a was not compatible with 802.11b, 5 GHz chips were expensive, and local regulations posed limitations.

In the early 2000s, the IEEE 802.11g working group started to develop a new standard that was supposed to offer a data rate of up to 54 Mbps and be backward compatible with 802.11b. The first IEEE 802.11g draft was approved in November 2001 and ratified in 2003. 802.11g was compatible with 802.11b and used the 2.4 GHz band. To offer a data rate of up to 54 Mbps, 802.11g made use of the technological innovations used in 802.11a, specifically OFDM. 802.11g met people's demand for high bandwidth at that time and promoted the development of WLAN.

You may wonder why IEEE did not adopt OFDM in 802.11b when developing the standard in 1999. If it did so, 54 Mbps could be offered in the 2.4 GHz band before the end of 2001 when 802.11g was introduced. Actually, OFDM was proposed for 802.11b during the development of this standard in 1999. However, the Federal Communications Commission (FCC) banned the use of OFDM in the 2.4 GHz band. This ban was lifted in May 2001. Six months later, the OFDM-based 802.11g draft was successfully approved.

Rates of 54 Mbps cannot meet user demand forever in the burgeoning network field. In 2002, a new IEEE working group, IEEE 802.11 Task Group N (TGn), was founded to research new WLAN technologies to offer a 100 Mbps rate. After many arguments in the group, the new protocol, known as 802.11n, was ratified in September 2009. With the 7-year effort, the rate in 802.11n was improved to a maximum of 600 Mbps from the original 100 Mbps. The 802.11n standard supports the dual-band mode (2.4 GHz and 5 GHz bands) and is backward compatible with 802.11b, 802.11g, and 802.11a.

Right after publishing the 802.11n protocol, IEEE started to work on the next-generation WLAN protocol: 802.11ac. The 802.11ac protocol was finalized in 2013. It is used only in the 5 GHz band and is backward compatible with 802.11a. The 802.11ac takes the best of 802.11n and makes improvements to offer a maximum rate of 1.3 Gbps.

The following table describes specifications in 802.11 series protocols.

Standard

Physical Layer Technology

Band (GHz)

Data Rate (Mbps)

Compatibility with Other 802.11 Standards

Commercial Use

802.11

FHSS/DSSS

2.4

1, 2

Incompatible

Earlier standard, supported by most products

802.11b

DSSS

2.4

1, 2, 5.5, 11

Incompatible

Earlier standard, supported by most products

802.11a

OFDM

5

6, 9, 12, 18, 24, 36, 48, 54

Incompatible

Rarely used

802.11g

DSSS/OFDM

2.4

1, 2, 5.5, 11, 6, 9, 12, 18, 24, 36, 48, 54

Compatible with 802.11b

Widely used

802.11n

OFDM/MIMO

2.4 and 5

A theoretical maximum of 600 Mbit/s, depending on the modulation and coding scheme (MCS)

Compatible with 802.11a, 802.11b, and 802.11g

Widely used

802.11ac

OFDM/MIMO

5

A theoretical maximum of 1300 Mbit/s, depending on the MCS, spatial stream quantity, channel bandwidth, and guard interval (GI)

Compatible with 802.11a and 802.11n

Widely used

 

Huawei products in V200R003C00 or earlier support 802.11n, 802.11g, 802.11b, and 802.11a. From V200R005C00 onward, Huawei products support 802.11ac. Huawei has introduced two 802.11ac-capable APs: AP5030DN and AP5130DN.

