Cisco Access points - Speed expectations

Posted on 2011-05-04
Last Modified: 2013-12-09

We have a site that uses 6 Cisco aironet 1200 access points, 4 of which are root points connected to ethernet and 2 are repeaters. Radio seems to be 802.11 G and A.

We are having issues where when a few computers are connected and browsing that everything seems to grind to a halt. This is greatly exaggerated if the computers are in the same room connecting to the same AP. Sometimes a reboot makes things better, sometimes not.

We have performed internal network speed tests at each access point and the average is around 20Mbps up and down with the repeaters giving slower performance at about 6-10Mbps.

Is it possible that these older access points can be becoming saturated with a few modern PCs streaming internet and transferring files over the network (such as a Mac backup using time machine)?

Also, when moving between access points and using skype phone over the wifi it often cuts off. Is this normal when switching APs?

Question by:jerryhatt
    LVL 20

    Accepted Solution

    First of all; moving between APs using skype or streaming media:
    - You can loose connectivity for some milliseconds if the station isn't already authenticated with the new AP. Then the ms is needed to re-authenticate.
    But you should be able to roam seamlessly between APs in most new models

    When it comes to speed, here's a few things to look into:
    - remeber that an Ap in 802.11a or 802.11g has theoretically 54 Mbps throughput - this is half duplex, which means you get 27 Mbps upload and 27 Mbps download. Since these are theoretical speed - you're morel ikely to see speeds around 20 Mbps. And that's for the AP all together. So if you have 10 users they would share this bandwidth.
    - And if there's any other AP's on the same channel within range - then the 2 APs will interfere, so if you have 10 users on AP1 and 20 users on AP2 - both APs on the same channel and within range of each other - then it would be 20/30 = 0,6 Mpbs each uses for DL or UL.

    Do a site survey using inSSIDer from and try changing channels. Remeber on 802.11g only channel 1, 6 and 11 do not interfere !
    LVL 2

    Assisted Solution

    legacy 802.11 and 802.11b devices can cause also cause impact on your network because they use different modulation techniques than 802.11g.  when one of these legacy devices wants to connect to the network it switches to a protected mode that requires all devices to request to send and get a clear to send message from the AP creating overhead.  dissallowing use of these devices by disabling the following data rates: 1, 2 5.5 and 11Mbps may help.

    Also for roaming between ap's, make sure your wireless cells overlap at least 30% by doing the site survey mentioned above.  If you don't have proper overlap, you are likely to drop calls when mobile.  Also look into a technology called mobile IP for devices that need to roam.  Make sure you aren't using EAP/802.1x with the network you need to roam with because the time it takes to perform authentication at the next AP is just enough to drop your call.  I'd recommend using a static key authentication like WPA personal for this network.

    Also when you say you have computers in the same room connecting to an AP, how many?  I typically recommend 10 maybe 15 at most PCs per AP.  20+ is pushing it.

    Author Comment

    Thanks for the responses, I believe that the repeaters may be causing a problem. There is never more that 5or 6 users across the access points or connected to any one of them. We have had a look and the APs are using the same channel (9) although the root APs do not overlap, only a repeater would overlap with the next root AP.

    Another question I have is that there are options for the different speed rates and all of them seem to be set to 1mbps as required (basic) and the other speeds are all set to enabled.
    They only use new laptops, macs and iphone/blackberries so I do not think that they jhave and B devices, should I switch off those speeds anyway?

    LVL 2

    Expert Comment

    If all of your devices are running at those speeds at the same time you are probably fine to leave them on.  It's when you get a mix of G speeds (above 11mbps) and B speeds (below 11mbps) that the protection method is used.  Idealy, having your clients transfer at a higher rate is better because of how wireless technology works, your ap can only hear 1 message at a time.  Using faster rates means less time to communicate that message and leave others waiting.  Sometimes you may not be able to achieve these higher speeds if other problems exist in your wireless network.  The client device chooses what speed to shift up or down to based on received signal strength(RSSI), and error rate.

    Doing a site survey would probably be best to make sure your channel usage pattern and wireless cell overlap is just right.  such as make sure 2 aps on channel 1 arent right next to each other, spread them out.  only use channels 1,6 and 11 since these dont interfere with each other but make sure 2 APs on the same channel are not right next to each other or you'll get interference and seriously degrade performance.

    Just kinda rambling here but hopefully some of this info helps.
    LVL 20

    Expert Comment

    by:Jakob Digranes
    Different send/receive rates:
    basic rate = 1 mbps. This is used for broadcast and beacon traffic. Should not be any higher.
    Supported rates = 1 - 54Mbps is how low you'll go on send speed. You can set this to higher speed (24Mbps) if you have APs that interferece, together with lower output power on AP's. If you set this to 24Mbps, then clients connecting at 12 Mbps (du to low signal strength) will be disconnected

    802.11b in 802.11a/g networks: it's true trat 802.11b clients (and even older legacy 802.11 clients) will force other clients into protected mode, as they don't use the modulation technology. However, most AP's nowadays will now use a more fair airtime algorithm, that will give 802.11a/g/n clients more air time than 802.11b - thus not influencing speed on 802.11a/g clients

    When looking at roaming:
    - you should, for a corporate network never use anything other than 802.1x. With 802.1x you authenticate to a 3rd party user database, like Microsoft AD via Radius or Cisco TACACS. The clue is that the wireless station will authenticate to the next AP before roaming, thus bbeing able to continue real time traffic. But then again, you need to have some degree of overlap between AP. overlap does not mean same channel (!!) but that AP's are visible.

    You can also try to force clients to 802.11a (5Ghz) band rather than 802.11b/g (2,4Ghz) since it's far less interference in 5Ghz.
    Even though a client is 5Ghz capable, it will connect with 2,4 Ghz as preferred band. The technique of tring to get cleint to connect on 5Ghz rather than 2,4 Ghz is controlled in AP's
    LVL 2

    Expert Comment

    I'd recommended not using 802.1x on the network if it is segmented (voice on its own vlan) to be vo-wifi only (not corporate data network).  Authentication takes longer with 802.1x than WPA personal because you are authenticating to an external database rather than locally on the AP.  This can cause calls to drop.  
    LVL 20

    Expert Comment

    by:Jakob Digranes
    @Zulazen: no no  --- not necessarily. you can authenticate with the next AP before switching over to it, thus you're already authenticated when roaming to that AP.
    802.11r -

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