Wireless Throughput Question

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justintime32
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PostPosted: Mon Dec 21, 2009 4:21    Post subject: Wireless Throughput Question Reply with quote
I have been trying to stream videos to my PS3 or Xbox over my wireless network (I have a WRT150N 1.1 running DD-WRT build 12426). Through some experimenting, I've noticed that the maximum throughput I get hovers between 1 and 1.5 MB/s, but never is faster than that.

This means that I generally can't stream high-resolution videos from my laptop (which is wireless) to my PS3 or Xbox (which are also wireless). The Xbox seems to handle streaming better, as the PS3 stutters like crazy; I'm not sure why this is, but the throughput is the same with both.

Since both clients are wireless, it looks like I'm getting 3 MB/s of total wireless throughput, multiplied by 8 that would be 24 Mb/s.

I guess my question is, is there a reason why I'm not getting something closer to 54 Mb/s? I understand there is some wireless overhead, but would it really account for 50-60% of wireless throughput? All my wireless clients report connecting at the full 54 Mb/s.

It's odd since I can download files over the internet at 15 Mb/s, which is much faster than I can stream something locally.
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GeeTek
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PostPosted: Mon Dec 21, 2009 5:48    Post subject: Re: Wireless Throughput Question Reply with quote
justintime32 wrote:
I guess my question is, is there a reason why I'm not getting something closer to 54 Mb/s? I understand there is some wireless overhead, but would it really account for 50-60% of wireless throughput?

That is normal. WIFI is half duplex. 27 Mbps EACH WAY = 54 Mbps.
justintime32 wrote:
It's odd since I can download files over the internet at 15 Mb/s, which is much faster than I can stream something locally.

WIFI has no co-ordination of the transmit time for the client. It is collision based and the clients wait a RANDOM period of time before re-sending a packet that collided with the other client's packet. That is why your single stream is so much faster than a stream between two radios transmitting across a single access point. You can improve this drastically by using two access points wired back-to-back on different channels and having the clients connect to their own access point, or you could upgrade your whole system to "N" MIMO hardware and get an air link rate of 270 Mbps.
justintime32
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PostPosted: Mon Dec 21, 2009 5:57    Post subject: Re: Wireless Throughput Question Reply with quote
GeeTek wrote:
That is normal. WIFI is half duplex. 27 Mbps EACH WAY = 54 Mbps.

I understand this. The effective throughput is 1.5 MB/s, which I doubled to get 3 since it is half duplex. That multiplied by 8 is 24 Mb/s. In an ideal situation I should be getting twice that, meaning 3 MB/s of effective throughput. I'm assuming my conditions aren't ideal, but I would think that I would get a bit more than I have.
GeeTek wrote:
or you could upgrade your whole system to "N" MIMO hardware and get an air link rate of 270 Mbps.

In theory, the router is capable of this (it is a WRT150N), however AFAIK it's impossible to upgrade the wireless card in a PS3.

On a somewhat-related sidenote, the WRT150N is supposed to be a MIMO router, however I noticed that with no clients connected, the rate is displayed as 270 Mb/s; as soon as my MacBook Pro connects (it has an AirPort extreme, with MIMO), it drops down to 130 Mb/s. Weird...
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PostPosted: Mon Dec 21, 2009 6:00    Post subject: Re: Wireless Throughput Question Reply with quote
justintime32 wrote:
In an ideal situation I should be getting twice that, meaning 3 MB/s of effective throughput. I'm assuming my conditions aren't ideal, but I would think that I would get a bit more than I have.

Both of your clients are connecting wirelessly. Did you not grasp what I just explained about collisions ?

With a single wireless client the transfer rate from LAN to client will be 27 Mbps under ideal conditions. I'm in a no interference environment and can clock that easily. With frame bursting enabled I can exceed 30 Mbps.
GeeTek
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PostPosted: Mon Dec 21, 2009 6:04    Post subject: Re: Wireless Throughput Question Reply with quote
justintime32 wrote:
...with no clients connected, the rate is displayed as 270 Mb/s;

I would pour coffee on a router that did that.
GeeTek
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PostPosted: Mon Dec 21, 2009 6:12    Post subject: Re: Wireless Throughput Question Reply with quote
justintime32 wrote:
I guess my question is, is there a reason why I'm not getting something closer to 54 Mb/s? I understand there is some wireless overhead, but would it really account for 50-60% of wireless throughput? All my wireless clients report connecting at the full 54 Mb/s.


justintime32 wrote:
...as soon as my MacBook Pro connects ...it drops down to 130 Mb/s.

WTF ?
justintime32
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Joined: 02 Mar 2008
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PostPosted: Mon Dec 21, 2009 6:15    Post subject: Re: Wireless Throughput Question Reply with quote
GeeTek wrote:
Both of your clients are connecting wirelessly. Did you not grasp what I just explained about collisions ?

With a single wireless client the transfer rate from LAN to client will be 27 Mbps under ideal conditions. I'm in a no interference environment and can clock that easily. With frame bursting enabled I can exceed 30 Mbps.


