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Multicast and Unicast Streaming Media

Bandwidth is a serious concern for applications using streaming media. The streaming media requires a high throughput and a delay in transmission can create an annoying delay for the user. The 802.11 standard has different options for how to deal with this situation.

In a multicast transmission, an access point (AP) has several nodes that it is transmitting to that require the streaming media. The AP will then transmit the data to all of the requesting nodes at once. Due to the fact that several nodes receive data at the same time, they are unable to send acknowledgment packets (ACK). If they did send out ACK, the acknowledgments would most likely collide, and even if they reached the AP, it would be unable to determine which nodes received the packet. Without the ACK, the AP has no way of determining if the nodes received the data. Thus, it must make an attempt to guarantee that every packet reaches the destination nodes blindly.

The solution is for the AP to transmit at its lowest allowable data rate for the minimal Quality of Service (QoS). In situations where the AP is expecting an ACK but does not receive one, the response is for it to lower its transmission rate and retransmit the data. The lower transmission rate in both scenarios increases the chance that a node will be able to receive the data. The drawback to this solution is that an 802.11g compatible AP has a throughput that drops from what could be up to 54 Mbps to 6 Mbps. This severely limits the possible quality of the transmitted media.

An alternative is to unicast the media. In this situation, The media is individually sent to each node that has requested it. The protocol for these transmissions include an ACK, so the AP can transmit at a higher bitrate than the minimal 7.2 Mbps. Thus, for a small enough number of nodes, unicast transmissions can result in higher throughput. The exact number of nodes where unicast becomes superior depends on what actual bitrate can be achieved by the AP with each node. If the AP supports 802.11n beamforming, then it is possible to achieve bitrates of up to 600 Mbps. This means that, in ideal situations, an AP could unicast to up to 100 nodes before multicasting became the superior option.

VOCAL Technologies, Ltd.
520 Lee Entrance, Suite 202
Amherst New York 14228
Phone: +1-716-688-4675
Fax: +1-716-639-0713
Email: sales@vocal.com