70 lines
2.6 KiB
Plaintext
70 lines
2.6 KiB
Plaintext
[[cell_broadcast]]
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== Cell Broadcast
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Normally, all user plane data in GSM/GPRS networks are sent in
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point-to-point channels from the network to the user. Those are called
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_dedicated_ radio channels which exist between the network and one
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given phone/subscriber at a time.
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Cell Broadcast is an exception to that rule. It permits user data
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(so-called SMS-CB data) to be broadcast by the network in a way that
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can be received by all phones in the coverage area of the given BTS
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simultaneously.
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More high-level information can be found at
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https://en.wikipedia.org/wiki/Cell_Broadcast and the related
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specification is <<3gpp-ts-23-041>>.
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=== Use Cases
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Cell Broadcast was used for various different use cases primarily in
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the 1990s and early 2000s, including
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* advertisement of the GPS position of the cell tower you're currently camping on
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* advertisement of the calling codes of your current "home zone", i.e. a
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"lower cost short distance" call zone travelling with you as you roam around.
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More recently, SMS-CB is seeing some uptake by various disaster
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warning systems, such as
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* CMAS (Commercial Mobile Alert System), later renamed to WEA (Wireless Emergency Alerts) in the US.
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* EU-Alert in the European union
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* Messer Ishi (Rocket Alert) in Israel
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* ETWS (Earthquake and Tsunami Warning System) in Japan
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* KPAS (Korean Public Alert System)
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=== Osmocom Cell Broadcast support
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* OsmoBTS implements the `SMS BROADCAST COMMAND` Message in A-bis RSL
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according to Section 8.5.8 of 3GPP TS 48.058
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* low-level testing/debugging features
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** OsmoNITB and OsmoBSC implement a VTY command
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`bts <0-255> smscb-command <1-4> HEXSTRING` to send a given hex-formatted cell
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broadcast message to a specified BTS. This can be used for low-level debugging
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* proper 3GPP-specified CBS/PWS elements and protocols
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** OsmoBSC supports routing and distribution of CBS and warning messages
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** OsmoBSC implements the BSC-CBC interface using the CBSP protocol
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** OsmoCBC implements the central function of a _Cell Broadcast Centre_,
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receiving cell broadcast and warning messages from external entities
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via a REST based HTTP interface, and distributing it throughout the
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cellular network.
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** OsmoCBC implements the BSC-CBC interface using the CBSP protocol (for
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2G/GSM RAN)
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** OsmoCBC implements the MME-CBC interface using the SBcAP protocol (for
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4G/LTE RAN)
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[mscgen]
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----
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include::osmocom-cbs.msc[]
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----
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=== Message Structure
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* Each message has a maximum of 15 pages
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* Each page is 82 bytes of data, resulting in 93 characters in GSM
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7-bit default alphabet
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* Messages are broadcast on logical channels (more like an address)
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* Subscribers can activate/deactivate selective addresses
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