BBC Technology Correspondent Rory Cellan-Jones testing mobile phone signals on a train from Cardiff to London
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3G mobile data network crowd-sourcing survey by BBC

A BBC crowd-sourcing survey to map the UK's mobile phone networks has revealed the parts of Britain where phones can access the fastest and slowest data speeds.

Users of more than 44,000 phones downloaded an app to measure mobile phone signals over a month.

The software helped build an interactive map that lets people check reception at specific postcodes.

Rory Cellan-Jones took a journey from Cardiff to London to find the "hot spots" and "not spots" in the mobile data network.

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