Best Practice for Video over WLAN - Part 2
In this part we will look at some of the key requirements on the WLAN infrastructure to deliver multicast video over wireless.
Multicast over-the-air transmissions can benefit from unicast transmissions depending on the number of clients in use. If only a small number of clients are subscribed to a multicast group, it can be more efficient to convert over-the-wire multicast to over-the-air unicast due to the faster data rates and prioritization capabilities of unicast connections. As this number grows, multicast gains in efficiency over unicast. With Aruba Dynamic Multicast Optimization (DMO), the conversion takes place at the controller or at the AP at the 802.11 layer, on a client-by-client basis, and is transparent to the higher-level client layers.
Benefits of DMO include:
- Ease of deployment: DMO is dynamic, providing automatic multicast-to-unicast traffic optimization in the controller without requiring ongoing monitoring and configuration. Multicast IP frames are transmitted over the air using 802.11 unicast headers dynamically based on a configurable threshold.
- QoS: Multicast video traffic utilizes the WMM video queue for prioritization.
- Higher capacity: Reliable multicast video over 802.11n at HT rates.
The DMO threshold has a default value of 6 clients, but it may be set higher. The DMO threshold specifies the number of HT WLAN clients per Virtual AP, per VLAN for video delivery mode. Video is delivered as multicast when the number of HT clients exceeds the threshold, and video delivered as unicast when the number of HT clients is below the threshold. For this computation, 1 legacy client (802.11a/b/g) has a penalty factor equal to 3 HT clients (802.11n). For example, if there are three 802.11n clients associated to a VAP and the threshold value is set to 4, DMO will take place. Once the fifth HT client associates to the same VAP, DMO will no longer take place. If two 802.11b clients are associated to the VAP and the threshold is set to 4, they will be treated as if they were 6 HT 802.11n clients and DMO will not take place. Consult the video scalability section below for recommendations regarding this setting.
In cases where Dynamic Multicast Optimization (DMO) determines that it is more efficient to send traffic over-the-air as multicast, Multicast RateOptimization (MRO) supports higher data rate multicast frame transmissions increasing multicast traffic capacity in a given channel. The AP transmits multicast traffic at the lowest common rate sustainable for all associated subscribers instead of using the lowest common supported rate for all clients. This allows conservation of wireless bandwidth and higher video density. It is recommended to enable this feature whenever deploying video, but note that DMO will always take precedence over MRO up to the configurable threshold value.
In the next section we will look at the role that RF plays in delivering high definition video over wireless.