Geekzone: technology news, blogs, forums
Guest
Welcome Guest.
You haven't logged in yet. If you don't have an account you can register now.




637 posts

Ultimate Geek

Subscriber

#273020 31-Jul-2020 09:24
Send private message quote this post

Hi

 

It is pretty common knowledge that if you have say a 4x4 MIMO AP you need a 4x4 MIMO Client to take advantage of that

 

The great area which i don't understand is if you have a 2x2 client on the network does that drag that whole access point down to 4x4 or just those clients?


Create new topic
7666 posts

Uber Geek

Trusted
Subscriber

  #2531895 31-Jul-2020 10:03
Send private message quote this post

Hi, no it will not drag the whole network down, MIMO streams are per client specific, ie 2x2 will not impact a 3x3 or higher devices ability.

 

Something to think about, a 4x4 or 3x3 AP serving a 2x2 client will perform better on Rx path to AP compared to a 2x2 AP with same 2x2 client, essentially the extra trains of the AP allow the AP to track and converge (think focus) better resulting in better stream orthoganal perfomance and therefore achieve better stream loading (higher throughput). Not sure if I have explained myself well there.

 

Cyril


'That VDSL Cat'
12460 posts

Uber Geek

Trusted
Spark
Subscriber

  #2531899 31-Jul-2020 10:22
Send private message quote this post

a 4x4 AP also can comunicaite to two 2x2 clients at once using MIMO ax enables the ability for that in the TX path too.





#include <std_disclaimer>

 

Any comments made are personal opinion and do not reflect directly on the position my current or past employers may have.

 


 
 
 
 


3455 posts

Uber Geek

Trusted

  #2532432 31-Jul-2020 21:18
Send private message quote this post

Im gonna ramble a bit but here goes....

 

 

 

MIMO can use multiple streams to send data to a client. It relies on multipath / bouncing off walls to create a slight delay in each stream. At the receiving end, some fancy processing splits out the streams and assembles the data in the original order again.    

 

To understand how a client capable of a smaller number of streams affects total throughput, its best to think of airtime. 

 

If each stream ran at 54mbits, then a 1x client can communicate at 54mbits per second under perfect conditions. 
I use some older common speed numbers but its just to illustrate the example.  
A 2x stream client can therefore communicate at 108mbits per second.

 

But consider the airtime impact. 

 

We will need to remove the concept of collision collapse, more on that later, and assume the clients use TDMA in a perfect world for this example....

Example 1 - 1x stream client1 + 2x stream client2 

If Client1 is communicating, the AP will in fact slow down to accommodate that client. 

 

A 2x stream AP which is capable of 108mbits per second, can divide its transmit time between the two clients. 
If we assume each client is communicating at the same time, then in our perfect world example, the time is divided in half equally between the two clients. 

The impact therefore is during single second of airtime, client1 gets 50% of a single streams throughput = 27mbits since it only has half the airtime to communicate.

Client2 also gets half of the second to communicate but due to it having two streams it has a higher starting point so 108mbits of a full second equivalent = 54mbits during its half second of airtime. 

 

 

The total throughput of the AP therefore becomes 81mbits during that full second of airtime. 

 

 

 

Once client1 stops requiring airtime, the AP can spend more airtime transmitting packets using both streams to client2. 

 

 

 

There are a couple of caveats....

 

CSMA - Collision Sense Multiple Access. 
This is the multi-user mechanism used by Wifi up until 802.11ac generation 2. 
Each packet sent has some extra error correction wrapped around it. Hence why you only get 50-60% TCP throughput compared to the advertised air-rates of the wifi equipment. A CRC check is performed on each packet and an acknowledgement sent between the wireless nodes at the wireless protocol layer. This is in addition to the CRC checking that the TCP protocol performs at the TCP layer. If a packet fails, it causes the CSMA recovery to take place. 

