Transmission methods for streaming media delivery

About this section

This section describes the two transmission methods for streaming media--unicast and multicast. One of your planning tasks for streaming media is to determine whether unicast, multicast, or both transmission methods are best suited for your organization. This section provides a high-level comparison of the two transmission methods.

Feature summary for transmission methods

The following table provides a high-level comparison of unicast and multicast transmission.

Element	Unicast	Multicast
Connections	One-to-one transmission	One-to-many transmission
Transport	Any of TCP, UDP, HTTP	IP multicast channel
Efficiency	Not as efficient as multicast for delivering streaming media to a large number of clients	More efficient than unicast for delivering streaming media to a large number of clients
Type of stream	VOD or live streams	Live streams only
Devices	The network devices use unicast	The network devices through which multicast transmissions are to be routed must support multicast.

Unicast described

As the following illustration shows, unicast transmission is point-to-point; that is, a separate copy of content is sent to each recipient that has explicitly requested the content.

NetCache support for live streams over unicast describes how NetCache optimizes transport of requests over unicast.

Multicast described

Multicast transmission is analogous to a radio frequency on which any device can listen. Any device that supports multicast can transmit on the multicast channel. One copy of the data is sent to a group address. Devices in the group listen for traffic at the group address and join the stream if clients in the routing tree are requesting the stream. Only the group participants receive the traffic at the address associated with the group. Broadcasts differ from multicast because broadcast traffic is sent to the entire network.

For multicast transmission to occur, the network devices through which the content is to be sent must support multicast. For example:

Content creators must explicitly set up their streaming servers to support multicast.
For example, for MMS, content creators can set up multicast-enabled stations, stations that are not multicast-enabled, or both. For RealNetworks, the configuration of the server includes specifying whether the server supports multicast and, if so, which clients (subnets) can use multicast.
Routers on the path must support multicast.
Clients must request a multicast transmission. Media players that are set for multicast transmission simply join the multicast channel to receive the streaming data, sometimes without establishing an explicit one-to-one connection to the device sending the transmission.

Benefits of multicast

The benefits of using multicast for streaming media include the following:

It alleviates network congestion.
For live streaming events that have a large audience, multicast significantly reduces network traffic compared to the traffic that would result from transmitting the same live event over unicast. If unicast transport is used, the same content must be sent across the network multiple times or it must be broadcast to all devices on the network.
It scales well as the number of participants expand.
It is well suited for efficient transmission over satellite links.
A company might, for example want to reserve WAN connections for business-critical traffic, such as stock trades, but it needs a way to deliver corporate broadcasts. The company could efficiently transmit corporate broadcasts over satellite by using multicast transmission and reserve the WAN for business-critical traffic.
It enables network planners to proactively manage network growth and control cost because deploying multicast is more cost-effective than alternatives for increasing LAN and WAN capabilities.

Limitations of multicast

The limitations of multicast include the following:

In general, multicast support is not yet available on the Internet. Therefore, using multicast to deliver content is limited to intranet-style deployments. However, Scenario: Multicast support for transmission over a satellite link describes how you can use NetCache multicast support to convert unicast traffic for live streams to multicast.
Not all networking equipment supports multicasting. In addition, not all network administrators enable the multicast functionality of their networking equipment.

Multicast transmission and switches

Switches do not understand multicast. When a multicast stream reaches a switch, the switch sends the multicast stream to all of its ports. A switch treats a multicast address as an Ethernet broadcast.

Details about NetCache support for unicast and multicast

Details about NetCache support for live streams over unicast and multicast and on-demand streams over unicast are provided in the following sections: