MAC

MAC is where scheduling actually happens — it's the layer that decides, slot by slot, which UE gets which resources, and it owns the time-critical machinery: the scheduler, HARQ, random access, and the multiplexing of logical channels into transport blocks. Everything above it (RLC, PDCP) is comparatively leisurely; MAC operates on the radio's real-time clock.

A few of its mechanisms come up constantly in practice. Logical Channel Prioritisation decides how a UE divides a limited uplink grant among its bearers, so latency-sensitive traffic isn't starved by a bulk transfer. Buffer Status Reports tell the gNB how much uplink data a UE is holding, and Power Headroom Reports tell it how much transmit power is left — both feed the scheduler. MAC Control Elements are the fast in-band signalling for things like activating an SCell, switching a BWP, or timing-advance updates, used when RRC would be too slow. The scheduler itself isn't standardised — it's a vendor's secret sauce — which is why two gNBs with identical specs can perform very differently under load.