Originally published on 4GNation, February 2016 — republished here as part of the Telco & Beyond Engineering Archive.
Radio link failure detection is one of those mechanisms that operators rarely think about until it goes wrong — a subscriber drops mid-call, and the root cause traces back to exactly this timer logic. It’s a short piece, but it’s a good example of how much of RF engineering comes down to three or four well-tuned parameters working in concert.
The full procedure for determining whether a link has failed due to loss of synchronization involves three main parameters and timers:
- N310 — the number of 200ms intervals during which the UE is unable to successfully decode the PDCCH due to low RSRP. In other words, it counts the number of times the UE fails to decode 20 consecutive downlink frames.
- T310 — a timer, in seconds, that gives the UE a window to get back in sync with the eNodeB.
- N311 — the number of 100ms intervals the UE must successfully decode the PDCCH to be considered back in sync. It counts the number of times the UE successfully decodes 10 consecutive downlink frames.
If the UE detects N310 consecutive out-of-sync indications, it starts the T310 timer. If that timer expires before recovery, the link has failed. If the UE detects N311 consecutive in-sync indications before T310 expires, the timer is stopped and the link is considered healthy.
It’s a simple state machine on paper, but getting N310, T310, and N311 tuned correctly for a given network’s RF environment is where the real engineering judgment comes in — set them too aggressively and you get unnecessary drops in marginal coverage; too loosely and you delay recovery for genuinely failed links.