Casting
The term casting is used to describe the different ways in which data can be sent to recipients. In IPv4 there are three methods called:
- Unicast
- Multicast
- Broadcast
Unicast
In unicast a single sender transmits data to a single receiver. Unicast is the simplest form of transmission and is the predominant form of data transfer on the majority of networks.
It is extremely efficient in its use of bandwidth. This is because network infrastructure equipment such as switches can ensure the packet is only transmitted over the specific parts of the network to which the intended recipient is connected.
The network switches learn the MAC addresses of the devices on the networks to which it is connected. It can then forward packets only onto the networks containing devices with the connected MAC Addresses. Routers perform a similar task based on IP Addresses.
Unicast gradually becomes less efficient as more receivers need to see identical data. In this scenario, many unicast packets are sent to unique destinations, hence the other two forms of transmission.
Multicast
In multicast, one or more senders transmit to a group of receivers. Devices ‘subscribe’ to become members of a multicast group. The multicast group is then allocated an IP address such that all members can receive data directed to that group.
Multicast can be more efficient that unicast when different groups of receivers need to see the same data. Multicast is the technique used in Internet streaming of video or audio. It is also used at the core of E1.31 (ACN Lite).
Some ethernet switches have limited capability to handle multicast and so convert it to broadcast. This limitation is rarely documented!
Broadcast
In broadcast, one or more senders transmit data to all receivers. This is very beneficial in some circumstances, particularly for network management packets such as ARP (Address Resolution Protocol) and RIP (Routing Information Protocol) where all devices must see the data.
However it can be very inefficient when a large percentage of devices do not need the data. Ethernet switches pass broadcast data to all their ports which can have the effect of saturating the network.
Broadcast is used extensively in a number of lighting protocols including ShowNet. It is used judiciously in a number of other protocols such as Art-Net 4 and E1.33.
There are two types of broadcast; Directed Broadcast and Limited Broadcast.
Limited Broadcast consists of a sequence of 32 ones (255.255.255.255). This style of broadcast is used to send to all hosts on the local network. Art-Net packets should not be broadcast to the Limited Broadcast address. It is primarily used in start up procedures when the NetId is unknown by the equipment. Limited Broadcast is so called because it is limited to the local network; routers will not route this packet.
Directed Broadcast consists of all ones in the HostId portion of the IP Address (for an Art-Net IP 2.255.255.255). When a network first connects, the controller does not know the number of nodes on the network, nor does it know their IP addresses. The directed broadcast address allows the controller to send an ArtPoll to all nodes on the network. Directed Broadcast is used in two key scenarios:
- The hosts are responsible for parsing data from broadcast packets
- All hosts require the same data
These terms are often confused. Many a network engineer has erroneously assumed that 255.255.255.255 ‘trumps’ all other addresses.
