What is a "handover" or "handoff" in a cellular network?
1 Answer
This is a core concept that makes mobile communication possible.
Let's break it down, from a simple analogy to the technical details.
The Simple Analogy: A Relay Race
Imagine you're a runner in a long relay race, and the "baton" is your phone call or data session.
- Each runner on the track is a cell tower.
- You are your phone, moving along the track.
- The handover is the moment one runner seamlessly passes the baton to the next without dropping it or slowing down.
The goal is for the baton (your connection) to keep moving smoothly, even though the entity carrying it (the cell tower) changes. You, the spectator, shouldn't even notice the exchange happened.
The Formal Definition
In a cellular network, a handover (or handoff) is the process of transferring an active call or data session from one cell tower (base station) to another without losing the connection.
This is essential because cellular networks are divided into geographic areas called "cells," each served by a specific tower. As a user moves from one cell to another, the network must pass the connection to the new cell's tower to maintain service.
How a Handover Works (The Steps)
The process is a constant, high-speed negotiation between your phone and the network.
Measurement: Your phone is never just talking to its current tower. It's also constantly "listening" to the signals from nearby towers, measuring their strength and quality.
Reporting: Your phone sends a "measurement report" back to the network, essentially saying: "Hey, I'm connected to Tower A with great strength, but I'm starting to hear Tower B's signal getting much stronger, and Tower C is also in range."
Decision: The network's "brain" (a controller) analyzes this report. It sees that you are moving away from your current tower (Tower A) and closer to a new one (Tower B). Based on signal strength, network load, and other factors, it decides that it's time to perform a handover to Tower B.
Execution: The network orchestrates the switch.
It contacts Tower B and reserves resources (a channel) for your phone.
It sends a command to your phone, telling it to switch to the new channel on Tower B at a precise moment.
* Your phone detaches from Tower A and immediately connects to Tower B.Release: Once the connection to Tower B is stable, the network releases the resources your phone was using on Tower A, making them available for another user.
This entire process happens in a few milliseconds, making it virtually undetectable during a voice call or video stream.
Types of Handovers
There are two main types, defined by how the switch is made:
1. Hard Handoff ("Break-Before-Make")
This is the most common type in modern 4G (LTE) and 5G networks.
- The phone completely breaks its connection with the old tower before establishing a new connection with the target tower.
- While this sounds risky, the "break" is incredibly brief (typically 30-60 milliseconds), so it's unnoticeable to the user.
- Analogy: Letting go of one trapeze bar just before you grab the next one.
2. Soft Handoff ("Make-Before-Break")
This was a key feature of older 3G (CDMA) networks.
- The phone establishes a connection with the new tower before it breaks its connection with the old one.
- For a short period, the phone is connected to two (or more) towers simultaneously.
- This provides a very smooth transition with no chance of a "dropped" connection during the handover itself, but it is more complex and uses more network resources.
- Analogy: Grabbing the next rung on a ladder before you let go of the previous one.
Why is a Handover Triggered?
It's not always because you're moving! A handover can be initiated for several reasons:
- Signal Strength (Most Common): You are moving out of the range of your current cell.
- Load Balancing: Your current cell might be too crowded with users. The network may hand you over to a nearby, less-congested cell to balance the traffic, even if your signal is still good.
- Quality of Service (QoS): The network might move you to a different frequency band (e.g., from a slower band to a faster 5G band) to provide better data speeds for an application you're using.
- Interference: To move you away from a source of radio interference.
In summary, a handover is the invisible, high-speed magic that allows you to use your phone seamlessly while walking, driving, or riding on a train, ensuring your connection stays stable as you move between different cell tower coverage areas.