George TomzaridisI spent my vacation in Athens from 10-18 August 2024. I had a great time there, went to many places and saw many friends. Unfortunately at the same time a large fire, strikes North-East Athens, so many people needed to evacuate their houses immediately, to survive and not be burned by the massive and quickly spreading fire. It was heartbreaking to see people running from their homes, unsure of what would happen next. Fortunately for them, they received an emergency message on their mobile phones, informing them about the nearby fire and what are the next steps to evacuate to a safer area. But... how this is possible? How we can use technology to save lives? How does this whole operation work?
I had the same questions and to be honest with everyone, also I had many technical questions about that subject, so I decided to analyze it and write an article about it. So let's get started step by step with some basic information first.
What is a Cellular Network?
Probably currently you have a smartphone... maybe you are using a smartphone to read this article. Imagine that some features on your smartphone, which you use daily, suddenly no longer exist. It will be completely useless to have one then (like holding a brick in your hands)... you will not be able to receive or make calls, receive sms, surf the internet wherever you go, use your favorite apps, etc. But why? The answer is that a smartphone relies of course on a network, for you to do basic things. To be more specific they need a cellular network to work.
A cellular network is a communication network where the last link is wireless. It is based on the division of a service area into small, overlapping regions known as cells. Each cell is connected to a wider network. This arrangement allows cellular devices, such as smartphones and tablets, to communicate with each other and with the Internet.
Each cell has its equipment and the whole cellular architecture has its own too. The basic structure of the cellular network, to proceed further and get a basic idea, is analyzed below:
Mobile User (User Equipment - UE): Mobile devices, or User Equipment (UE), are the starting point of any cellular network. These include smartphones, tablets, and other portable communication devices. The UE is equipped with a SIM card that identifies the user and their subscription details. These devices communicate wirelessly with the nearest cell tower, sending and receiving data in the form of radio waves.
Base Stations (BTS): Base Stations, commonly known as cell towers, are the central communication points within each cell. A Base Transceiver Station (BTS) is located at each tower, containing the equipment necessary to transmit and receive radio signals to and from the mobile devices within its coverage area. The BTS converts these radio signals into digital data that can be processed and routed through the network.
Base Station Controller (BSC): The Base Station Controller (BSC) manages the radio resources and controls multiple BTS units. It handles tasks such as handover management (when a mobile device moves from one cell to another), frequency allocation, and the establishment of communication channels. The BSC ensures that each device is connected to the appropriate BTS, optimizing the network's efficiency.
Mobile Switching Center (MSC): The Mobile Switching Center (MSC) is the core component responsible for routing voice calls, text messages, and data within the cellular network. It connects the BSCs to the wider telecommunications network, managing call setup, termination, and routing. The MSC also handles the billing and subscriber management functions.
Home Location Register (HLR) and Visitor Location Register (VLR):
The Home Location Register (HLR) is a central database that contains information about each subscriber's profile, including their services, billing information, and current location within the network. When a subscriber moves to a different cell, the Visitor Location Register (VLR) temporarily stores their information to facilitate local communication without constantly querying the HLR.
Understanding Signaling in Cellular Networks
Now that we understand the basic structure of cellular networks, let's talk about signaling. If we want to describe signaling in one sentence, this would be: It's critical and has an important role in the system and communications in general. Without that, the network will not work as expected or at all.
Signaling is responsible for managing, controlling, and coordinating communication between devices and network infrastructure. Unlike user data (voice or internet traffic), signaling refers to the control information exchanged to establish and maintain communication sessions.
Signaling encompasses all the non-user data communication that is necessary for the network to operate. This includes:
Call setup and teardown: Establishing and terminating voice or data connections.
Handover management: Transferring a call or data session from one cell to another as a user moves.
Authentication and security: Verifying the identity of users and securing communications.
Network resource management: Allocating and managing bandwidth, frequencies, and channels.
Roaming: Enabling users to move across different networks while maintaining service.
Emergency Messages: Establishing and broadcasting emergency alerts to mobile phones.
In modern networks, signaling typically uses separate channels from user data (voice, internet). Protocols vary by network type, so we will just mention some of them per network type, but will not deep dive and analyze them one by one:
- 2G: SS7, MAP, IS-41
- 3G: SS7/MAP, RANAP, SIP
- 4G: S1-AP, X2-AP, Diameter, GTP
- 5G: NGAP, HTTP/2, PFCP
How Emergency Cell Broadcast Works
Now that we understand some basics about cellular networks and learn some new terminologies, we can finally move on.
Emergency alerts are sent using a technology called Cell Broadcast (CB), which is different from SMS or internet-based messaging. Cell Broadcast allows messages to be sent to all phones within the range of specific cell towers. The technology is inherently location-based and can reach many users simultaneously without overloading the network.
When an emergency alert needs to be issued, authorities like government agencies or emergency services decide on the content and the geographic area that should receive the alert. The message is then sent to the cellular network operators, who broadcast the alert via their cell towers. These towers transmit the message to all phones connected to them within the designated area. Phones within the range of these cell towers automatically receive the alert, typically accompanied by a distinct sound or vibration.
The area to be notified is determined by the geographical coordinates and radius or by selecting specific cell towers covering the desired region (each cell tower has a unique ID - The mobile network operator maintains a database that includes detailed information about each cell tower). Authorities provide this information to the network operators, who then ensure that the broadcast is only sent through the towers covering that area. If we use only coordinates or polygons/radius the system will contact the Geographic Information System (GIS) of the network provider, which stores information about each cell tower location, to find which cell towers are covered by the marked area and need to get the message.
