Specifications

Cisco Unified Wireless IP Phone 7921G Series Deployment Guide 29
where the user may experience choppy audio with the weak signal, followed up with a small second audio gap before looking
for the least preferred band. Then once it has failed over to a less preferred band (i.e. associated to 802.11b/g when phone
configured for Auto-a), there was no mechanism in place to check to see if the preferred band is available again or not in order
to roam back to the preferred band.
It is recommended to perform a spectrum analysis to ensure that the desired bands can be enabled in order to perform seamless
interband roaming.
Channel Parking
Channel Parking is a sub-feature of interband roaming, which is designed to conserve idle battery life when the Cisco Unified
Wireless IP Phone 7921G is configured for an Auto 802.11 mode and continuous scan mode is enabled in the Cisco Unified
Communications Manager.
Continuous scan mode enables constant scanning of all channels regardless of call state, which can also help with location.
When configured for auto scan mode, typically the phone is only scanning when on call and not in idle unless the current signal
drops below a certain RSSI threshold.
When channel parking is active, the Cisco Unified Wireless IP Phone 7921G will discontinue the scanning of the 5 GHz band
and potentially roam to a 2.4 GHz neighbor, but also may stay on the currently connected 5 GHz AP if that is the strongest
signal received.
Channel parking will occur when the phone is in idle mode and there are at least four 2.4 GHz access points available where at
least one of those four access points has met the RSSI threshold to enable channel parking.
If configured for Auto-a and continuous scanning is enabled, the phone can potentially roam to 2.4 GHz when in idle even when
the phone is in good 5 GHz coverage as the Auto-RSSI logic is used when in idle, regardless of whether 5 GHz channels are
parked or not. When on call the local configured mode will be used (i.e. Auto-a), so it will attempt to associate to the preferred
band if available.
Channel parking will become inactive either when there are less than four 2.4 GHz access points, the RSSI for all 2.4 GHz
access points is low enough to meet the RSSI threshold to disable channel parking or there is a current inbound or outbound
call. When channel parking becomes inactive, the phone will rapidly scan the 5 GHz band, where it can potentially roam back
to 5 GHz if configured for Auto-a, as roaming is based on band preference and then RSSI when on call.
When the call is terminated, channel parking may become active again.
Multicast
When enabling multicast in the wireless LAN, impacts on battery life, performance, and capacity must be considered.
The Cisco Unified Wireless IP Phone 7921G uses the DTIM period to receive the queued broadcast and multicast packets.
If there are many packets queued up, then they client may have to stay awake longer thus potentially reducing battery life.
With multicast, there is no reliability that the packet will be received the by the client.
The multicast traffic will be sent at the highest basic data rate enabled on the access point, so will want to ensure that only the
lowest enabled rate is configured as the only basic rate.
The client will send the IGMP join request to receive that multicast stream. The client will send the IGMP leave when the
session is to be ended.
The Cisco Unified Wireless IP Phone 7921G supports the IGMP query feature, which can be used to reduce the amount of
multicast traffic on the wireless LAN when not necessary.
Ensure that IGMP snooping is also enabled on all switches.