Deploying Wi-Fi With Multiple SSIDs
Enterprise Wi-Fi networks often need to run multiple virtual networks on their physical Wi-Fi network. This means that each AP will use multiple SSIDs that users can connect with.
What is an SSID?
SSID stands for service set identifier and name given to a wireless network. They can be created by default by the manufacturer or set by the user. It is made up of up to 32 case-sensitive numbers or letters, with no low character limit.
If two devices transmit on the same frequency at the same time, the signals collide and the data is corrupted or lost. For this reason, devices perform a Clear Channel Assessment before transmitting their signal, to ensure the channel is clear.
It is important to deploy APs on non-overlapping channels to remove interference from other nearby channels. Wireless networks use a reliable transport mechanism where each frame must be ACK’d by the receiver. If the sender does not receive an ACK, it will retransmit the data, which will result in slower speeds.
Every SSID enabled on an AP is a called a Virtual AP (VAP). APs can use multiple SSIDs, which enables you different types of Wireless access for different devices or users. It allows more flexibility but increases channel utilisation because of overhead.
The more SSIDs you use, the more management frames are needed, which increases channel utilisation. Two different management frames increase channel utilisation.
Beacon frames are used by the VAP to display the SSID and are sent at a rate of one every 100ms at the minimum data transfer rate of 1Mbps on 802.11b/g/n or 6Mbps on 802.11a/n.
Probe requests are sent by client devices to the APs. The AP responds with a probe response at the minimum data transfer rate.
The overhead is made up of beacon frames and probe responses. The more SSIDs per access point, the more VAPs and the more beacon frames and probe responses are sent. Having too many SSIDs reduces performance due to channel utilisation so it is recommended to have no more than three SSIDs per access point.
Improving Performance with Multiple SSIDs
You can take several steps to improve performance when using multiple SSIDs. Two configurations can help improve performance dramatically:
1. Disabling legacy bit rates
You can set the lowest supported data rate to 6Mbps on an SSID. This will affect the connectivity of 802.11b devices but will mean that beacons and probe responses will consume less airtime.
2. Band steering
With band steering enabled, dual-band APs only reply to probe requests on the highest supported frequency band (2.4GHz or 5GHz). This reduces probe responses to the 5GHz band where applicable.
Example SSID Deployment
An example of a typical multiple-SSID deployment is below:
This SSID will normally have no encryption and will keep clients isolated from the internal network. It will not use legacy data rates, speeding up the network.
This will use encryption and is for internal users, such as staff. It also would not use legacy data rates
Legacy devices can connect to this network, which will have a lower minimum data transfer rate.
Book a Wi-Fi Health Check
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About the Author
Albert Clarke | Sales & Marketing Executive
Albert writes about all topics relating to barcode scanning, mobile computing, RFID and enterprise Wi-Fi. You'll also see him regularly present videos on our YouTube Channel. He's passionate about new technology and the impact it can have.