G devices connect when "up to 300mpbs" selected

[Zackary]

I have just bought a WNDR3800. I have several N devices on my network plus 2 802.11g devices. Well the way I would have set up a router in the past would be to have it in mixed mode n/g, so all devices can connect. However, with my new faster router I planned to use it just for n and an old router just for g, so basically 2 wireless routers on the same LAN. I've set the 2 routers up to work together on different channels, without a problem. Obviously I'd rather not have 2 routers on different channels placed close to each other but I've been experimenting with various options to see what's best.

Well the settings in the 3800 aren't like other routers I've used in that you cannot select mixed mode or fixed on g or n, but according to the user manual, setting it on "up to 300mbps" is only compatible with n devices. So that is how I set it.

Today while setting things up, I connected my 802.11g radio to the 3800 by mistake, and it worked. I then tried my printer which also worked. There is no doubt that they are both only 802.11g devices, yet the router is set to "up to 300mbps" so surely that's the same as "n only".

So firstly, I wonder why the older devices are able to connect. Secondly, will the fact that I have 802.11g devices on the wireless network make any appreciable difference to my n connections? It's a question that many in forums ask, and the general consensus seems to be that it does make things a little slower, but by how much is unsure. For now I'm really just trying to future-proof things as I don't have any 300mbps wifi adapters in other equipment (my main PC is linked on gigabit Ethernet), and the adapter's maximum of 150mbps is being achieved on one of the other PC's in the house. I don't know though if the fact I'm using 802.11g devices is causing the flow of data to be slower despite the connection rate, and I also cannot tell how it might affect performance once I upgrade that and other adapters to 300mbps.

At the moment I have 5GHz turned off (there is no need for it with my setup) and 2.4GHz set to channel 10. When looking at the router status, it shows the channel to be "10(P) + 6(S)". Is this a clue to what's going on? I assume the 10 is the main channel, but what is the 6, and what's the P and S all about?

Things are working but I would like to understand why, and whether there is a usable 300mbps network in operation at the moment or not. Any ideas on this would be appreciated please.

[Mars Mug]

Firstly, we are just considering the 2.4GHz band here, not the 5GHz band which has a different set of conditions.

Your router has to operate in a legacy mode on the 2.4GHz band even when a rate of ‘up to 300Mbps’ is selected. I’m not sure what the manual says exactly but this is still compatible with B/G devices, but only N devices can achieve the higher rate.

The router has to transmit a preamble which can be understood by any B/G devices and networks within range, if it did not transmit the preamble and only operated in a high throughput ‘Greenfield’ mode, then any B/G devices in the area would not be able to properly avoid transmitting at the same time as your N devices, take a look at these links;

http://en.wikipedia.org/wiki/Carrier...sion_avoidance
No two devices can transmit at the same time, that’s devices not simply networks, so applies also to devices on your network (only one device at a time can transmit on your network). This is the reason why the router needs to operate in a legacy mode, to protect the band from multiple transmissions at the same time where B/G devices could operate in range of N devices.

http://www.ni.com/white-paper/7131/en#toc3
Scroll down to Figure 10. It shows the effect of adding legacy data to the preamble, there are other sites that give timing figures for this which show how much latency this adds. This will exist regardless of whether you are operating B/G devices in the area or not, I don’t think the option is there yet to run only in Greenfield mode. If that option was provided, and people enabled it when inappropriate to do so, then all networks (including the Netgear one) would suffer the consequences of multiple transmissions / collisions / re-transmissions.

The extra delay added to your LAN by operation if the B/G devices are not so easy to quantify, it depends on relative use, e.g. are you printing to a ‘G’ printer while at the same time transferring files to a NAS using an ‘N’ computer?

This might help also;

http://www.nle.com/literature/Airmag..._on_80211n.pdf
See particularly pages 4, 6, 7

[Zackary]

Thanks for all that great info Andy. I've read through the links, but my brain is still digesting it so I'll take another look after sending this.

After reading what you wrote again though, the main thing I'm trying to understand makes more sense. So in a nut shell, as it's working on the 2.4GHz band only, then there is so such thing as "N only" mode because the router still has to check for b and g devices before transmitting. Is that about right?

