Which online service added internet trading and battling to Pokémon Diamond and Pearl?

Pokémon Diamond and Pearl added online trading and battling through Nintendo Wi-Fi Connection.

Released for Nintendo DS in Japan on September 28, 2006, the games expanded Pokémon multiplayer beyond the local link cable and nearby wireless play used by earlier generations. Players could connect through Nintendo’s Wi-Fi service to trade and battle over the internet.

The fourth-generation games also introduced the Global Trade Station, commonly called the GTS. It allowed players to request particular Pokémon or offer one for trade, making worldwide exchanges a defining feature of the generation.

Nintendo Wi-Fi Connection was later discontinued in 2014. It should not be confused with Pokémon HOME, a modern storage and transfer service, or Pokémon Global Link, which supported different online features in later games.