dual radio's

Answered Question
Nov 21st, 2007
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I currently have a wireless network setup with 2 1230AG access points. I have started to recieve complaints about speed issues. I have done a little research and cannot determine if it is better for me to add more access points or get antenna's for, and turn on the 802.11a radio on my exisiting access points. any suggestions? I also have 2 follow questions. If I add 802.11a radio's and set it up with the same setup (SSID and Key) as the 802.11g radio's, will everyone who has a/b/g card wireles cards connect to the A or do wireless cards know to connect to the best quality (speed/signal) network? on the other hand, if I add more access points and set them up with the same setup (SSID adn Key) is there a possibility that a majority of the users will connect to the same access point? what i am trying to get at is do wireless network cards have the ability to chose the best access point if they are in range of 2 that are configured identically?

any help or if anyone has a good article about this it would be appreciated.


Correct Answer by Richard Atkin about 9 years 6 months ago

John, I'm right I promise ;o)


The reason for sending broadcast / multicast traffic at the lowest supported datarate is so that you can guarantee all clients assocaited to the AP can receive the information. If they were sent at 54Mbps, then hardly anybody would receive them, as a client with a 48Mbps or less connection wouldn't be able to interpret the data.


When you use TKIP / AES (for example) you actually have two encryption schemas, one for unicast traffic between AP and client - this is unique per client. Then there is also a group schema for broadcast traffic, having a group encryption key means that all clients receive broadcast / multicast traffic at the same time, and that the AP doesn't have to encrypt the broadcast/multicast packet several times (once for each unique client). The fact that you've got a group key brings us back to the lowest supported data-rate part, ie - everybody needs to be able to receive the inforamtion, so it is sent using the lowest common denominator (ie, data rate).


HTH,


Richard.

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Richard Atkin Wed, 11/21/2007 - 12:40
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How many users do you have associated to the Access Points, and what are they using it for? First thing to do is to set an expectation that wireless is slower than a wire - much slower. On a very good day, you'll get no more than 19Mbps of real throughput via a 54Mbps radio. Don't forget that 19Mbps is contended by every associated client.


There are a number of ways to increase the speed of your wireless network;


1 - Reduce (IP) broadcast traffic. Broadcast traffic is transmitted at the lowest available data-rate, which is typically 1Mbit. By reducing the amount of broadcast traffic (smaller VLANs typically, or perhaps some ACLs? or adjusting client configs), you will improve overall throughput.


2 - Adjust minimum supported data-rate. Broadcast and multicast traffic happens at the lowest support data-rate, typically 1Mbit. By increasing this data rate (to say, 11Mbps), you will improve network throughput.


BUT


If you do this, clients at the edge of your WiFi cell may not be able to connect to the WLAN. You need to establish what the lowest data-rate is, and adjust accordingly. If a clients link quality is too low to support 11Mbps (for exmaple), then it won't be able to connect to the WLAN.


3 - Increase AP density. Depending upon what part of the world you're in, APs can have rather large coverage areas, but typically only have three none-overlapping channels. If you increase AP density, you must be careful to reduce your cell sizes (ie, lower the tx power) and set the channels accordingly. 1, 6 & 11 are fine in most parts of the world.


4 - Adding 5GHz radios. This will effectively double the max throughput of your network from 108 Mbps (2x 54Mbps) to 216 Mbps (4x 54Mbps), providing the clients distribute themselves evenly. You will find that 2.4GHz travels 'better' than 5GHz, which by its nature has less penetrating power. Clients closer to your AP will probably use A, while clients at distance will probably use B/G. "Yes, The A/B/G cards will roam between the APs / Radios as necessary."


5 - Controlling Client Associations. This is largely a client-side thing, but the good folks at Cisco have invented something called CCX, which depending upon the version in use on both the AP and the Client, can influence roaming decisions, which effectively helps with load balancing. AP CCX is controller through IOS releases. Client CCX is controller through client-side driver releases.


Hope this helps.

Richard.

pmccubbin Wed, 11/21/2007 - 13:19
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Richard,


It was a pleasure to read your response. I peruse this forum to keep my knowledge current and to see what issues people are experiencing. Thanks for not only providing an answer but some of the reasoning behind your answer. This rates a "5" in my book.


Best,


Paul

lockheedmartin69 Wed, 11/21/2007 - 13:43
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thanks for the excellent answer. I do have one questions regarding adjusting minimum supported data-rate. If i decrease the minimum supported data rate to 11Mbps are you saying that this will stop any Broadcast and multicast traffic because this traffic uses very low data rates?

Richard Atkin Wed, 11/21/2007 - 14:36
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No, it means that all of your broadcast / multicast traffic will then happen at 11Mbps, as that's your new minimum. Happening at 11Mbps instead of 1Mbps is how you get your performance incrase.


The problem is that if a client's connection is too poor to support an 11Mbps connection, then it will have no connection at all to the WLAN.


HTH,


Richard.


lockheedmartin69 Wed, 11/21/2007 - 16:46
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Can you explain how having broadcast/multicast trafic happening at 11Mbps rather then 1Mbps increases performance?

john.preves Wed, 11/21/2007 - 19:58
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however many milliseconds it takes to deliver a particular size packet at 11Mbps, will take twice as long at 5.5Mbps, and then twice as long again at 2Mbps and then again at 1Mbps. The performance increase is not having to wait so long to deliver the same size packet.


Richard, an excellent, lucid post. However, I have always been under the assumption that broadcast traffic and such travels at 1Mbps anyway, regardless of what the datarate is set to. Nobody has ever been able to give me a straight answer and I'm hoping for another lucid post. Damn, I hope your right.

Correct Answer
Richard Atkin Wed, 11/21/2007 - 23:46
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John, I'm right I promise ;o)


The reason for sending broadcast / multicast traffic at the lowest supported datarate is so that you can guarantee all clients assocaited to the AP can receive the information. If they were sent at 54Mbps, then hardly anybody would receive them, as a client with a 48Mbps or less connection wouldn't be able to interpret the data.


When you use TKIP / AES (for example) you actually have two encryption schemas, one for unicast traffic between AP and client - this is unique per client. Then there is also a group schema for broadcast traffic, having a group encryption key means that all clients receive broadcast / multicast traffic at the same time, and that the AP doesn't have to encrypt the broadcast/multicast packet several times (once for each unique client). The fact that you've got a group key brings us back to the lowest supported data-rate part, ie - everybody needs to be able to receive the inforamtion, so it is sent using the lowest common denominator (ie, data rate).


HTH,


Richard.

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