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Voice Over Wireless LAN (VoWLAN)

 

Introduction

The Cisco Unified Wireless Network incorporates advanced features that elevate a wireless deployment from a mean of efficient data connectivity to a reliable, converged communications network for voice and data applications.

 

Voice services place stringent performance requirements on the entire network. Because digitized voice is a sampling of an analog signal (verbal communication), its transmission is very sensitive to delays during transit. In fact, in order for voice to work correctly over any infrastructure, the end-to-end transit time (cumulative time encoding the packet, leaving the sending client, traversing the network, and then being decoded at the receiving client) must be less than 150 ms. Issues encountered during transit result in imperfections in the reconstituted signal; also known as jitter. The jitter is basically the variation in delay that the system is experiencing.

Best Practice

  • Make sure an AP can be seen from the phone at -67 dBm or better in all areas to be covered. Also verify that the AP sees the phone at -67 dBm or better in all areas as well.
  • The Signal to Noise Ratio  should always 25 dB or more in all areas to provide coverage.
  • Channel utilization should be under 50%.
  • When using EAP authentication, make sure that fast roaming is supported such as CCKM.
  • Voice WLAN should be marked with Platinum QoS.
  • Make sure the switch ports used to connect to the controller are set to trust CoS and ports to APs and uplinks are set to trust DSCP.
  • Make sure that Call Admission Control is enabled globally for the radios.
  • The Load-based CAC should be enabled under Call Admission Control.
  • Ensure that Load Based CAC (7920 AP CAC) under the WLAN is enabled for the voice WLAN if the network has a mix of 7920 and 792xG Series wireless IP phones.
  • Ensure that Client Based CAC (7920 Client CAC) under the WLAN is disabled for the voice WLAN.
  • Make sure that the EDCA profile on the controller is set to Voice Optimized.
  • The Low Latency MAC should be disabled.
  • Verify that the 12 Mbps data rate is enabled (default PHY rate of the phone).
  • Verify coverage is designed for 24 Mbps to maximize throughput. Optionally disable 36-54 Mbps.
  • The Aggressive Load Balancing should be disabled.
  • Verify that DTPC is enabled so that the client and AP match tx power levels.
  • Verify the Beacon interval is set to 100 ms.
  • Session timeout for the WLAN should not be too short (300 seconds or more).
  • Verify that the radio of the AP has multiple antennas and that diversity is enabled.
  • Validate the virtual interface address is the same across all controllers in the same mobility group.
  • Validate that the mobility status shows as UP between all controllers in the same mobility group.
  • Enable Traffic Stream Metrics collection on the controller.
  • DCA Channel Sensitivity set to High to reduce chance of channel changes during business hours.

Frequently Asked Questions

Question 1

On a voice over wireless deployment, what is the ideal / practical size of the single voice VLAN? If that is  exceeded what is the best possible subnet size for AP Groups?

Answer 1

We do not have a hard and fast rule on how large a voice client subnet should be in our VoIP or Voice over Wireless LAN documentation.

But rule of thumb is to not exceed a /22.  /24 would be a more preferred size from a wired VoIP perspective, but with VoWLAN, that number can be exceeded very quickly in a single deployment.

Use of multicast could also influence the subnet size as well.

Question 2

Cisco Unified IP Phone to reset during heavy network usage. Why?

Answer 2

It looks like you do not have a voice VLAN or the appropriate QoS settings configured. By isolating the wireless phones on a separate auxiliary VLAN, you can use QoS to prioritize the voice traffic over data traffic and improve the voice quality. Please refer to Voice QoS in wireless network section in VoIP wireless network document for additional information.

Question 3

During the active call and walking from one location to another (roaming), the voice quality deteriorates or the connection is lost. Why?

Answer 3

Verify the signal strength is good by checking the RSSI value on the destination access point. This value should be -67 dBm or higher. Also check if the channel overlap is adequate for the phone and the access point to hand off the call to the next access point before the signal is lost from the previous access point.

Verify if noise or interference in the coverage area is too high which impacts the voice quality.

Verify the signal to noise ratio (SNR) levels are 25 dB or higher for acceptable voice quality.

Resolving Voice Quality and Roaming Problems

Cisco Unified Wireless IP Phone 7921G users might have problems with voice quality and connectivity when roaming with their phones. See the following sections for troubleshooting information:

Symptom: Cisco Unified Wireless IP Phone Resets Unexpectedly

If users report that their phones are resetting during calls or resetting while idle on their desk, you should investigate the cause. If the network connection and Cisco Unified Communications Manager connection are stable, a Cisco Unified Wireless IP Phone 7921G should not reset on its own.

Typically, a phone resets if it has problems connecting to the access point and LAN or to Cisco Unified Communications Manager. These sections can help you identify the cause of a phone resetting in your network:

Source

Reference