Voice and video traffic on the wireless LAN, like data traffic, is
susceptible to delay, jitter, and packet loss. These issues do not impact the
data end user, but can seriously impact a voice or video call. To ensure that voice and video
traffic receives timely and reliable treatment with low delay and low jitter,
you must use Quality of Service (QoS).
By separating the devices into a voice VLAN and marking voice
packets with higher QoS, you can ensure that voice traffic gets priority
treatment over data traffic, which results in lower packet delay and fewer lost
packets.
Unlike wired networks with dedicated bandwidths, wireless LANs
consider traffic direction when implementing QoS. Traffic is classified as
upstream or downstream relative to the AP as shown in the
following figure.
The Enhanced
Distributed Coordination Function (EDCF) type of QoS has up to eight
queues for downstream (toward the 802.11b/g clients) QoS. You can allocate the
queues based on these options:
-
QoS or Differentiated Services Code Point (DSCP) settings for the
packets
-
Layer 2 or Layer 3 access lists
-
VLANs for specific traffic
-
Dynamic registration of devices
Although up to eight queues on the AP can be set up, you
should use only three queues for voice, video, and signaling traffic to ensure the best possible QoS. Place voice in the Voice queue (UP6), video in the Video queue (UP5), signaling (SIP) traffic in the Video
queue (UP4), and place data traffic in a best-effort queue (UP0). Although 802.11b/g EDCF
does not guarantee that voice traffic is protected from data traffic, you
should get the best statistical results by using this queuing model.
The queues are:
-
Best Effort (BE) - 0, 3
-
Background (BK) - 1, 2
-
Video (VI) - 4, 5
-
Voice (VO) - 6, 7
Note |
The device marks the SIP signaling packets with a
DSCP value of 24 (CS3) and RTP packets with DSCP value of 46 (EF).
|
Note |
Call Control (SIP) is sent as UP4 (VI). Video is sent as UP5 (VI) when
Admission Control Mandatory (ACM) is disabled for video (Traffic Specification
[TSpec] disabled). Voice is sent as UP6 (VO) when ACM is disabled for voice
(TSpec disabled).
|
The following table provides a QoS profile on the AP that gives
priority to voice, video, and call control (SIP) traffic.
Table 1 QoS Profile and Interface Settings
Traffic Type
|
DSCP
|
802.1p
|
WMM UP
|
Port Range
|
Voice
|
EF (46)
|
5
|
6
|
UDP 16384-32767
|
Interactive Video
|
AF41 (34)
|
4
|
5
|
UDP 16384-32767
|
Call Control
|
CS3 (24)
|
3
|
4
|
TCP 5060-5061
|
To improve reliability of voice transmissions in a
nondeterministic environment, the device supports the IEEE
802.11e industry standard and is Wi-Fi Multimedia (WMM) capable. WMM enables
differentiated services for voice, video, best effort data and other traffic.
For these differentiated services to provide sufficient QoS
for voice packets, only a certain amount of voice bandwidth can be serviced or
admitted on a channel at one time. If the network can handle
"N" voice calls with reserved bandwidth, when the amount of voice
traffic is increased beyond this limit (to N+1 calls), the quality of all calls
suffers.
To help address issues with call quality, an
initial Call Admission Control (CAC) scheme is required. With SIP CAC enabled
on the WLAN, QoS is maintained in a network overload scenario by limiting
the number of active voice calls so as not to exceed the configured limits on the
AP. During times of network congestion, the system maintains a small bandwidth
reserve so wireless device clients can roam into a neighboring AP, even when the
AP is at
"full capacity." After the voice bandwidth limit is reached, the next
call is load-balanced to a neighboring AP so as not to affect the quality of the
existing calls on the channel.
Note |
Cisco DX Series devices use TCP for SIP communications, and Cisco Unified Communications Manager registrations can potentially be lost if an AP is at full
capacity.
Frames to or from a client that has
not been "authorized" through the CAC can be dropped, leading to Cisco Unified Communications Manager
deregistration.
Therefore, Cisco recommends that you disable SIP CAC.
|
Note |
The DSCP, COS, and WMM UP markings correctly display for the optimum
transmission of video frames.
The device does not support
Voice and Video CAC; Cisco recommends that you implement SOP CAC.
|
The devices use the Flexible DSCP and Video Promotion feature to resolve inconsistent QoS and inconsistent bandwidth accounting when a video occurs with a different type of device.