Cisco Cisco Aironet 350 Mini-PCI Wireless LAN Client Adapter Guida Alla Progettazione
5-13
Enterprise Mobility 4.1 Design Guide
OL-14435-01
Chapter 5 Cisco Unified Wireless QoS
Wi-Fi Multimedia
of client power than the regular listening for beacons method, at a period controlled by the delivery
traffic indication message (DTIM) interval, because the latency and jitter requirements of voice are
such that a WVoIP client would either not be in power-save mode during a call, resulting in reduced
talk times, or would use a short DTIM interval, resulting in reduced standby times. The use of
U-APSD allows the use of long DTIM intervals to maximize standby time without sacrificing call
quality. The U-APSD feature can be applied individually across access categories, allowing
U-APSD can be applied to the voice ACs in the AP, but the other ACs still use the standard power
save feature.
traffic indication message (DTIM) interval, because the latency and jitter requirements of voice are
such that a WVoIP client would either not be in power-save mode during a call, resulting in reduced
talk times, or would use a short DTIM interval, resulting in reduced standby times. The use of
U-APSD allows the use of long DTIM intervals to maximize standby time without sacrificing call
quality. The U-APSD feature can be applied individually across access categories, allowing
U-APSD can be applied to the voice ACs in the AP, but the other ACs still use the standard power
save feature.
•
The secondary benefit of this feature is increased call capacity. The coupling of transmission
buffered data frames from the AP with the triggering data frame from the WLAN client allows the
frames from the AP to be sent without the accompanying interframe spacing and random backoff,
thereby reducing the contention experience by call.
buffered data frames from the AP with the triggering data frame from the WLAN client allows the
frames from the AP to be sent without the accompanying interframe spacing and random backoff,
thereby reducing the contention experience by call.
shows a sample frame exchange for the standard 802.11 power save delivery process.
Figure 5-11
Standard Client Power-Save
The client in power-save mode first detects that there is data waiting for it at the AP via the presence of
the TIM in the AP beacon. The client must power-save poll (PS-Poll) the AP to retrieve that data. If the
data sent to the client requires more than one frame to be sent, the AP indicates this in the sent data
frame. This process requires the client to continue sending power-save polls to the AP until all the
buffered data is retrieved by the client.
the TIM in the AP beacon. The client must power-save poll (PS-Poll) the AP to retrieve that data. If the
data sent to the client requires more than one frame to be sent, the AP indicates this in the sent data
frame. This process requires the client to continue sending power-save polls to the AP until all the
buffered data is retrieved by the client.
This presents two major problems. The first is that it is quite inefficient, requiring the PS-polls, as well
as the normal data exchange, to go through the standard access delays associated with DCF. The second
issue, being more critical to voice traffic, is that retrieving the buffered data is dependent on the DTIM,
which is a multiple of the beacon interval. Standard beacon intervals are 100 ms, and the DTIM interval
can be integer multiples of this. This introduces a level of jitter that is generally unacceptable for voice
calls, and voice handsets switch from power-save mode to full transmit and receive operation when a
voice call is in progress. This gives acceptable voice quality but reduces battery life. The Cisco Unified
Wireless IP Phone 7921G addresses this issue by providing a PS-Poll feature that allows the 7921G to
generate PS-Poll requests without waiting for a beacon TIM. This allows the 7921G to poll for frames
when it has sent a frame, and then go back to power-save mode. This feature does not provide the same
efficiency as U-APSD, but improves battery life for 7921Gs on WLANs without U-APSD.
as the normal data exchange, to go through the standard access delays associated with DCF. The second
issue, being more critical to voice traffic, is that retrieving the buffered data is dependent on the DTIM,
which is a multiple of the beacon interval. Standard beacon intervals are 100 ms, and the DTIM interval
can be integer multiples of this. This introduces a level of jitter that is generally unacceptable for voice
calls, and voice handsets switch from power-save mode to full transmit and receive operation when a
voice call is in progress. This gives acceptable voice quality but reduces battery life. The Cisco Unified
Wireless IP Phone 7921G addresses this issue by providing a PS-Poll feature that allows the 7921G to
generate PS-Poll requests without waiting for a beacon TIM. This allows the 7921G to poll for frames
when it has sent a frame, and then go back to power-save mode. This feature does not provide the same
efficiency as U-APSD, but improves battery life for 7921Gs on WLANs without U-APSD.
shows an example of traffic flows with U-APSD. In this case, the trigger for retrieving traffic
is the client sending traffic to the AP. The AP, when acknowledging the frame, tells the client that data
is queued for it, and that it should stay on. The AP then sends data to the client typically as a TXOP burst
where only the first frame has the EDCF access delay. All subsequent frames are then sent directly after
the acknowledgment frame. In a VoWLAN implementation there is only likely to be one frame queued
at the AP, and VoWLAN client would be able to go into sleep mode after receiving that frame from the
AP.
is queued for it, and that it should stay on. The AP then sends data to the client typically as a TXOP burst
where only the first frame has the EDCF access delay. All subsequent frames are then sent directly after
the acknowledgment frame. In a VoWLAN implementation there is only likely to be one frame queued
at the AP, and VoWLAN client would be able to go into sleep mode after receiving that frame from the
AP.
153867
Beacon
with TIM
Beacon
with TIM
AP
Client
ACK
ACK
ACK
ACK
ACK
Sleep
PS-Poll
PS-Poll
PS-Poll
Sleep
Client Full Power
DATA
(more=0)
(t)
DTIM Period
Access Delay
DATA
(more=1)