URLLC
Ultra-Reliable Low-Latency Communications: a 5G service category targeting 99.999% reliability with sub-1 ms latency for mission-critical applications.
URLLC is one of the three pillars 5G was built around, and it's the awkward one — because reliability and latency usually pull against each other. The classic way to make a link more reliable is to retransmit, but retransmission costs time, and URLLC's whole point is to be both fast and dependable: on the order of 99.999% packet success within a 1 ms budget.
Hitting that needs a different toolbox than ordinary mobile broadband. Mini-slots shorten the transmission unit so you don't wait for a full slot boundary. Grant-free (configured) uplink lets a device transmit immediately without asking for resources first. Diversity and packet duplication send the same data over multiple paths so a single failure doesn't break the deadline. The motivating use cases — factory automation, remote control, vehicle safety — are ones where a late packet is as useless as a lost one, which is why latency is treated as a hard reliability requirement rather than a nice-to-have.
Frequently asked questions
- Why is URLLC so hard to deliver?
- Because reliability and low latency normally conflict. The usual way to boost reliability is retransmission, but that adds delay, and URLLC has almost no delay budget to spend. Meeting both at once requires shorter transmission slots, pre-allocated uplink resources, and sending data over multiple paths simultaneously — all of which cost spectrum and scheduling complexity. It is achievable, but mostly in well-engineered private and industrial deployments rather than the public macro network.
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