How efficient one-to-many video distribution is achieved over IP networks using multicasting.
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In traditional broadcast SDI systems, we use a point-to-point distribution system with one-to-one mapping. That is, to connect the output of a camera to a production switcher we take the SDI output of the camera and connect it directly to one of the production switchers inputs. If we then want to monitor the camera feed going to the production switcher we must either disconnect the input or introduce a Distribution Amplifier which has multiple outputs. Each one of these then forms a one-to-one mapping with monitors, routing matrices and other processing equipment.
By introducing a DA we have effectively provided a one-to-many mapping system.
In the IP world, to provide one-to-many mappings, we substitute the physical DA with an abstracted logical model using IP routers. Instead of having individual SDI cables connecting the equipment together, we take advantage of the IP-router’s ability to be able to duplicate packets and create a multicast system.
IP routing generally works on the unicast model; when a computer requests data from a server, there will be just one source and one destination. However, if a playout server wants to send an online film to many users, a mapping of one-to-many is used and this is called multicast.
Multicasting is much more flexible than SDI routing using matrices and DA’s as we are not limited to working in the SDI domain. If a user wants to watch the studio output from their office, or the green room, then we don’t have to worry about running new SDI cables to them, or adding the studio to RF distribution, we just set up another multicast user in the IP-router.
The IP-router provides the one-to-many mapping by distributing the video at the IP packet level. Each IP datagram is copied and then sent out on the appropriate port of the router to the user.
Figure 1 - Multicast distribution is more efficient than the unicast distribution for streaming to multiple destinations.
The source device, such as camera, production switcher or sound console, has no knowledge of which devices are receiving their output in an analogous way to traditional broadcast equipment. The sound console has no knowledge of the destination of its outputs, only that they are seeing a high impedance load to a DA at the end of a twisted pair. The Ethernet output of a camera has no knowledge of where its packets are going and can only see the network interface card of the IP-router or switcher it is connected to.
An alternative method to multicasting would be to set the camera’s destination address of each device requiring the stream. This would cause two problems, firstly, the overhead of having to set the IP destination addresses of six cameras in a studio, each connected to potentially ten different destinations, is an administrative nightmare and would be unworkable. Secondly, we would increase the network load by the number of destinations set in the camera’s routing table, as each packet would be individually sent to each device requiring the stream.
Multicasting solves both problems as it relies on the receiving equipment to opt-in to the feed, and IP packets are only sent once to each group destination, with the routers providing duplication only on the network branches that need the stream. There are three key concepts in a multicasting system: the group address, reverse path forwarding and IGMP.
The multicast group address is the destination IP address of the equipment creating the audio or video stream, i.e., a camera or microphone. Each camera, microphone, production switcher output or sound console aux send will require its own group address. The multicast group addresses are special IP addresses constrained in the range 126.96.36.199 to 188.8.131.52, approximately 248 million groups. Some of these are reserved and a full list can be found at the Internet Assigned Numbers Authority (IANA).
Internet Group Membership Protocol (IGMP) provides the mechanism for equipment receiving the stream (monitors etc) to tell its router that it wants to receive a particular group. Each IP-router passes this information along the line to the source equipment so that only the networks that require the stream relay it, thus reducing network congestion.
In figure 2 the router will periodically send an IGMP Host Membership Query message to all devices connected to its networks, in this case the sound console responds with an IGMP Host Report Group 1 and Group 2 message, advising the router that it wants to receive Mic-1 and Mic-2 audio streams. Mic-3 (Group 3) is not sent to the sound console as the sound console did not opt-in to receive that group.
Unicast routing works by looking at the destination IP address of the packet and comparing it to its look-up table so the router knows where to forward the packet to. Multicast routing uses the source address in the IP-datagram to route back to find out where the source of the packet is, a system called Reverse Path Forwarding (RPF).
The beauty of IP networks in general, and multicasting specifically, is that the data can move in both directions simultaneously. For example, a sound console could have all its aux sends and returns on one Ethernet cable, potentially saving tons of twisted pair cable. The sound console becomes both a multicast receiver, and multicast broadcaster. If sufficient capacity IP routers are used the production switcher can do the same with its video inputs and outputs, with hundreds of feet of coax cable being substituted by CAT6 Ethernet and fiber optic cable.
If a producer wants to watch the studio output in their office, their desktop computer or tablet can be easily configured to receive the group address of the studio output. No configuration will be required by the IT department, and no additional cables will need to be run to their office. However, this introduces the topic of network security. It might be that the producers do not want the whole station to be able to see the studio output, so the routers would have to be configured to stop them sending certain streams to certain networks.
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