Cell broadcasting in 2G/4G

Cell Broadcast (CB) is a free service that allows the transmission of messages to mobile phones in a certain geographical area.
It is defined by the ETSI’s GSM committee and 3GPP and is part of the 2G, 3G, 4G LTE (telecommunication) and 5G standards.

Furthermore, CB is a one-to-many geo-targeted messaging system. This is different from the conventional Short Message Service-Point to Point system (SMS-PP) used by most mobile networks.

Cell broadcasting in 2G/4G 1

The Process involved in Cell broadcasting

As established before, cell broadcast uses a messaging service in which messages are sent point-to-area to users within a certain area or what is referred to as a cell. A cell is a geographical area in the region of a certain mobile phone antenna or a set of antennas. All mobile phones connected with those antennas will receive the broadcast message.

Cell broadcasting service is therefore anonymous and free of cost for the receivers involved. With cell broadcast, the messages are not stored in a buffer before sending – just like with radio broadcasting, but messages are only received in real-time by mobile phones that are switched on. If a phone is not switched on, the message is not received even when the phone is switched on later. This problem can be solved partly by a repeat rate messaging – the same message being sent out several times with a certain repeat rate. The broadcast message can only be received once by anyone’s mobile phone. Due to a lack of standardization, some mobile phones will receive repeated messages more than once.

A broadcast message can be sent out from one cell, a set of cells within one area, or an entire network. For each mobile phone to be able to receive a broadcast message, certain channels on the mobile phone need to be activated, similar to tuning in to a radio station.

Cell broadcasting places a very low network load on mobile networks. A cell broadcast message to a geographical area in the network is equivalent to sending just one SMS message to a single mobile phone. Network loading problems can cause severe issues during an emergency. This is caused when network usage is likely to be very high, and in these conditions, SMS messages can be delayed for hours or days or even be lost altogether.

To receive a cell broadcast warning message, a mobile phone user has to: 

  • Have predefined the channel for which the message is sent (citizens warning channel);
  • Have his phone switched on at the time of the cell broadcast sending;
  • Have to be in the geographical area or region in which the message is sent at the time it has been sent.

Technology

CB Messaging technology needs to pass through a series of steps involving the following components:

  • 1
    A CBE (Cell Broadcast Entity)
    This is the originator or creator of a Cell Broadcast message to be sent out. CBE determines the message text, the destination of the message, and the message scheduling. CBE is normally operated by a government authority.
  • 2
    CBC (Cell Broadcast Center)
    This is found in the mobile operator's core network. The CBC manages the sending of a CB message received from the CBE. CBC then routes the CB message to the target cells through the RAN controllers and links it to the RAN controllers.
  • 3
    RAN (Radio Access Network) Controller
    This is in the operator's radio network, and it manages the group of cells. The RAN controller disseminates the Cell Broadcast message to the cells in the target region. The RAN Controllers for 2G, 3G, and 4G networks are BSC, RNC, and MME, respectively.
Cell broadcasting in 2G/4G 2
  • 4
    Radio node
    This is the component that sends the Cell Broadcast message wireless to the mobile devices within the geographical area.
  • 5
    End-User Device
    This is the user’s mobile phone that responds to the Cell Broadcast protocol to receive and display the CB message to the users in the target region.

The maximum length allowed for a cell broadcast message is only 1395. This cannot be exceeded, and the CB message parameters must contain the schedule of the broadcasting. The start time must be indicated – if the start time is left open, the system assumes it should start immediately. This also applies to the end-time; it must be defined. If the end-time is left open, the message is repeated indefinitely. This can be canceled using a subsequent cancel message. Repetition rate can be put at 2 seconds up to 30 minutes for each CB message with the same serial number and a message identifier. The message identifier indicates the source of the message. With the identifier and serial number, a mobile phone can ignore a message already sent.

A cell Broadcast Center can be connected to either a Base station Controller (BSC) for GSM networks, a Radio Network Controller (RNC) for UMTS networks, a Mobility Management Entity (MME) for LTEs, or a Core Access and Mobility Management Function (AMF) for 5G networks.

The technical implementation for Cell broadcasting is defined in the 3GPP standards. There are also specific 3GPP standards for each network as follows.

  • 3GPP standard TS 48.049 for 2G-CBC (BSC) interface
  • 3GPP standard TS 25.419 for 3G-CBC (RNC) interface
  • 3GPP standard TS 29.168 for 4G-CBC (MME) interface
  • 3GPP standard TS 29.518 for 5G-CBC (AMF) interface

Benefits of using Cell Broadcast for customers and public warnings

The main benefit is that it gives the customer a way to test emergency broadcast without actually delivering emergency messages to the general public. They can do a very small limited test in the laboratory of emergency broadcast and still involve the real core network without the danger of accidentally releasing a test message and annoy everyone in the region.

Advantages of cell broadcasting during public warnings include:

  • It takes less than 10 seconds to send a cell broadcast message to a few or even over a million people.
  • Cell broadcast can only be sent out by authorized authorities and serving mobile networks.
  • 99% of mobile phones of today support Cell broadcasting
  • Over 1395 characters available for messaging in Latin and 615 characters in Universal Coded Character Set (UCS-2)
  • Supports multi-languages
  • URLs and web-links are supported
  • Supports device based Geo-Fencing
  • Network congestion does not affect cell broadcasting
  • There are no effects from SIM class baring and access class Baring
  • There are data protection constraints as personal data is not required to deliver a CB message

Difference between 2G and 4G cell broadcasting

In 2G cell broadcast, there is a special radio channel that only carries cell broadcast information, and that radio channel carries information in small segments. So, when you send cell broadcasts in 2G, the cell broadcast message has to be broken up into small segments that would fit on this 2G cell broadcast radio channel.

LTE (4G) cell broadcast messages are delivered in what is called the System information messages. There is no special channel for cell broadcast in LTE. It is carried along with other systems information, not on a special channel. In LTE, there is no fixed size limit for these messages, so they don’t need to be segmented.