Wednesday, October 20, 2010

The Multicast Option

In the quest to preserve precious wireless bandwidth, multicast can be a beautiful thing; especially for video if you have the right paradigm.  Anytime video content is destined for the masses and is streamed live or its delivery can be scheduled a la the broadcast television model, multicast can save you bandwidth big time.  For example, if I were to stream CNN live, I could deliver standard (not HD) broadcast quality using MPEG-2 at 2Mbps.  If I have 50 people associated with an AP and 20 of them chose to watch CNN, a unicast delivery scenario would mean each user would receive their own copy of the broadcast.  That's 20 users times 2Mbps or 40Mbps of wireless bandwidth being steadily consumed.  If you’re running 802.11n, you'll probably be able to accommodate the load but at the expense of available bandwidth for other users.  Running 802.11b/g/a means you're dead in the water without off-loading that traffic to other APs.  In a multicast scenario, the content is streamed only once and every user accesses that same stream.  Thus only 2Mbps is consumed leaving plenty of bandwidth remaining.

Seems like an ideal scenario doesn't it?  However, multicast suffers from one key disadvantage. Multicast traffic is not acknowledged. Clients cannot indicate to the transmitter that they missed a packet, and because there is no retransmission mechanism, any errors due to lost packets cannot be corrected.  Multicast over Wi-Fi compounds this difficulty. Wireless frames are subject to loss and corruption over the air.  These factors are addressed in a unicast connection using 802.11 protocol features such as acknowledgements, retransmission and rate adaptation.  But with 802.11 multicast, there is no acknowledgement or adaptation, and therefore some level of frame loss is inevitable.  The error rate can be reduced by adjusting the modulation rate, given a constant over-the-air signal to noise ratio (SNR). For instance, if the rate is reduced from 48 Mbps to 24 Mbps, the error rate will be improved provided that the noise level does not change. Because of the lack of acknowledgements, 802.11multicast traffic is usually transmitted at a much lower rate than would be used for unicast traffic. This takes more time on the air, consuming more of the network’s data capacity, but provides a margin of safety in case RF conditions deteriorate.

An additional challenge with multicast over Wi-Fi is that the modulation rate is set for the worst-case among the client population; normally the client most distant from the access point. For example, if four clients on an access point subscribe to a multicast group, and they would connect with unicast traffic at 36, 36, 24, and 18 Mbps, then the multicast stream must be transmitted at a maximum of 18 Mbps. As noted above, a safer figure would be 12 or 9 Mbps giving a better SNR to improve error rates and thus throughput.

So, while multicast can preserve wireless bandwidth, the reality is it may take some away due to increased errors rates.  To effectively propagate the content, the modulation rate must be lowered at the expense of network capacity.  Fortunately, wireless infrastructure is becoming more flexible and accommodating allowing you to select the best methodology for multimedia propagation.  Here are some of the key multicast optimization techniques for supporting multimedia in a campus environment:
  • The infrastructure should automatically adapt by keeping track of the transmit rates sustainable for each associated client and using the highest possible common rate for multicast transmissions.
  • Wireless needs to support IGMP snooping and IGMP proxy, ideally at the central controller, so that it can identify which APs and clients need particular transmissions, blocking all others. This adds significant efficiency to the overall network.
  • The network should automatically select the best transmission mechanism based on real-time network and video usage information. When multicast is transmitted as unicast over the air, it can be transmitted at much higher speeds and has an acknowledgement mechanism to ensure reliability. The network should make this conversion when appropriate and then automatically switch back to multicast when the client count increases high enough that the efficiency of unicast is lost.


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