76 lines
2.7 KiB
Plaintext
76 lines
2.7 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 RSL
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according to Section 8.5.8 of 3GPP TS 08.58
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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
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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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==== What's missing
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What's missing (for production operation in larger networks)
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* mechanism to broadcast one (set of) cell broadcast messages from the
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BSC to multiple/all BTSs, rather than one BTS individually
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* OsmoBTS reporting of current CBCH load
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* BSC scheduler scheduling multiple alternating sets of CBCH messages
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based on the current CBCH load reported by BTS
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* external interface from BSC to a Cell Broadcast Center (CBC), e.g.
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according to 3GPP TS 48.049
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* an Osmocom implementation of the Cell Broadcast Center (OsmoCBC)
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which can manage and distribute messages to multiple BSCs and which
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has an external interface by which cell-broadcast can be entered
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into the network
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If you would like to contribute in any of those areas (by means of
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code or funding), please reach out to us any time.
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=== Message Structure
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* Each message has a meximum 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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