Multicast vs. Unicast

IP traffic can be classified based on different parameters. This document uses classification built upon data-sending methods: unicast (a distinct copy of the data is sent from the source to each client that requests it), broadcast (a single copy of the data is sent to all clients on the network), and multicast (in a one to many environment, a single copy of the data is sent across the network and only those clients who request the data, receive it). Since Window Media Services does not use broadcast, this section focuses only on unicast and multicast.

The definitions indicate the strengths and weaknesses of each method. Unicast sends multiple copies of the same data for each request (in streaming media terms, unicast is a one-to-one connection between the Windows Media server and each client; every client gets its own stream). Multicasting is more efficient in its bandwidth usage because multiple copies of data are not sent across the network. Data is not sent to clients who do not want it (in streaming media terms, multicast is a one-to-one connection between the Windows Media server and a group of clients; every group receives just one stream). The user is simply instructing the network card on the computer to listen to a particular IP address for the multicast. The client does not have to be identified to the computer originating the multicast. Any number of computers can receive a multicast transmission without bandwidth saturation (again, only one copy of the data is sent over the network). For streaming media across an enterprise network, which generates a considerable amount of traffic, multicast is an affordable and effective solution.

Multicasting has its drawbacks: clients have no control of the data stream – they cannot stop, pause, rewind, or advance it. Multicast streams must be scheduled rather than offered on-demand. Clients can only connect to the multicast stream, or disconnect from it. In addition, to enable multicast, you may have to update your routers and make changes to router configurations.

Currently, multicast is enabled on the Internet on what is called the Internet multicast backbone (MBone). MBone consists of multicast-enabled islands connected by unicast (tunneling). However, most of the Internet is not yet on the MBone. Therefore, private enterprise networks are best suited for multicast.

In summary, plan to use multicast and unicast streaming on your intranet, and unicast streaming over the Internet.

 
.
 
 Back