help understanding wifi speed ratings

[Special K]

I am in the market for a new router and am having trouble deciphering all of the speed ratings that I'm finding. For reference, the last router I bought was the Linksys WRT54GL in 2006.

I started by reading this article:

http://www.cnet.com/how-to/home-networking-explained-part-1-heres-the-url-for-you/

The article says:

On each band, the Wireless-N standard is available in three setups, depending on the number of spatial streams being used: single-stream (1x1), dual-stream (2x2), and three-stream (3x3), offering cap speeds of 150Mbps, 300Mbps and 450Mbps, respectively.

Here are my follow up questions:

1. Are those cap speeds quoted the total amount of wifi bandwidth available to all clients connected to the router, or could each client simultaneously see those speeds (at least in theory)?

2. How do I know how many streams a given client uses?

3. Is there a limit to the total number of clients that can connect using each number of streams (1, 2, or 3 in the case of a 3x3/450 Mbps router)? For example, if I have 5 clients connected each through 1 stream, does that impose any limit on the number of additional clients that could connect using 2 or 3 streams?

The article also says:

With two separate 5Ghz bands, both high- and low-end clients can operate in their own band at their respective top speeds without affecting each other.

Here are my questions:

1. What is a "low-end client" in this context? Is it one with a lower number of streams than the other clients? Is it one with a lower signal strength than other clients?

2. How/why does a low end client affect a high-end one?

3. Has this been true for every wifi standard, or is this new with 802.11ac?

 

[azazel1024]

To answer some of them.

Wifi is a shared medium. All devices share airtime. Having some slow devices on your network will slow down the fast ones too, because they take up talk time. The speed is limited by each device and the router.

So if you have a 450Mbps client and a 450Mbps router, that is the maxium modulation rate (I'll get in to that in a second). If you have a 150Mbps client, that is the fastest that client can connect, no matter how fast the router is.

If both clients were trying to talk with the router at the same time, you'd divy up the air time (and it likely won't be even), so the slow client might get 75Mbps, but the faster client will get 225Mbps if they both divied up the air time 50/50. That means the max performance of the router is now 300Mbps instead of 450Mbps.

On modulation, that is the speed advertised on the box. This is the FASTEST it can communicated data (with a supporting client of that speed). It is NOT the payload speed/the speed you'll get. Because wireless is a highly interference prone medium, forward error correction is included (parity, so if I send you 56 bytes of data, here is another 12 bytes of data representing that 56 bytes, if the 56 bytes doesn't add up to the 12 bytes, the transmission was in error, resend). This takes up roughly 24% overhead. So the MAXIMUM in the BEST case scenario you can get is 76% of the box figure.

Actual interference, distance from the router, etc. will diminish this also.

As for what each client can handle, you'll have to research the client. Most phones only have a single spatial stream radio in them. That means at best 150Mbps on 802.11n or 433Mbps on 802.11ac. In a lot of cases, the interface that the wifi adapter in the phone/tablet has can't actually handle full data rate, so it might be limited to more like 100Mbps no matter what it claims it can do.

For the number of clients, sky is the limit, but most home routers/access points are going to start crumbling after about 20 low use wireless devices connect. Call it maybe 30-40 for the latest and greatest home wifi routers/APs. This is LOW USE. Most of them just sitting occasionally getting some push notifications or something. If you had 20 clients trying to stream youtube and netflix, even if you had the internet connection to handle it, pretty much all home routers would crumble under that kind of workload.

 

[pauldun170]

http://www.smallnetbuilder.com/wireless/wireless-basics/32175-how-fast-can-your-wi-fi-go
Some additional resources.

http://www.smallnetbuilder.com/wire...-smallnetbuilders-wireless-faq-the-essentials

http://www.smallnetbuilder.com/wire...316-how-to-buy-a-wireless-router-2014-edition

 

[Special K]

Thanks for the information.

I read the posts and the provided links and still have a few follow up questions:

If both clients were trying to talk with the router at the same time, you'd divy up the air time (and it likely won't be even), so the slow client might get 75Mbps, but the faster client will get 225Mbps if they both divied up the air time 50/50. That means the max performance of the router is now 300Mbps instead of 450Mbps.

I'm not sure I follow the math here. If the total bandwidth of the router is 450 Mbps, and the slower client gets 75 Mbps, why wouldn't the fast client get 450-75 = 375 Mbps?

One more question that still isn't clear to me: a wireless N router with 3x3 streams has a max theoretical throughput of 450 Mbps (150x3). A client must also support 3x3 streams to even get the theoretical throughput of 450 Mbps when connected to this router.

Suppose a client that only supports 1x1 stream connects to this router. The max speed will be 150 Mbps. Now suppose another 1x1 client connects to the router. Does that client get its own 150 Mbps stream, or does it share the 150 Mbps with the first client?

UPDATE: This article appears to answer my question:

http://arstechnica.com/information-...but-multi-user-beamforming-will-save-the-day/

In particular:

The soon-to-be-deployed technology is called MU-MIMO (multi-user, multiple-input and multiple-output), which is like a wireless "switch" that sends different data to different receivers at the same time.

It's powered by multi-user beamforming, an improvement over the single-user beamforming found in first-generation 11ac products. MU-MIMO will let wireless access points send data streams of up to 433Mbps to at least three users simultaneously, for a total of 1.3Gbps or more. First-generation 11ac equipment without MU-MIMO could send those streams of data simultaneously, but only to one device—and only if that device was capable of receiving multiple streams. Many computers could handle the influx of data, but smartphones and tablets generally couldn't. That meant they could only receive one stream (occasionally two) because of power limitations.

So it sounds like the double, triple, and quad stream modes only apply to one client at a time. It still isn't clear whether the remaining single stream clients would each get their own 150 Mbps or would all share the same 150 Mbps.

UPDATE 2:

http://www.anandtech.com/show/7921/...increasing-the-efficiency-of-80211ac-networks

The difference between SU-MIMO (Single User MIMO) and MU-MIMO (Multi User MIMO) is pretty simple. In the SU case, each device gets full exclusive access to the AP's bandwidth for a given timeslice before moving on to the next one. This works beautifully for many-stream MIMO devices (e.g. a 3x3 3-stream notebook) as each device can easily use up all of the bandwidth the AP has to offer. In reality though, you're likely to have a number of devices attached to the network that support only 1 or 2 streams (think: tablets or smartphones). In the case of a SU-MIMO network with a 3 or 4 stream 802.11ac AP and a bunch of 1 or 2-stream clients, you'll end up with a lot of unused bandwidth capacity on the AP for any given timeslice. Each device is served sequentially, and a single stream device can only use a fraction of the AP's total bandwidth.

The problem seems to be a lack of consistent terminology from one article to the next. For example, neither the CNET article nor the small network articles ever use the term MIMO.