If an AP and client1 are talking, and client2 wants to communicate, it will wait until the air is clear to transmit. If it takes too long, or it cant hear client1 because its too far away at the opposite end of the house (called the hidden node issue) then it will transmit anyway. 
This causes the packets to fail their CRC check due to the interference or a "collision", and the CSMA recovery will take place. 

The recovery involves each station choosing a random wait time, waiting out the chosen amount of time, then retransmitting with the hope that its packet will get through. Repeat as necessary. This can waste a lot of airtime. 

 

So this explains why two devices on a wifi network streaming video will often have problems. Due to CSMA collisions, the throughput capability of the access point will "collapse" and having multiple devices on the network does not mean that each will get 50% of the throughput - the random wait times will often drop throughput capability to 20% or less of what the AP is capable of when communicating with only a single client. 

 


TDMA - Time Division Multiple Access
This is the multi-user mechanism used in new versions of the Wifi protocol. It was introduced as an optional extra in what is often called the second generation of 802.11ac but not all manufacturers use it and if used, the AP and all stations must support it. 
I understand it is universally adopted by 802.11ax or future standards coming after 802.11ac.  

 

When using TDMA, the AP will assign specific airtime slots to each client. The clients will only transmit during their allocated time slots. As a client starts transferring data, the AP will allocate more time slots to the client and announce a revised timing plan at regular intervals. 
This removes the dead-airtime caused by a packet collision under the old CSMA standard and thus enables much higher throughput when multiple clients are communicating at the same time.   


MU-MIMO or Multiple User- MIMO

 

With multiple streams being transmitted from the AP, its possible to go further and dynamically reallocate the streams to specific clients. 
For example, our client1 is a single stream device, and client2 is a dual stream device. 
Before we learned how the throughput of the AP would drop from 108mbits to 81mbits because of how client1 would use extra airtime per megabit of throughput by not using the second stream. 

With some fancy processing, its possible that stream 1 is allocated to client 1, while stream 2 is allocated to client 2. 
The AP is able to send data to both clients at the same time. Client 2 just ignores the first stream, and client 1 ignores the second stream. 

 

So Client 1 doesnt have to share its airtime, and gets the full 54mbits of throughput. 
Client 2 meanwhile will indeed drop in speed from its 108mbits using 2 streams to 54mbits using 1 stream. 
However the total throughput of the AP is back up at 108mbits (if we ignore the possibility of collision collapse) 
By reallocating the streams and not airtime, its much more efficient. 

 

MU-MIMO is another feature of the latest revision of 802.11ac and isnt backward compatible with older clients. 
To make use of it, the AP and clients must support it. 

 

Like TDMA, MU-MIMO is moving to the core feature set of the newer wifi standards such as in 802.11ax. Currently its optional and only some manufacturers support it in 802.11ac.

 

 

 

So to summarize...

 

If you have the latest "spider-style" AP and multi-stream capable clients with the latest generation 802.11ac then you can achieve some very high throughput rates - especially noticable when multiple clients are transferring large amounts of data compared to the same setup on an 802.11n network. 

 

But if you have an 802.11g/n device on the 802.11ac network then assume everything will slow down to accommodate that device, and anything faster than dual stream 802.11n and mu-mimo will be disabled for the duration of that legacy device being connected to the AP. 

 

In any case, if you have a device that has a low signal usually due to distance from the AP, or interference, then the AP will waste a lot of airtime having to re transmit packets to that device so everything will slow down. 

 



 





Ray Taylor
Taylor Broadband (rural hawkes bay)
www.ruralkiwi.com

There is no place like localhost
For my general guide to extending your wireless network Click Here




Mad Scientist
22616 posts

Uber Geek

Trusted
Lifetime subscriber

  #2532545 1-Aug-2020 07:27
Send private message quote this post

raytaylor:

 

So to summarize...

 

If you have the latest "spider-style" AP and multi-stream capable clients with the latest generation 802.11ac then you can achieve some very high throughput rates - especially noticable when multiple clients are transferring large amounts of data compared to the same setup on an 802.11n network. 