The systems that play a critical role in transmitting successfully emergency alerts are:
Cell Broadcast Center (CBC): This is the core component responsible for creating and managing the broadcast messages. The CBC receives the content of the emergency alert from authorized sources, such as government agencies, and determines which areas should receive the message.
Base Station Controller (BSC): These are responsible for controlling multiple cell towers or base stations. The CBC sends the broadcast message to the BSC/RNC, which in turn instructs the relevant cell towers to transmit the message.
Base Transceiver Station (BTS): These are the actual cell towers or radio nodes that transmit the message to mobile devices. The message is broadcast over the air interface to all devices within the range of the cell tower.
Steps for transmitting an Emergency Alert
The Cell Broadcast Center (CBC) creates the message, specifying the content, the language, and the geographical area (e.g., using cell IDs or geographical coordinates).
Cell Broadcast messages are structured into "pages," each of which can carry a maximum of 82 bytes of information. A single-cell broadcast message can be composed of one or more pages, depending on the content length.
The Cell Broadcast Center (CBC) sends the message to the Base Station Controller (BSC), which controls the specific cell towers that cover the target area.
The message is broadcast over the air to all connected devices of each cell tower.
Phones within range of the targeted cell towers receive the message automatically. These alerts are usually accompanied by a distinct sound or vibration. Since Cell Broadcast is transmitted over signaling channels (like the Paging Channel (PCH) or Control Channels (CCH)), even if the network is congested or the phone is not in use, the message will still be delivered and displayed to the user.
Cell Broadcast messages are carried over signaling channels, not the same channels that handle regular user data (voice, internet). This makes the delivery of emergency alerts reliable even under heavy network congestion. In 2G, for instance, CB messages use the Broadcast Control Channel (BCCH), while in 4G/5G, they utilize the Physical Downlink Control Channel (PDCCH).
Since signaling channels are always monitored by mobile phones—even when idle or in low signal areas—this ensures that alerts can be received in almost any condition, including in some underground or low-coverage areas.
Example of a Cell Broadcast Message (for reference only)
Message ID: 4371
Message Type: Emergency Alert
Page Count: 2
Language: English (en)
Geographic Scope: Polygon
Polygon Coordinates: [(37.7749, -122.4194), (37.8044, -122.2711), (37.6879, -122.4702)]
Cell IDs: [1024, 1025, 1031, 1035]
Expiration Time: 15 minutes
Priority: High"EMERGENCY ALERT: Severe weather warning in your area. Seek shelter immediately and follow instructions from local authorities."
Message ID: A unique identifier for the broadcast message.
Message Type: Specifies the type of alert (e.g., Emergency Alert, Weather Warning).
Page Count: The total number of pages if the message exceeds the 82-byte limit for a single page.
Language: The language of the message content (e.g., en for English).
Geographic Scope: This defines the area as a polygon using a series of latitude and longitude pairs. For instance, the coordinates [(37.7749, -122.4194), (37.8044, -122.2711), (37.6879, -122.4702)] represent points that outline a region in San Francisco, California.
Cell IDs: A list of specific cell towers that should broadcast the message. This is usually determined by matching the geographic area (e.g., the polygon or coordinates) with the cell towers that cover that area.
Expiration Time: The time after which the message should no longer be broadcast.
Priority: Indicates the urgency of the message.
This message has 135 characters. Assuming 1 byte per character (for simplicity, as we're dealing with ASCII), this gives us 135 bytes of content. Since each page can carry 82 bytes, the message will be split into two pages.
Will transmit Page 1 first, followed by Page 2. These pages are sent through the Cell Broadcast Channel (CBCH). Your phone will receive Page 1, store it, and wait for Page 2. Once it has both pages, it reassembles them into the complete message and displays it on your screen.
What if I have No Signal / No Service? How I will receive the message?
When your phone displays "No Service," it typically means that your phone cannot establish a standard connection for voice or data services. However, this does not always mean that your phone is entirely disconnected from the network. In some cases, your phone might still be able to receive low-level signaling information, like emergency alerts, through the signaling channels. This could happen if there is enough connectivity for basic signaling but not enough for regular communication services. However, if your phone is completely out of the coverage area or in a dead zone (like deep underground without repeaters), it will not be able to connect to any signaling channels, and you will not receive any broadcast messages.
Why do I receive an emergency alert for a far-away / different emergency area from the area where I am currently located?
Sometimes we receive emergency alerts for areas that are not even close to us. Let's be honest, it can happen. If the government officials don't include our area in purpose in the emergency alert, this can happen due to a few reasons:
Cell Tower Coverage: Mobile phones can connect to cell towers that are relatively far away, especially in rural areas or places with fewer towers. If you're connected to a tower that is within the emergency broadcast area, you might receive the alert even if you're not in immediate danger.
Overlapping Coverage: Cell towers often have overlapping coverage areas. If you're near the edge of the broadcast zone, you might receive the message due to the overlap, even if you're slightly outside the intended area.
Network Propagation Effects: Sometimes, the broadcast might be received in unintended areas due to signal propagation conditions, such as reflections or atmospheric effects, causing the signal to travel farther than usual.
Conclusion
Cell Broadcast is a robust, efficient, and geographically targeted method of sending emergency messages. It operates across all cellular networks, including 2G, 3G, 4G, and 5G, and uses specialized channels to transmit small packets of data that are continuously monitored by mobile devices. These messages are sent with high priority to ensure they reach as many users as possible, even under challenging conditions (low bandwidth usage).
I want to thank publicly my friend Dimitrios Collier for giving me the idea to write a whole article about it, with all of our discussions / my research as we walked in the city center and just talking about it due to recent events with the wildfires in Attica.
