Sunday, October 17, 2010

How Not To Kill Your Wireless Network Capacity

[Part 1]
This post of mine has nothing to do with wireless network security. But I have experienced people making mistakes and killing the overall network capacity by making mistakes during deployment. And hence, this time I am going to talk about how you should not kill your wireless network capacity.

The part 1 is basically focussed on a few tips presented here on how to maintain network performance. In future posts, I will present few more interesting stuff about the wireless network performance and monitoring.

Let's first recall and memorize rules of the game:

1. The overall network throughput of an IEEE 802.11 based wireless network is fixed. It depends on the protocol (a/b/g/n) you are using.

2. Should avoid deploying APs on the same channel. It cuts down the throughput offered by an AP present on the channel.

3. Should avoid deploying APs on adjacent channels (non-overlapping channel in 2.4 GHz band). It cuts down the throughput offered by an AP present on the channel.

4. Should understand the WiFi deployment requirements (Number of clients per AP, Average throughput per client or coverage area) and stick to these requirements while making any changes in the network.

5. Should try to minimizing the overhead of creating multiple wireless network on a channel without affecting the network deployment requirements

Though the observation presented by Mr. Andrew in his post is absolutely correct, the tips presented by him has some penalty that needs to be learnt in advance.

Though there should not be APs operating in the proximity of each other on the same channel, yet creating of multiple SSIDs does waste the bandwidth and only in 2.4 GHz band in mixed mode (b/g) deployment and not in pure 'a' or 'g' mode deployment. About 2% loss can probably be ignored. Further, the suggestion made here would help reduce the wastage of bandwidth due to multiple beacon transmission in 'g' and 'a' band as well. Let's take a loot at the penalty that we have to pay:

1. By limiting the active SSIDs per AP, you are basically limiting the support of segmented wireless networks created on an AP.
2. Disabling low data rates means reducing the range of network. This can create coverage hole. You may have to revisit your network deployment requirements and see if coverage was the important requirement.

Network bandwidth waste can be reduced by configuring APs to transmit beacons less frequently. For example loss of bandwidth in 2 SSIDs scenario can be compensated by doubling the beacon interval. By default APs are configured to transmit beacons every 100ms. The beacon interval can be increased or decreased. So just by doubling the beacon interval you should be able to support 6 SSIDs per-AP while limiting the bandwidth waste equivalent to the use of 3 SSIDs per-AP.

In fact, you can also compensate the data rate reduction by increasing the beacon interval to 400ms in a non-VOIP network deployment.

Point is, use of multiple SSIDs do cause waste of network bandwidth. There are many ways to compensate this loss. You should know all and choose the one which does not affect the operation of your existing network deployment, In the new deployment, you must include support for mutiple-SSIDs as deployment requirements and design the network accordingly.

Supporting low link speed network (11b or mixed mode ) itself is a big challenge as legacy wireless clients do kill significant network bandwidth by transmitting data frames at lower rates.

I would be happy to learn about your experiences and share my thoughts from a wireless researcher and developer perspective.

No comments:

Post a Comment