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\section{Conclusion}
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This paper provides a comprehensive analysis of intraday volatility transmission across equities, foreign exchange, commodities, and cryptocurrency markets using 5-minute OHLCV data spanning 2008--2026. Employing the \citet{DieboldYilmaz2012} generalized variance decomposition framework within a VAR(4) model, we document several important findings.

The full-sample analysis reveals a total spillover index of 55.44\%, with U.S.\ equity index futures serving as the dominant volatility transmitters and the VIX acting primarily as a volatility receiver. Cryptocurrencies remain largely isolated from the traditional asset volatility network, though this isolation is diminishing over time.

Regime-specific analysis uncovers dramatic shifts: during the COVID-19 crash, USD/JPY emerges as the dominant shock originator, while during the 2022 rate-hiking cycle, Bitcoin transitions to a net volatility transmitter ($+7.9\%$). The rolling connectedness analysis demonstrates the TSI's potential as a systemic risk indicator, with the index rising before and during major market dislocations.

Bitcoin's 24/7 market structure generates a remarkably uniform volatility profile across global trading sessions, contrasting with the session-dependent patterns of equity and VIX markets. Granger causality tests confirm the primacy of equity futures in driving VIX movements and the causal independence of Bitcoin.

Future research could extend this analysis by incorporating frequency-domain decompositions \citep{Barunik2018} to distinguish short-run from long-run spillovers, employing TVP-VAR models \citep{Antonakakis2020} to avoid rolling-window smoothing, and examining the role of crypto-specific events in shaping the crypto--traditional market volatility nexus.