Cellular Network Infrastructure

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h1. Service Area Broadcast

Normally, all user plane data in UMTS networks are sent in point-to-point channels from the network to the user. Those are called "dedicated" radio channels which exist between the network and one given phone/subscriber at a time.

Service Area Broadcast is an exception to that rule. It permits user data (so-called SMS-CB data) to be broadcast by the network in a way that can be received by all phones in the coverage area of the given Service Area (group of cells) simultaneously.

More high-level information can be found at https://en.wikipedia.org/wiki/Cell_Broadcast and the related specification is found at 3GPP TS 23.041 Technical realization of Cell Broadcast Service

Service Area Broadcast is the UMTS successor of the GSM [[Cell_Broadcast]]

h2. Use Cases

More recently, SMS-CB is seeing some uptake by various disaster warning systems, such as

* CMAS (Commercial Mobile Alert System), later renamed to WEA ("Wireless Emergency Alerts":https://en.wikipedia.org/wiki/Wireless_Emergency_Alerts) in the US.

* EU-Alert in the European union

* Messer Ishi (Rocket Alert) in Israel

* ETWS (Earthquake and Tsunami Warning System) in Japan

* KPAS (Korean Public Alert System)

h2. Architecture as per 3GPP specs.

See Section 6.2 of 3GPP TR 25.931.  Basically, the CBC (Cell Broadcast Centre) is issuing SABP (Service Area Broadcast Protocol, 3GPP TS 25.419) requests via the Iu-BC interface towards the RNC.  The RNC then schedules the transmission of CBS messages on the CTCH (Common Traffic Channel)

h3. Network Architecture

{{graphviz_link()

digraph G {

  BTS0 [label="NodeB"];

  BTS1 [label="NodeB"];

  BTS10 [label="NodeB"];

  BTS11 [label="NodeB"];

  BSC [label="RNC"];

  BSC1 [label="RNC"];

  User -> CBC [label="proprietaty"];

  CBC -> BSC [label="SABP"];

  CBC -> BSC1 [label="SABP"];

  BSC -> BTS0 [label="Iub"];

  BSC -> BTS1 [label="Iub"];

  BSC1 -> BTS10 [label="Iub"];

  BSC1 -> BTS11 [label="Iub"];

  BTS0 -> UE [label="BMC/CTCH"];

}

}}

h3. Ladder Diagram

!osmodevcon2019-cell_broadcast__4.png!

h2. Osmocom Service Area Broadcast support

Currently it is not yet supported by any of the Osmocom components.  We would love to implement it, but somebody would have to contribute related code and/or funding.

Assuming that the NodeB+RNC connected to [[OsmoMSC:]] or the hNodeB connected to [[OsmoHNBGW:]] implement SABP, all we'd need to do is to implement a kind of SABP multiplexor inside [[OsmoHNBGW:]] so that the CBC has to talk only once to the HNBGW, which then takes care of distributing that information to all hNodeB in the service area.

We would also want to implement a simplistic CBC which implements SABP on Iu-BC towards the RNC/HNBGW, as well as 3GPP TS 48.049 for GSM.  This way one could easily configure certain CBS / SMS-CB messages on the CBC, and the CBC would distribute it to all BSCs/RNCs/HNBGWs in the respective service area, who in turn communicate the messages to the MSs.

h2. Further Reading

* "3GPP TR 25.931: UMTS UTRAN functions, examples on signalling procedures":https://www.etsi.org/deliver/etsi_tr/125900_125999/125931/14.00.00_60/tr_125931v140000p.pdf (See Section 6.2)

* "3GPP TS 25.419: UTRAN Iu-BC interface: SABP Protocol":https://www.etsi.org/deliver/etsi_ts/125400_125499/125419/15.00.00_60/ts_125419v150000p.pdf