 

But if you have an 802.11g/n device on the 802.11ac network then assume everything will slow down to accommodate that device, and anything faster than dual stream 802.11n and mu-mimo will be disabled for the duration of that legacy device being connected to the AP. 

 

In any case, if you have a device that has a low signal usually due to distance from the AP, or interference, then the AP will waste a lot of airtime having to re transmit packets to that device so everything will slow down. 

 

 

wow this is gold. though my brain will need the entire weekend to digest everything!





Involuntary autocorrect in operation on mobile device. Apologies in advance.


29124 posts

Uber Geek

Moderator
Trusted
Biddle Corp
Lifetime subscriber

  #2532558 1-Aug-2020 09:28
Send private message quote this post

Batman:

 

wow this is gold. though my brain will need the entire weekend to digest everything!

 

 

Just wait until you hear about WiFi6..




637 posts

Ultimate Geek

Subscriber

  #2535022 5-Aug-2020 11:47
Send private message quote this post

Batman:

 

raytaylor:

 

So to summarize...

 

If you have the latest "spider-style" AP and multi-stream capable clients with the latest generation 802.11ac then you can achieve some very high throughput rates - especially noticable when multiple clients are transferring large amounts of data compared to the same setup on an 802.11n network. 

 

But if you have an 802.11g/n device on the 802.11ac network then assume everything will slow down to accommodate that device, and anything faster than dual stream 802.11n and mu-mimo will be disabled for the duration of that legacy device being connected to the AP. 

 

In any case, if you have a device that has a low signal usually due to distance from the AP, or interference, then the AP will waste a lot of airtime having to re transmit packets to that device so everything will slow down. 

 

 

wow this is gold. though my brain will need the entire weekend to digest everything!

 

 

Mine too, just finally caught up on this thread thanks @raytaylor for your detailed explanation

 

 

 

 


Create new topic




News »

Freeview On Demand app launches on Sony Android TVs
Posted 6-Aug-2020 13:35


UFB hits more than one million connections
Posted 6-Aug-2020 09:42


D-Link A/NZ extends COVR Wi-Fi EasyMesh System series with new three-pack
Posted 4-Aug-2020 15:01


New Zealand software Rfider tracks coffee from Colombia all the way to New Zealand businesses
Posted 3-Aug-2020 10:35


Logitech G launches Pro X Wireless gaming headset
Posted 3-Aug-2020 10:21


Sony Alpha 7S III provides supreme imaging performance
Posted 3-Aug-2020 10:11


Sony introduces first CFexpress Type A memory card
Posted 3-Aug-2020 10:05


Marsello acquires Goody consolidating online and in-store marketing position
Posted 30-Jul-2020 16:26


Fonterra first major customer for Microsoft's New Zealand datacentre
Posted 30-Jul-2020 08:07


Everything we learnt at the IBM Cloud Forum 2020
Posted 29-Jul-2020 14:45


Dropbox launches native HelloSign workflow and data residency in Australia
Posted 29-Jul-2020 12:48


Spark launches 5G in Palmerston North
Posted 29-Jul-2020 09:50


Lenovo brings speed and smarter features to new 5G mobile gaming phone
Posted 28-Jul-2020 22:00


Withings raises $60 million to enable bridge between patients and healthcare
Posted 28-Jul-2020 21:51


QNAP integrates Catalyst Cloud Object Storage into Hybrid Backup solution
Posted 28-Jul-2020 21:40



Geekzone Live »

Try automatic live updates from Geekzone directly in your browser, without refreshing the page, with Geekzone Live now.


Support Geekzone »

Our community of supporters help make Geekzone possible. Click the button below to join them.

Support Geezone on PressPatron



Are you subscribed to our RSS feed? You can download the latest headlines and summaries from our stories directly to your computer or smartphone by using a feed reader.

Alternatively, you can receive a daily email with Geekzone updates.