Configuration Tips for VoWLAN Access Points

By Jim Geier

September 03, 2008

The support of voice applications on a wireless LAN is much different than traditional data. In this excerpt from his new book, "Deploying Voice Over Wireless LANs," Jim Geier offers tips on how to configure access point radios for voice applications.

 

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This tutorial is excerpted with permission from the book, Deploying Voice over Wireless LANs  (Cisco Press), written by Jim Geier.

 

 

Download Chapter 1, VoWLAN Applications and Benefits, free of charge. This chapter explains how to justify deploying voice over wireless LANs in various environments. 

 

Download Chapter 2, VoWLAN System Components, free of charge. This chapter describes the various components that comprise a voice over wireless LAN system.

 

When deploying voice over wireless LANs, consider the following tips for configuring access points:

 

Firmware

 

The access points that you receive may have been manufactured a while ago, and there may be updated firmware available. If this is the case, upgrade the access point firmware before configuring and using the access point. Be certain, however, to check vendor recommendations on which firmware release to utilize for voice applications. The recommended release may not be the most current version.

 

VLANs

 

If possible, establish a separate virtual local area network (VLAN) for the voice traffic. This will allow much better performance and ease of management of the voice applications when data applications are sharing the same access points. With voice traffic on a separate VLAN, special quality of service (QoS) mechanisms especially tailored for voice applications can be deployed on the voice VLAN. Be sure, however, to enable applicable QoS mechanisms for the voice VLAN. If you have a choice, give data traffic a lower priority. This will help maximize the performance of the voice solution.

 

SSID

 

Assign a Service Set Identifier (SSID) to the access points that will distinguish the voice network (VLAN) from other networks. Select a name that is recognizable to ease configuration of the wireless IP phones, but consider not naming it something that gives away the application. For example, a SSID of "WiFiVOIP-LAN" is an example of a SSID that you should avoid. This SSID would give a hacker some good information on which type of tools to use to exploit the application. 

 

You'll have the option to broadcast or not broadcast the SSID for the voice network. The nonbroadcasting of SSIDs does not add much more security to the network because packet sniffers easily detect SSIDs from 802.11 frames other than beacons. So, it's probably best to go ahead and broadcast the SSID of the voice system to make installation much easier. In fact, some VoWLAN system solution providers may require that the SSID not be broadcasted.

 

Transmit Power

 

Be sure to adjust the transmit power of the access point to the value used when conducting the RF site survey. This is critical to ensure that the signal coverage is optimum. In some cases, the transmit power of the access point will need to be set lower than the maximum level in order to compensate for weaker radio devices in the wireless IP phones. Ideally, the radios in the access points and clients should be set to the same transmit power to avoid one-way voice conversations. Some wireless IP phones have automatic power control to ensure that the power levels remain the same.

 

RF Channel

 

Avoid using auto RF channel settings. For example, Cisco access points allow you to enable searching for the least congested channel. If you enable this, the access point will periodically search for better channels on the selected channels, such as channels 1, 6, and 11. If another channel has less congestion, the access point will change channels. Even though this might offer a higher throughput because of operation over a channel with less congestion, the changing of channels forces the wireless IP phones to roam, which can cause dropped calls. As a result, when deploying VoWLAN systems, assign the RF channels manually.

 

Data Rates

 

In most cases, set the data rate of the access point to the highest value, and don't allow auto rate shifting. Also, disable all lower data rates. This will ensure that the wireless IP phones will have the higher performance connections with the access point. Just be sure that the RF site survey was conducted with data rates set the same way. For example, Cisco recommends that the access points be set to allow only 11Mbps data rates for their 802.11b-based 7920 Wireless IP Phone.

 

Filtering

 

It's best to filter nonessential wired network traffic to ensure that it doesn't flow over the wireless LAN. This minimizes unnecessary traffic over the wireless LAN, which can erode capacity. In order to improve performance of the access points, attempt to filter this traffic on Layer 3 switches. For example, there may be 3270 terminal emulation applications running over the wired network infrastructure. In this case, consider blocking the associated Systems Network Architecture (SNA) traffic from getting onto the wireless LAN.

 

Final Thoughts

 

These tips will help you get off in the right direction. Before configuring access points for voice applications, however, be certain to refer to the manufacturer of the specific wireless IP phones that you’re using to fully understand the most effective access point configuration.

 

Jim Geier provides independent consulting services and training to companies developing and deploying wireless networks for enterprises and municipalities. He is the author of a dozen books on wireless topics.



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