They may look similar because the serving cell can change, but they occur under different conditions and are controlled differently.
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① CELL SELECTION
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When does it happen?
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• When the device is switched on
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• After leaving airplane mode
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• When no suitable serving cell is available
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What happens?
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• The UE searches supported frequency bands
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• It detects available cells
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• It reads essential system information
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• It checks whether a cell is suitable
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• It camps on the selected cell
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Who controls it?
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• The UE
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Typical state:
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• Idle mode
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② CELL RESELECTION
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When does it happen?
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• The UE is already camped on a cell
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• The UE is in idle or inactive operation
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• A more suitable cell or frequency becomes available
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What happens?
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• The UE measures the serving and neighbouring cells
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• It applies broadcast priorities, thresholds and ranking criteria
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• It evaluates whether the reselection conditions remain satisfied
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• It camps on the new cell
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Who controls it?
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• The UE, using parameters broadcast by the network
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Typical state:
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• Idle or inactive mode
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③ HANDOVER
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When does it happen?
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• The UE is connected
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• An active call or data session is in progress
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• The serving cell is weakening or another cell is more appropriate
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What happens?
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• The UE performs configured measurements
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• It sends measurement reports
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• The network evaluates the serving and target cells
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• The target cell is prepared
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• The network commands the UE to move
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• The data path is updated
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Who controls it?
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• The network
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Typical state:
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• Connected mode
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THE SIMPLE DIFFERENCE
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Cell Selection:
Find a suitable cell when no serving cell exists.
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Cell Reselection:
Change the camped cell while the UE is not actively connected.
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Handover:
Move an active connection to another cell with service continuity.
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KEY TAKEAWAY
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Selection and reselection are primarily UE-driven procedures.
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Handover is a network-controlled mobility procedure supported by UE measurements.
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The exact criteria, messages and behaviour depend on the radio technology, UE state and network configuration.
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