AXIS Media Control - FAQ

MPEG-4 transmission methods

Question:

Which are the transmission methods available for MPEG-4 streaming and when to use which method?

Answer:

Retrieving the media produced by the server over a network can be done in a number of ways. Axis has chosen to follow the ISMA Implementation Specification for the supported network protocols. This means that RTP (Real-Time Protocol) is used for the media delivery, and RTSP (Real-Time Streaming Protocol) is used for the media control. Information about available media presentations is provided via SDP (Session Description Protocol).

As network environments in which streaming is supposed to be performed can vary, the following transmission methods are available:

1. Multicast RTP (RTP+RTSP) - suitable for a subnet or an intranet.
2.  Unicast RTP (RTP+RTSP) - suitable for both an intranet and the Internet where there is no NAT firewall.
3.  RTP over RTSP (RTP/RTSP) - suitable for the Internet where firewalls are used and an RTSP proxy is available.
4.  RTP over RTSP, which in turn is tunneled over HTTP - tunneling via HTTP can be used to pass a firewall located between the camera/video server and the client.

Multicast
The simplest case of media transmission is multicast RTP, for which the server only needs to send the media stream to one predefined multicast address - a Class D IP addresses. For more information about multicast addresses, see http://www.iana.org/assignments/multicast-addresses . A client wishing to acquire the stream retrieves the SDP description and connects to the indicated multicast address.

The benefit of this is that large numbers of clients can retrieve the media without increased bandwidth usage, and also more than the product’s standard capacity of 20 unicast viewers.
The drawbacks are that routers and switches are not usually configured to allow multicast traffic to pass. The protocol taking care of the announcement of services to the router is IGMP (Internet Group Management Protocol), which will need to be enabled on the router for multicast traffic to pass from one network segment to another.

Unicast
To better accommodate the users on an intranet or on the Internet, unicast RTP support is required. The client can then inform the server via RTSP where and how it wants to retrieve the RTP stream, which can then be unicast to the client.

Unicast means the media stream goes independently to each user, i.e. the required bandwidth for several simultaneous users is usually greater than that required for multicast. This means the maximum of 20 clients connecting to the AXIS product needs to be considered.

Reliability of the delivery is increased with RTP over RTSP and RTP over RTSP over HTTP as TCP/IP is used for media delivery. UDP/IP protocol (connectionless protocol, no resending upon error in transmission) is used for Multicast RTP and Unicast RTP methods which can reduce performance of the stream delivery in harsh and environments with limited bandwidth.

How to select which method to use when accessing the product using Internet Explorer? Right-click on the video in the Live View page, select Settings and go to the Network tab of the AXIS Media Control control panel applet. In the 'MPEG-4 connection order' area, you can select/unselect used methods and change their order as well.

This FAQ applies to the AXIS 207/207W/210/211/210A/211A/212 PTZ/213 PTZ/214 PTZ/216FD/221/225FD/231D/232D/241S(Blade)/Q(Blade)/241SA/QA.