Advanced Question: How does 802.11n bandwidth handle a/b/g mix-ins?

[chuck]

Questions 3 and after are my main concern, please skip to that if you must. : )
1) Say you have a router that operates on n. This gives you a max bandwidth/speed of 300 Mbps when communicating with other n devices, right?
2) If you have 4 x n devices linking to this n router at the same time -> Device A requires 100Mbps, B requires 70Mbps, C requires 70Mbps, D requires 100Mbps (total simultaneous req of 340Mbps). Let's say they all req the same QOS priority. How does the router deal with the additional 40Mbps or more that it cannot accommodate?
3) If, say, you have 3 x n devices in your router's 802.11n LAN - which it can accommodate easily - but you add an 802.11b device and also a an 802.11g device, what happens to the overall bandwidth/speed? Does everything go down to the lowest common denominator? Does the router suddenly open a special new communication channel for the b and then another for the g device? (I'm assuming the overlapping frequencies would make things even more complicated, so I guess we should first theorize the answer as if frequency interference is not an issue, and look only at data transfer issues.)
4) What would your setup options be for creating such special new communications channels?
5) How would such new comm channels affect the n channel's bandwidth? I assume it would eat into the MIMO radios' ability to send/receive bits on the n range?
6) Say your wireless LAN operates nicely on 802.11n with bandwidth up to 300Mbps, but your 802.11n router is connected to the web via an ADSL modem that has a max throughput of 24Mbps. What would be the value of the 300Mbps LAN if the uplink is only 24? I'm assuming we are talking a major bottleneck here?

 

[stevech]

Questions 3 and after are my main concern, please skip to that if you must. : )
1) Say you have a router that operates on n. This gives you a max bandwidth/speed of 300 Mbps when communicating with other n devices, right?

Yes, but only if the mode is set to 40MHz rather than 20MHz. And some routers decline 40MHz mode if any 20MHz mode WiFi SSID's are in range, on any of the channels in the 40MHz swath. To be useful, of course, the client devices must support 40MHz mode. In 11n, 40MHz is an option

2) If you have 4 x n devices linking to this n router at the same time -> Device A requires 100Mbps, B requires 70Mbps, C requires 70Mbps, D requires 100Mbps (total simultaneous req of 340Mbps). Let's say they all req the same QOS priority. How does the router deal with the additional 40Mbps or more that it cannot accommodate?

First, the xxx bps you mention are the WiFi "air link" rates. The useful (net of overhead) rates are about 60% of that. Second, the air link rate (bps) varies quickly according to the signal strength for each client. So two client devices are likely at different rates. Each data frame from each client, or to each client, can be a a data speed (bps) that is unique to that client based on signal conditions - and often limited by the client's transmitted signal, not the AP/router's signal. Finally, consider that WiFi 802.11anything is half-duplex, meaning there is one channel and all devices, AP/router included, transmit only when no other is. OK- if all client had the same signal strength at the router (not vice-versa), then ideally all would burst their frames at the same rate, in different times. So the client with the most traffic to send would tend to "hog" more air time than others, until that traffic surge finishes (say, a complex web page). If a client is constantly streaming, say, a video or file transfer, then we HOPE that the client is polite in not hogging all the air time. But 802.11 has little useful bandwidth fairness itself. That is optionally done by QoS mechanisms in the network layers well above 802.11 in consumer gear, as a rule. And it's more about high priority streams' packets cutting-line in the queue of packets waiting to be transmitted (by 802.11 or by ethernet, same same). Fully prioritizing the WiFi bandwidth itself is not well standardized and often absent in consumer routers. Some have QoS but it's at the IP layer, independent of WiFi versus ethernet.

3) If, say, you have 3 x n devices in your router's 802.11n LAN - which it can accommodate easily - but you add an 802.11b device and also a an 802.11g device, what happens to the overall bandwidth/speed? Does everything go down to the lowest common denominator? Does the router suddenly open a special new communication channel for the b and then another for the g device? (I'm assuming the overlapping frequencies would make things even more complicated, so I guess we should first theorize the answer as if frequency interference is not an issue, and look only at data transfer issues.)

The 11b device would need to be on the same channel as the 11n devices for a single router/AP. The Router/Access point does not channel hop or change frequencies according to 11b vs. 11n -it uses the channel number you choose. The speed, as in the earlier response, varies by packet/frame by client. And so the 11b client has the lowest bit rate frame of all. The 11n clients may transmit at their desired bit rate when they get air time. There is a hitch, though, with 11b/n mix: That is that the 11b client cannot know how to properly observe the transmissions of the 11n clients because of the different IEEE standards. To cope, the proper thing is for all clients to resort to "request to send/Clear to send", meaning one transmits a request to get the channel reserved for a short time, and the 11b clients can know this. This is why 11b/n tends to slow down everything - and why it's best to put in an Access Point in the 11b mode, on a different channel (1, 6 or 11) from the 11n clients, in 20MHz mode. In 40MHz mode (300mbps at most), this isn't easily done because 40MHz uses 2/3 of the entire set of overlapping channels.

4) What would your setup options be for creating such special new communications channels?

As above, you'll need to have a dedicated 11b mode access point on a channel 3 removed from the 11n WiFi router/access point.

5) How would such new comm channels affect the n channel's bandwidth? I assume it would eat into the MIMO radios' ability to send/receive bits on the n range?

I think the answers above treat this question.

6) Say your wireless LAN operates nicely on 802.11n with bandwidth up to 300Mbps, but your 802.11n router is connected to the web via an ADSL modem that has a max throughput of 24Mbps. What would be the value of the 300Mbps LAN if the uplink is only 24? I'm assuming we are talking a major bottleneck here?

Of course, WiFI clients cannot achieve any faster speeds on the Internet than provided by that connection. The good news is that when the DSL speed is much less than the WiFi speed, the amount of air time used by WiFi clients is less - because they burst at, say, 150 or 300Mbps x 60% and queue up packets in the router to be emptied more slowly out the WAN port to the DSL/cable modem. And vice-versa for the downstream.
Remember too that in WiFi, the rule is listen before transmitting (carrier sense multiple access, collision avoidance or CSMA/CA), so sharing the channel spectrum with others' WiFi, and other 2.4GHz stuff can add delays in getting some air time. If the delays are just channel-busy due to a neighbor (or in an airport), the delay is typically a few miliseconds, depending on how air-time hoggy the other guy is, on the channel in common. So choose the channel you use based on air time busyness and not signal strength. We wish more consumer gear would measure/show the channel busyness (utilization) but that takes time to dwell and make an estimate.

As 2.4GHz gets more crowded, the use of 5.8GHz WiFi (was 802.11a; is now an option in 11n) - is more popular.

Hope this helps. There are some good 802.11 tutorials and books to be had.

 

[chuck]

Wow, thanks Stevech! Thanks for the quick and detailed reply. I may have some more clarification questions later, but this gives me plenty to think about for now. Great info delivered quickly. Thanks again!

 

[stevech]

you're welcome. I hope some of that drivel will help others here with common questions.

 

[teknojnky]

for #6

What would be the value of the 300Mbps LAN if the uplink is only 24? I'm assuming we are talking a major bottleneck here?

the value would be for local traffic not going to the internet.

ie file sharing/media streaming from within your network