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Question is about the speed of data transferring when several devices are connected to AP

Posted on 2013-11-02
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Last Modified: 2013-11-04
My wi-fi device is Ubiquity Bullet M2HP and it's works in AP bridge network mode and it's supports 802.11n. I was reading somewhere that the max speed is achieves when all client devices have 802.11n wireless network adapters. And if at least one of them has 802.11b/g the AP will work in b/g-mode, but not in n-mode. Is that true?
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Question by:dmitrij75
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by:aadih
ID: 39618836
"Even though your fancy new router may be super-fast with 802.11n (or even 802.11ac), as soon as a device connects using an older protocol, say, 802.11g, the entire network slows down. The fix to this problem is to set the router to only broadcast newer wireless modes. For your reference, the speeds from slowest to fastest are: b, g, n, ac."

From "7 Tips to Boost Wireless Speed, Range, and Reliability" at:

< http://www.audioholics.com/home-theater-connection/increase-wireless-speed-and-range >
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by:Craig Beck
ID: 39618854
It's not exactly true, but it doesn't actually work like most people think either.

The network will drop to the speed of the slowest connection, but only when the slowest client sends or receives data.

Basically, each user WILL connect at their highest achievable data-rate (config permitting).  Wireless is half-duplex by nature so only one client is ever actually connected to the network at a time.  It just happens so quickly that it appears that all clients are actually connected simultaneously.

The issue is that clients connected with slower data-rates need more airtime to send the same amount of data as a client connected at a faster data-rate.  That gives the effect of slowing down everyone else.

One of the things we can do to ensure that people at lower data-rates don't impact the throughput for other, faster users is to limit which data-rates clients can connect at.  This will decrease the size of the area in which users can connect, but it will mean that users have to connect at faster data-rates, and therefore require less airtime.
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by:Akinsd
ID: 39618917
In addition to the two comments above.

Wireless network is 1 domain and the number of users connected determines your overall speed as they are sharing bandwidth.
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by:pgm554
ID: 39618937
The way to avoid the slowdown is to have the n clients connect to the 5 ghz range and the g/b clients connect to the 2.4 ghz range.

Hence the reason for dual band nics and ap's.
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by:Craig Beck
ID: 39618942
Well again not entirely true...

802.11n is an extension of both 802.11a and 802.11g, so it's not correct to say 802.11n users should use 5GHz only.  Also not all client devices have support for the 5GHz frequency so it's not always possible to do that anyway.

The reason for dual-band NICs is to allow connections to 2.4GHz or 5GHz APs.  This means there's less chance of not being able to connect to an AP.  It's nothing to do with 802.11n.

The reason for dual-band APs is multi-faceted.  Some APs will let you use either 2.4GHz or 5GHz simultaneously, allowing for more users to connect to the AP as a general advantage.  Some dual-band APs only allow you to use either 2.4GHz or 5GHz at a time, so it's only dual-band in the sense that you 'could' use either frequency.
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by:Akinsd
ID: 39619023
Your network is as fast as your weakest link!

If you remember token ring technology and how it works, it will help answer your question. Wireless can't detect collision and therefore, it is designed to avoid it (Carrier Sence Multiple Access / Collision Avoidance - CSMA/CA). Meaning, devices take turns to communicate. This is part of the reason also why the communication is half-duplex.

In essence, your overall speed will be affected as devices take turns.

It's like running a relay-race, with a 4-team member. The overall speed is the average of all 4 members. The more people that join the team, the more the time it takes taking turns


Additional info

http://compnetworking.about.com/cs/wireless80211/a/aa80211standard.htm
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by:Craig Beck
ID: 39619045
Not in wireless...

It's a difficult concept to explain and understand, but I could still achieve a 54Mbps wireless link while an 802.11b device is connected.  Sure, my overall throughput would reduce, but I wouldn't be limited to 11Mbps.

The focus of this question is will all devices be limited to legacy data-rates if legacy clients connect.  The answer is no.
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by:Akinsd
ID: 39619056
There are 2 things in here.
1. Speed per device
2. Overall Speed

The type of devices connected and the number of devices connected will eventually determine the speed.

Imagine a Single Highway.

A Lamborghini can travel at 200MPH if it desires (let's say off peak hours)
A Ford Pinto may manage 100MPH (off peak also)

The Lamborghini will have to slow down when trailing behind the Pinto until the Pinto exits, after which it can now accelerate.

Even the Pinto will have to slow down as more cars get on the freeway than cars exiting.
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by:Craig Beck
ID: 39619061
I know what you're saying... but it really doesn't limit the device in the same way.

If two users connect at 11Mbps, it takes them both 5s to send/receive their data.  That's 10s total.