For Huawei products in V200R003C00 or earlier, you need to configure the radio type using the following command:

[6605_v2r3_111-wlan-radio-prof-test] radio-type ?                                       

  80211an   802.11an                                                           

  80211bgn  802.11bgn                                                          

  80211gn   802.11gn                                                            

  80211n    802.11n                                                            

  80211b    802.11b                                                            

  80211a    802.11a                                                             

  80211bg   802.11bg                                                           

  80211g    802.11g                                                            

 

In the setting, 80211n indicates that the radio type is 802.11n. If STAs support 802.11b or 802.11g only, they cannot access the WLAN. 80211bgn indicates that 802.11n, 802.11b, and 802.11g are supported. Any 802.11n, 802.11b, or 802.11g STAs can access the WLAN. This rule applies to other values in this command. The radio-type command has been abolished since V200R005C00. The radio type is automatically adjusted according to the protocols supported by STAs with no need for commands from users. The procedure for configuring data rates supported by radios is different in V200R005C00 and V200R003C00. For details, see the WLAN product documentation:

http://support.huawei.com/enterprise/productNewOffering?idAbsPath=7919710|21782164|21782201|21782208|7974000&pid=7974000&productname=AC6605

802.11ac was only recently introduced. 802.11ac-capable STAs are rare, and 802.11n-capable products are commonly used. Compared with earlier 802.11 protocols, the 802.11n has the following improvements: more sub-carriers, higher code rate, shorter GI, wider channel, more spatial streams, and frame aggregation. The 802.11ac leverages these features. To make the best use of these improvements, Huawei products need to be configured as follows.

More sub-carriers: The 802.11n offers four more data sub-carriers than 802.11a/g. (The 802.11b is not compared with the 802.11n because it does not use OFMD.) 802.11n users can enjoy benefits of this improvement without the need for additional configuration. The following figure shows the data increase provided by the more sub-carriers in 802.11n.


576d10558d233.png 

Higher code rate: Data transmitted on WLANs consists of payload data and forward error correction (FEC) overhead. When errors occur in payload data due to attenuation, interference, or other factors, FEC overhead can correct errors. 802.11n improves the code rate to 5/6 (from 3/4 in earlier 802.11 protocols), which improves the physical connection rate by 11%. 802.11n users can enjoy benefits of this improvement without the need for additional configuration.


576d1067e4fce.png 

Shorter GI: The 802.11a/b/g standards require a GI, a period of 800 ns, between each OFDM symbol that is transmitted to protect against intersymbol interference (ISI). 802.11n adds an optional for a 400 ns guard interval, which provides an 11% increase in data rate (about 72.2 Mbps). 802.11n uses the 800 ns GI by default, but the 400 ns GI in good radio environments. Users can run the 80211n guard-interval-mode short command in the radio profile view to configure the 802.11n short GI function.

[AC6605] wlan

[AC6605-wlan-view] radio-profile name 80211n 

[AC6605-wlan-radio-prof-80211n] 80211n guard-interval-mode short

To configure the 802.11ac short GI function, run the 80211ac guard-interval-mode short command.

system-view

[AC6605] wlan

[AC6605-wlan-view] radio-profile name 80211ac 

[AC6605-wlan-radio-prof-80211ac] 80211ac guard-interval-mode short

Note that the short GI function is not suitable in all environments. In complex spatial environments, radio reflection caused by obstacles may result in multipath between APs and STAs. In this case, data arrives at the receiver at different times, which may generate interference. A proper GI can avoid interference. An improper GI will reduce link efficiency.


576d1079a3d10.png 

It is a good practice to disable the short GI function in complex environments using the 80211n guard-interval-mode normal command on an 802.11n device or the 80211ac guard-interval-mode normal command on an 802.11ac device.

Wider channel: Section 2 WLAN Radio Frequencies and Channels describes channel bonding in the 802.11n that combines two 20 MHz channels into a 40 MHz channel. This can provide greater than twofold increase in data rates. Users can run the following command in the radio view to configure a 40 MHz channel and specify a primary channel.

[AC6605] wlan

[AC6605-wlan-view] ap 0 radio 0

[AC6605-wlan-radio-0/0] channel 40mhz-plus 1  //Configure a 40 MHz channel and specify channel 1 as the primary channel.

802.11ac supports 80 MHz bandwidth.

[AC6605] wlan

[AC6605-wlan-view] ap 0 radio 1

[AC6605-wlan-radio-0/1] channel 80mhz 149

More spatial streams: 802.11a/b/g APs and STAs can only use single-input single-output (SISO) with a single spatial stream to transmit data. 802.11n supports up to multiple-input multiple-output (MIMO) with four spatial streams, and 802.11ac supports 8x8 MIMO. Huawei's multiple-antenna APs support MIMO. For example, the AP5130, AP7110, and AP5030 support 3x3 MIMO, while the AP3010, AP6510, and AP6610 support 2x2 MIMO.