It's strange in that case that my previous Linksys WRT150N router, which only worked on 2.4GHz, had options for mixed mode as well as just N. Also my new Netgear router has the different speed options for both the 2.4GHz and 5GHz bands as separate setting. The only purpose of the "up to 300mbps" setting appears to be to allow the speed to go higher, as it doesn't stop slower devices connecting. So why have that option? Surely anyone would want to go faster if possible.

Obviously there is still something I haven't grasped.

[StratmanX]

Originally Posted by Zackary

So in a nut shell, as it's working on the 2.4GHz band only, then there is so such thing as "N only" mode...?

Correct. There is only mixed mode operation.

Also my new Netgear router has the different speed options for both the 2.4GHz and 5GHz bands as separate setting. The only purpose of the "up to 300mbps" setting appears to be to allow the speed to go higher, as it doesn't stop slower devices connecting. So why have that option? Surely anyone would want to go faster if possible.

The Mode setting tackles several issues, but, as you now know, does not allow for just N-only transmission.

Mode allows you to set link rate up to 54, 130, or 300 Mbps. Legacy client devices (and even newer telecommunication devices I've read about) may not operate at certain Mode settings, requiring the 54 Mbps Mode. (Security settings can also affect these legacy devices Mode functioning as well)

For the next part, a little background info is useful. In North America there are 11 channels for use on the 2.4 GHz band. A channel occupies 5 Mhz on the band. However, whatever Channel you select on your router, it actually overlaps with 2 channels to either side for a total of 20 MHz occupied during use. See here for a graphical representation - scroll down to Channels and international compatibility.

When you select Mode 54 or 130 Mbps, you are selecting a "single" channel that is 20 MHz in width. The idea is to select a channel that is either not used by anyone else in your neighborhood, or if also occupied is lightly used by others compared to the other channels. You also want to choose a non-overlapping channel - 1, 6, 11 in North America. But your neighbors should as well otherwise you get overlapping channels which will result in delays as all people whose channel selections overlap or are the same channel will have slowdowns in transmissions when trying to use their wireless signal at the same time.

The best you can hope for is a channel no one else has chosen in your area. The next best is a channel that someone else has also selected or overlaps with yours but they are not using the wireless at the same time as you, or at least very lightly (ie not downloading bog files or sending media files to their TV), otherwise you both will be slowed down when using your wireless at the same time.

Using Mode = 300 Mbps uses "bonded" channels that is 40 MHz in width. Because of channel bonding you are more likely to overlap into a neighbor's signal (their channel selection) and cause slowdowns for both of you on the 2.4 GHz band. However, your router will automatically change to Mode = 130 (or lower) if it senses another router conflicting with transmission on those channels you have selected. This is part of the Good Neighbor Policy. Google if interested.

Channel bonding on the 5 GHz band is unlikely to cause issues with neighbors as the 5 GHz signal is only propagated a short distance compared to the 2.4 GHz signal.

Put another way, N devices are backwardly compatible and can function at any Mode selection - 1 radio / 1antenna N devices can function up to 130 Mbps Mode while 2 radio / 2 antenna N devices may function up to 300 Mbps Mode. Legacy devices that use B or G only are not forward compatible with Modes of 130 and 300 Mbps, and will force the router to operate in 54 Mbps Mode, even if Mode is set to 130 or 300, when transmitting or receiving data. When these legacy clients are NOT transmitting or receiving data, then the router can again function at a setting of Mode 130 or 300 Mbps, depending upon which you've chosen and the ability of your other client devices on your LAN network.

[Mars Mug]

Originally Posted by Zackary

After reading what you wrote again though, the main thing I'm trying to understand makes more sense. So in a nut shell, as it's working on the 2.4GHz band only, then there is so such thing as "N only" mode because the router still has to check for b and g devices before transmitting. Is that about right?

Kind of right.

As I said the rules apply to all devices within range of each other both on your network and on other networks, access points (your router) have a few other jobs to do, but essentially the access points and all clients within an area need to listen for transmissions before they can transmit, and as you see in those links the protocols for that are quite complex. There are also other protocols in action that are mentioned in that last link I provided. All devices on all networks should report what they are capable of and what capabilities they are aware of in other devices. So a PC on the far side of a neighbouring network that can only operate at B/G rates will report this in its messages to the router and this will propagate to all other devices on the network, your neighbouring network will also see those messages and your router will act accordingly.