If two users connect, one at 11Mbps and one at 54Mbps, it takes one user 5s to send/receive the data, but only 1s for the other user to send/receive the same data.  Therefore you're not limiting ALL users to 100MPH as in your example- you're just making the car drive that fast for a portion of the time.

In wireless, one user can connect at one data-rate while another can connect at another.  It's a totally different concept to links-in-the-chain.

The thing to remember here is that the faster client isn't actually connected at the same time as the slower client, so it's only waiting for the duration and therefore nothing is being sent/received.
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by:Akinsd
ID: 39619085
The connection is not waiting for all traffic for 1 user to be sent before switching to another user. These transmissions are in microseconds and don't become noticeable to humans.

To go more technical, install and configuring monitoring devices or applications to see the flow of traffic.

The transmission is in packet chunks, not by data.

There are reasons
1. Why network traffic is slow when tons of users connect at the same time?
2. For congestion management
3. Why Quality of Service (QoS) is implemented in a network
4. Why we prioritize traffic with QoS
5. For Weighted Queuing or Prioritization in Congestion Management?

Electronics are based on Computer programming.
Computers multitask by default.
This is also the reason Computers run slow when you open multiple files on your computer. It manages everything altogether but it is one single processor or group of processors.
The reason we now have quad cores, octa cores to speed up computers is to allow the processor to attend to multiple things at a time.



Back to the author's question
My wi-fi device is Ubiquity Bullet M2HP and it's works in AP bridge network mode and it's supports 802.11n. I was reading somewhere that the max speed is achieves when all client devices have 802.11n wireless network adapters. And if at least one of them has 802.11b/g the AP will work in b/g-mode, but not in n-mode. Is that true?

Yes, but the impact is not noticeable until you exceed the optimum!
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Craig Beck earned 500 total points
ID: 39619146
The connection is not waiting for all traffic for 1 user to be sent before switching to another user. These transmissions are in microseconds and don't become noticeable to humans.
In my examples I'm being as general as I can.  I'm fully aware that an AP doesn't wait for ALL of the user's data to be sent/received before moving on to the next client.  To put it as basically as I can, if one user was streaming a video from YouTube that would render all other clients' connections useless as they'd never get a slot to send or receive data.

And if at least one of them has 802.11b/g the AP will work in b/g-mode, but not in n-mode. Is that true?

Yes, but the impact is not noticeable until you exceed the optimum!
That is completely and absolutely incorrect.

The AP will allow the b/g user to connect at b/g data-rates, while the n user will connect at n-rates.  Try it!

In a mixed-mode network, tests by multiple vendors determine that there is a noticeable drop in performance for 802.11n clients when a non-n client is connected to the network, but performance isn't nearly the same as the legacy client.  Instead on average the perceived difference in throughput for the 802.11n client is around 30-50% worse than in a strict 802.11n-only environment.  That's hardly matching the 20Mbps you'd get as a 802.11g client connected at 54Mbps - more like 60-70Mbps throughput instead when connected at 300Mbps.

The data-rate and the achievable throughput are NOT the same thing.  Whether a b/g client slows a n client down in pushing traffic or not, the rate at which the client connects to the AP is totally different to that.  802.11 defines a maximum RADIO data-rate at any given standard.  That rate is completely independent of the maximum achievable throughput rate (which is always at least 50% lower than the radio data-rate).

Honestly, I understand where you're coming from, but I get paid to do this as a wireless consultant.  It's my day-job.  I get asked questions like this all the time.  I design WLAN systems for massive multi-billion pound corporates so I know the concepts, the technical pros and cons, the problems users see, etc...
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by:Akinsd
ID: 39620946
I build multiple site networks from ground up also.  I am amazed at the outcome but someone will find out the truth someday when their eyes open.

FYI, the reason a video streamer (udp traffic by the way) holds up traffic is not because every other user queues up behind them. Knowledge of theory is VERY IMPORTANT but most people still use trial by error and get stuck when something goes wrong. I try to educate people as much as possible but not everyone as I have found cares about the intricate details which limits their growth.

Getting something to work or getting it to work at peak performance is a different ball game
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by:Craig Beck
ID: 39621191
I am amazed at the outcome but someone will find out the truth someday when their eyes open.
??

FYI, the reason a video streamer (udp traffic by the way) holds up traffic is not because every other user queues up behind them. Knowledge of theory is VERY IMPORTANT
I know this, but evidently you didn't understand my point...

In my examples I'm being as general as I can.  I'm fully aware that an AP doesn't wait for ALL of the user's data to be sent/received before moving on to the next client.  To put it as basically as I can, if one user was streaming a video from YouTube that would render all other clients' connections useless as they'd never get a slot to send or receive data.
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