Many system designers and architects digitise and distribute radar video over Ethernet networks. This approach, rather than the use of older analogue splitter and distribution technology, makes use of common infrastructure to carry many different types of data (for example, voice communications and sensor inputs as well as radar signals). It is possible to configure a Cambridge Pixel product, for example SPx Server, to send this radar video data, or other data such as track position reports, to a single specific client (unicast distribution) or to send data to any number of clients using multicast. Multicast distribution offers a number of advantages, one of the most important being the efficient use of bandwidth.
Multicast v unicast
Unicast transmission involves sending IP packets to a single specific recipient on a network. Multicast transmission allows IP packets to be sent simultaneously to a group of hosts on a network. The multicast protocol has been part of Internet standards for over thirty years. It is typically used when higher bandwidth streams, for example video streams, need to be made available to multiple subscribers at the same time. The basic operational distinction between unicast and multicast is shown below.
Advantages of using multicast distribution
One significant advantage of multicast distribution is that network traffic is reduced by sending data once and permitting it to be received by several clients. The volume of network traffic depends only on what the server sends out, not how many clients are receiving it, and therefore its use is very attractive particularly where higher data rates are in play. Furthermore, multicasting helps to provide a flexible, scalable system architecture where the number of consumers of the data streams may need to be increased at a later point, for example when a new operator console is added. However, we frequently see problems with the configuration of multicast distribution, so some understanding of the process is useful.
To keep in mind when configuring multicast distribution
The network switch or router has a role to play when multicast data is being used. A client wishing to receive multicast data on a specific address sends a “join” request for that group using the IGMP protocol. A network router will receive such requests and will send a copy of any received multicast packets for that address to all hosts that have requested membership; further distinctions between streams of different packet types can be achieved by the use of different ports. A related function is implemented in network switches, using a technique known as “IGMP snooping” to detect membership requests. IGMP (Internet Group Management Protocol) is used to control multicast group membership. Switches typically operate at layer 2, the data link layer, and would not normally process IGMP (layer 3) messages. IGMP snooping, or listening, ensures that the switch is aware of multicast membership and manages the traffic on its ports appropriately. The switch may also send IGMP query messages, prompting all hosts to send their current multicast memberships.
Time to live
Every IP packet has a time-to-live (TTL) field in its header. This is normally used to control the duration of a datagram as it traverses networks. The TTL value is decreased by 1 by every router; a packet with TTL of 0 is discarded. However, the TTL field has an extra meaning for multicast packets. It defines a threshold which is matched with a threshold setting on the routers through which the packet would normally pass. A packet needs to have a threshold greater than the router’s threshold to pass through it. By this means, the effective scope of a multicast packet can be set by assigning a threshold. A value of 0 means that the packet will not leave the source host. A value of 1 restricts its scope to the local subnet. Any value less than 32 restricts the packet to the same site, while values of less than 64 or 128 limit it to the originating region or continent (the meaning of site and region in this context is not formally defined). Cambridge Pixel applications that generate multicast packets allow the TTL to be assigned to such packets to be set in a configuration file....
What can go wrong with multicast distribution?
The complications of multicast arise with the role of the network switch....
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