I'm sorry... maybe I misunderstood. Since the server is sending data at 1.5 MB/s, and the client is receiving data at 1.5 MB/s, I assumed that meant the throughput was 12 Mb/s in each direction, for a total of 24 Mb/s. So what you're saying is that the combined throughput in a wireless network won't exceed ~27 Mb/s? If so, I guess that explains my observations.

GeeTek wrote:
justintime32 wrote:
...with no clients connected, the rate is displayed as 270 Mb/s;

I would pour coffee on a router that did that.


Well, that was in the DD-WRT web interface. And if I ruin this one, I'll have to throw in a POS D-Link, which I definitely don't want to do :)

GeeTek wrote:
justintime32 wrote:
I guess my question is, is there a reason why I'm not getting something closer to 54 Mb/s? I understand there is some wireless overhead, but would it really account for 50-60% of wireless throughput? All my wireless clients report connecting at the full 54 Mb/s.


justintime32 wrote:
...as soon as my MacBook Pro connects ...it drops down to 130 Mb/s.

WTF ?


I shut off N mode on the router to see if that was affecting anything, so ATM all my clients are at G speed. Normally my Mac is at N speeds.
GeeTek
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PostPosted: Mon Dec 21, 2009 6:50    Post subject: Reply with quote
justintime32 wrote:
So what you're saying is that the combined throughput in a wireless network won't exceed ~27 Mb/s?

No. What I said in reference to that was this...
GeeTek wrote:
WIFI has no co-ordination of the transmit time for the client. It is collision based and the clients wait a RANDOM period of time before re-sending a packet that collided with the other client's packet. That is why your single stream is so much faster than a stream between two radios transmitting across a single access point.

Ignoring collisions and re-sends, if you were to stream from each end-point to the other simultaneously, the combined bandwidth would tally to your long lost 54 Mbps link rate.

Also, if you wired one of the end-points to the LAN port of the router and then streamed from each end-point to the other, then you would eliminate the collisions and protocol in-efficiencies, and again achieve a combined bandwidth of the 54 Mbps link rate.
phuzi0n
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PostPosted: Mon Dec 21, 2009 11:25    Post subject: Re: Wireless Throughput Question Reply with quote
justintime32 wrote:
So what you're saying is that the combined throughput in a wireless network won't exceed ~27 Mb/s? If so, I guess that explains my observations.

This is precisely right for 802.11g spec. Wifi is indeed half duplex (only one thing can transmit at a time) but in addition to that, there is lots of overhead for error correction that makes it only achieve roughly half the link rate as throughput. That ~27 mbps throughput for G spec is then divided among everything competing for their share of the bandwidth.

Using mixed mode is still more beneficial than using a slower spec because the faster spec will be used for clients that do support it. For instance, an N client connected at 130mbps could transmit 1MB in ~1/8 of a second to the AP, but then the AP would have to spend ~1/3 of a second to transmit it to a G client at connected at 54mbps. The more air time the slower spec clients use, the more mixed mode becomes like the slow spec. The more air time faster spec clients use, the more it becomes like the faster spec.

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Nomad Ninja
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Joined: 25 Dec 2009
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PostPosted: Fri Dec 25, 2009 0:40    Post subject: Re: Wireless Throughput Question Reply with quote
justintime32 wrote:

On a somewhat-related sidenote, the WRT150N is supposed to be a MIMO router, however I noticed that with no clients connected, the rate is displayed as 270 Mb/s; as soon as my MacBook Pro connects (it has an AirPort extreme, with MIMO), it drops down to 130 Mb/s. Weird...


Hello, I'm having exactly the same issue described here. I just recently moved to the DD-WRT firmware cuz some bugs started popping up in the Linksys original firmware (wireless-n speeds dropping to 1 Mb/s). Using the original firmware, my wireless-N client (yes, only one wireless device on this router, with wan port wired to FIOS router/modem) connected at speeds of 243-270 Mb/s. After flashing to the DD-WRT firmware (first the initial mini, then the standard generic (v24 pre-sp2 [beta] build 13064)) the highest available speed when connected wirelessly is only 130 Mb/s, but when connected thru ethernet it shows 270 Mb/s as an option for the MIMO wireless. Is this all the DD-WRT firmware is capable of producing? or is there some settings I need to tweak further?

Other info you may wish to know:
WLAN Card: Gigabyte Aircruzer N300 (GN-WS30-RH) MIMO 300 Mb/s, using 3 antennae (2T/3R)
Router in question: Linksys WRT150N v1.1 (receiving WAN from FIOS router/modem)
Connected to router: 1 wireless-n laptop, 1 wired xbox360
OS: Windows 7 Ultimate 64-bit as well as Windows Vista Home Premium 32-bit (dual-boot config)

Any help resolving this question is greatly appreciated, I've been pouring through the forums for days trying to find more info on this, and this post is first place I've seen this particular issue. Thank you in advance.


*edit* noting the time on this post, I'd like to wish everyone a Merry Christmas
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