Controller Based WLANs

Do wireless VoIP phones have to stay within range of a base station or can they travel and connect to wireless access points?

by on ‎07-03-2014 11:15 AM

Product and Software: This article applies to all ArubaOS versions.

 

Protocols like the real-time transport protocol (RTP) are designed to operate over UDP/IP datagram services, which allows for changes in network routing and connectivity during the lifetime of a VoIP call. 802.11 stations automatically assess signal strength and reassociate to the AP with the strongest signal. When reassociation occurs, connectivity is briefly interrupted. Jitter buffers and many other techniques are used to smooth out the consequences of packet loss. If reassociation occurs reasonably fast, the results are satisfactory. If the station moves completely out of reach of any access point (AP), packet loss persists and the call is eventually dropped.

 

This behavior is somewhat like a cellular call if there is blanket 802.11 coverage in the area (floor, building, or campus) where calls are occurring. Of course, if you had only one 802.11 AP, you would need to stay within range of that AP throughout the call.

 

802.11k, Radio Resource Measurement, provides the mechanism that makes clients aware of the RF environment and optimizes mobility. Without 802.11k, roaming between APs can have an impact on service because of things like scanning or probe delays, delay in selecting an appropriate AP to move to, reassociation delay, and delay in setting up a data path. Delay-sensitive applications like voice can be adversely affected by the delays that are common with roaming between APs.

 

802.11k provides mechanisms to do these tasks:

  • Request and report radio measurements.
  • Perform radio measurements on supported channels.
  • Provide information about neighboring APs.

When 802.11k is enabled, the AP sets "Radio Measurement (bit12)" in the capability field of beacon, probe response, and association response.

 

To configure 802.11k, use the CLI or WebUI to configure the dot11k-profile, which is linked to the virtual AP profile.

 

Using the CLI

 

1) Enable 802.11k.

(MC3200) #configure t

Enter Configuration commands, one per line. End with CNTL/Z

(MC3200) (config) #wlan dot11k-profile test_profile

(MC3200) (802.11K Profile "test_profile") #dot11k-enable

2) Enable the force-disassoc option to disconnect on-hook voice clients after a period of inactivity. This option allows APs to free up Call Admission Control (CAC) capacity to serve active clients.

(MC3200) (802.11K Profile "test_profile") #force-disasso

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