QCI
QoS Class Identifier: a scalar value in LTE/EPC that defines the QoS characteristics (delay budget, error rate, priority) for an EPS bearer. Superseded by 5QI in 5G.
QCI is the LTE scalar that does the same job 5QI does in 5G: a single number that indexes a standardised set of QoS characteristics — resource type (GBR or non-GBR), priority, packet delay budget and packet error loss rate — for an EPS bearer. Assign a bearer a QCI and every node in the path knows how to treat it without the parameters being signalled each time. QCI 1 is conversational voice (GBR), QCI 5 is IMS signalling (non-GBR, top priority), QCI 9 is the typical default best-effort bearer.
The reason it still matters in a 5G world is interworking and the sheer size of the deployed LTE base. The 5QI table was built to line up with QCI values so familiar mappings carry over (QCI 1 ↔ 5QI 1 for voice), which keeps voice continuity sane when a call falls back from NR to LTE. So you'll meet QCI any time you touch the 4G estate, EPS Fallback, or VoLTE — it's superseded by 5QI in native 5G, not erased from the network. The 5QI reference carries both tables and the QCI-to-5QI mapping, which is the lookup you actually need when a bearer crosses between the two.
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