576d108e6a0f8.png 

Frame aggregation: At the MAC layer, the 802.11 protocols define a significant amount of overhead for every frame, especially acknowledgment overhead. At the highest data rate, this overhead consumes larger bandwidth than the payload data frame. For example, the theoretical data rate in 802.11g is 54 Mbps, but the actual throughput is only 22 Mbps. The rest is overhead. The 802.11n MAC Protocol Data Unit (MPDU) aggregation function aggregates multiple MPDUs into an aggregated MAC Protocol Data Unit (A-MPDU), so that N MPDUs can be transmitted through one channel contention or backoff. This function saves channel resources to be consumed for sending N-1 MPDUs. Users can run the 80211n a-mpdu enable command to enable the 802.11n MPDU aggregation function, and the 80211n a-mpdu max-length-exponent command to set the maximum length of frames aggregated into an A-MPDU. The maximum length of frames aggregated into an A-MPDU is 65,535 bytes in 802.11n.

system-view

[AC6605] wlan

[AC6605-wlan-view] radio-profile id 0 name rp01

[AC6605-wlan-view] 80211n a-mpdu enable

[AC6605-wlan-radio-prof-rp01] 80211n a-mpdu max-length-exponent 3  //3 indicates that the maximum length of frames aggregated into an A-MPDU is 65,535 bytes.


576d1099dbce5.png 

By default, 802.11ac supports A-MPDU with the maximum length of 1,048,575 bytes. Users can run the following command to set the length without the need to enabling this function:

[AC6605-wlan-radio-prof-rp01]80211ac a-mpdu max-length-exponent 7  //7 indicates that the length of frames aggregated into an A-MPDU is 1048575 bytes.

Moreover, 802.11ac supports MAC Service Data Unit (MSDU) aggregation, increasing the data transmission rate. Users can run the following command to configure this function:

system-view

[AC6605] wlan

[AC6605-wlan-view] radio-profile name rp01

[AC6605-wlan-radio-prof-rp01] a-msdu send enable

[AC6605-wlan-radio-prof-rp01] a-msdu send max-subframes 2  //2 indicates the number of frames aggregated into an A-MSDU.


576d10b5d689a.png 

In addition to 802.11 series protocols, another common concept in the WLAN field is Wi-Fi.

Wi-Fi is short for Wireless Fidelity. In the 802.11b era, although all 802.11b products adhere to the 802.11b standard, it is still possible for one manufacturer to build a product based on one interpretation of a standard feature, while another manufacturer works with a different interpretation. In 1999, WLAN device manufacturers around the world established the Wireless Ethernet Compatibility Alliance (WECA), which changed its name to Wi-Fi Alliance later, to ensure compatibility of WLAN products. The Wi-Fi Alliance introduced the Wi-Fi CERTIFIED program to test and certify compatibility of 802.11b products. If a product passes these tests, it is certified and receives a Wi-Fi CERTIFIED stamp. Wi-Fi CERTIFIED gradually extended to 802.11a, 802.11g, and 802.11n. Because the 802.11n standard publication took a long time and there were market demands, Wi-Fi Alliance certificated hundreds of 802.11n products based on the IEEE 802.11n Draft 2.0 standard before the 802.11n standard was ratified. This is the reason why we could buy 802.11n products with good compatibility before the 802.11n standard was published.

Question:

Can a 2.4 GHz STA access an 802.11ac WLAN?

The post is synchronized to: From Beginner to Expert-WLAN Fundamentals

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user_235153
Created Jun 24, 2016 12:34:26

顶一下。。
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wissal
MVE Created Apr 11, 2018 10:07:23

useful document, thanks
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Pogo
Created Jul 5, 2019 07:39:25

Nice documents, but should be updated with 802.11ax/WiFi 6. There are some new technical issues which are interessting for sure.
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