I don’t know about your old router, but the 802.11n standard was drafted over quite a long period of time. Some manufacturers built in user selectable features that really should not be user selectable, such as Greenfield Mode and forced 40MHx bandwidth. An inconsiderate user who lacks the knowledge to understand the consequences of applying either of those features will not only cause problems for other networks in the area, but will also damage their own throughput. What they are after though is that magic Link Rate number of 300 or 450 (or 600) Mbps, they are fooled into thinking they have achieved the fastest network by applying those settings.

The main point of all this is that the basic media access control (the protocols that decide when a device can transmit) is based on CSMA/CA, and anything that breaks that such as operating a Greenfield configured network within range of a non-Greenfield network, will harm throughput for everyone, including the speed greedy poorly configured Greenfield network.

And as you can see from StratmanX's post and those links, the details of how networks are configured and how they interact are extremely complex.

[Zackary]

Many thanks to both Andy and StratmanX for all that info. I appreciate you both taking the time to write it all out for me. You've explained many of the things about wireless networking that I had questions about, and have answered all the questions I specified in my original post. I'll bookmark this thread as I'm sure I will need to read parts again in future.

It's odd that despite being very sure of myself with computers, PC's and having qualifications in electronics, the networking side of things is something I have never taken the time to learn properly - Mostly because I haven't needed to know. But I'm taking time now to get a better understanding of all aspects of networking.

Thanks again.

[Mars Mug]

Changes made after the early 802.11n;

“Meanwhile Draft 1.0 needed some additional work to detect legacy devices and back off
to single channel transmission to other N devices if necessary. It seems that 802.11n
would play nicely with 802.11g’s single OFDM channel while still being able to use 40
MHz when needed. The presence of 802.11b, which does not use OFDM and in fact
uses a slightly wider channel (22 MHz), would confine N to a single channel distanced
from the 802.11b channel. There are no such restrictions in the 5 GHz band since any
802.11a devices already use 20 MHz channels and OFDM.”

(from here https://learningnetwork.cisco.com/se...20Analysis.pdf )

So devices released against the early draft may well now breach the current requirements, and not play well since those requirement changes were made for good reason and not just to keep people in a job.

Zackary, as a few of those linked documents may demonstrate, wireless networking is an incredibly complex field, but with time (and without trying to understand every single word or term) you can build up a basic understanding of the major issues in play and have a better understanding of the limitations and effects of the environment on the Network.

[Zackary]

My old router was, I believe, only draft N so that does explain why it had a "N only mode", which from all you and StratmanX have told me goes against the full N standard now. I did use that router just for G after connecting my new router, but now I understand things better I've turned wireless on that router off and am using older G devices on the new router, along with my N devices.

[StratmanX]

If you want to separate your legacy G devices then consider using your old router and assign those G devices to the old router's SSID. Assign a different channel to the old router than is used by your 3800 if possible.

I suggest you download and install inSSIDer to look at all the broadcasted wireless signals in your location. People who do not broadcast their SSID will not be detected bu inSSIDer but you may notice slower wireless throughput (data transfer). InSSIDer will display what channels people so you can get an idea of what channel may be st for you to select. InSSIDer also displays RSSI - which is a gauge of signal intensity (not signal strength) and can be used to help determine best location for your router and client devices orientation in space - higher/lower placement, turned to right/left, best spot in the room, etc. InSSIDer does not display electrical interference from electrical devices or measure use of a channel (data throughput).

To determine data throughput you will need two computers networked on your LAN and use an application like iperf/jperf, LAN Speed Test, or something else. I can help you with instructions if you want to use iperf.

Networking is fun.

[Zackary]

Thanks StratmanX. Setting my old router up is what I planned to do, and did so as soon as the new router arrived. It was when I accidentally set either my G radio or printer to the new router that I discovered they actually worked, which is what got me asking questions. So I'm currently debating with myself whether to take up another 2.5GHz channel or if the slower data rate caused by having legacy devices on the N network really matters.

I installed inSSIDer yesterday and have set my channel accordingly. I'd prefer to use channel 1 for the increased range of a lower frequency, but unfortunately everyone locally has the same idea, so I'm on channel 6.

I have seen iperf mentioned by many others too, so I will download that soon.

I agree, just like all computing, networking is fun.