EMZETT.Geocast
In short: A transmission type in which messages are sent specifically to all recipients within a particular geographical area.
In more detail: A special form of multicast in which group membership isn’t defined by registration but by the physical location of the devices. Relevant, for example, in vehicle-to-vehicle communication (warning of a hazard only to vehicles nearby) or location-based emergency alerts.
In Depth
How it differs from multicast
The decisive difference from classic multicast lies in HOW the recipient group is defined: with multicast, devices actively register for a group (e.g. a particular streaming channel, by registering with a multicast address); with geocast, membership results purely from the physical position — a device doesn’t have to register for anything; it automatically receives all messages intended for its current area as soon as it’s there, and just as automatically stops receiving them as soon as it leaves the area.
Typical areas of use
In practice this is relevant above all for time-critical, location-based warnings. Vehicle-to-vehicle communication (V2V, often via the Wi-Fi standard IEEE 802.11p or mobile-network-based C-V2X) uses geocast to send, for example, an emergency braking, an accident or a black-ice warning only to vehicles in the relevant stretch of road (typically a few hundred metres to a few kilometres), instead of flooding the entire road network with broadcast messages that would be irrelevant to most recipients and would needlessly use up radio capacity. Emergency warning systems (cell broadcast during natural disasters, Amber Alerts for missing children, Germany’s “Cell Broadcast” disaster warning system since 2023) work on the same basic principle: the message goes specifically to all mobile network cells in an affected area, not nationwide — unlike a classic SMS, the recipient’s phone number doesn’t even have to be known; every device logged into the target cell receives the message automatically.
Technical challenge: dynamic routing
The technical challenge with geocast lies in routing: the network has to know (or actively find out) which devices are currently in which geographical area, which is considerably more complex with fast-moving participants (vehicles on the motorway) than with static multicast groups with a fixed membership list known in advance. Vehicle networks usually solve this via so-called “flooding with area limitation”: a message is passed on from vehicle to vehicle, but each vehicle checks, based on its own GPS position, whether it’s still within the target area — outside of it, forwarding is stopped so that the message isn’t distributed endlessly throughout the whole network.
How it differs from location-based services
Geocast isn’t the same as a location-based service, in which an application actively asks for content nearby (e.g. “show me restaurants around me”) — that’s a classic client-initiated request. Geocast, on the other hand, is a pure transmission type at the network level, in which the sender takes the initiative and recipients are selected passively according to their geographical position.