Call-setup & accessibility optimization, written as a book chapter: the access door, the admission lobby, the fragile first seconds, the resurrection, the search, and the sacred path — closing with six KPI-optimization case studies you can run on a real network.
Every SIM holds a random access class 0–9; AC 10 is emergency, AC 11–15 are priority users (TS 22.011). SIB2 carries, per traffic type, an ac-BarringFactor and ac-BarringTime (TS 36.331 §5.3.3.2): a UE draws rand(), proceeds if below the factor, else waits (0.7 + 0.6·rand) × barringTime and retries — probabilistic crowd control.
On the live cells: acBarringForMoData/MoSignalling/Csfb all staged at factor 95, time 4 s; the 17-member acBarringPresence struct runs AUTO on MoData/MoSign/CSFB — automation owned by Load-Based Access Barring (FAJ 121 3093 / CXC4011807, ACTIVATED), which starts barring from the RRC reject:attempt ratio, recalculates periodically, and — per the FD — withdraws the cell from inter-frequency load-balancing candidacy until barring probability returns to 0. Progressive Access Barring (FAJ 121 4746) adds three priority tiers over five traffic classes; this operator parks it (golden: Deactivated) and maps all five classes to PRIORITY0 with special ACs at PRIORITY4.
The VoLTE skip — Prioritization of VoLTE in Access Barring (FAJ 121 4329 / CXC4011942, High Load Handling FAJ 801 0415): three cell booleans (acBarringSkipForMmtelVoice/Video/Sms) inject Rel-12 SIB2 IEs (ac-BarringSkipForMMTELVoice-r12…) that let voice originations bypass the barring check. Two FD footnotes: skips broadcast only while barring broadcasts; only Rel-12 UEs read them (Rel-9/10/11: SSAC only; Rel-8: neither). On this network the skip is off and golden-compliant — a deliberate engineering position: skipped attempts still cost PRACH/PDCCH/RRC and fail deeper, where failures are expensive; barring is the only tool protecting already-connected calls. Emergency is never barred: acBarringForEmergency=false, presence MANUAL.
Historically a VoLTE origination arrived as mo-Data — invisible at the moment of rejection. Rel-12 (TS 36.331 V12.8.0) added the mo-VoiceCall-v1280 establishment cause, broadcast-enabled by SIB2 voiceServiceCauseIndication-r12, and copied into the S1AP InitialUEMessage (core must support TS 36.413 ≥ V13.2.0).
Dynamic UE Admission Control (FAJ 121 4301) defends against connection storms with three states — Admit All → Initial Blocking → Block All — crossed at a differentiation and an overload threshold. Optimized VoLTE Access at High Load (FAJ 121 5062 / CXC4012368) makes exactly one move, quoted from the FD: “a Mobile Originating voice request is given the same priority level as Reestablishment and Handover requests” — admitted in Initial Blocking, rejected only in Block All. New counters split voice out (pmRrcConnEstabFailDynUeAdmCtrlMoVoice), and the FD warns the Initial E-RAB Establishment SR formula must incorporate the new moVoice counters. On this network the whole DUAC chain is parked by documented policy (both CXCs DEACTIVATED; golden: Deactivated, “Not GA'd as off”) — this operator defends with reservations and barring instead. Two schools; mixing them carelessly multiplies state machines.
| Layer | Identity | Adds | This node |
|---|---|---|---|
| Basic | FAJ 121 1857 · unlicensed | PA reservations + transport NW admission (dl/ulTransNwBandwidth=1000) | base behavior |
| Differentiated | FAJ 121 1787 / CXC4011062 | ARP blocking + pre-emption (optionally SPID) | ACTIVATED |
| Dynamic GBR | FAJ 121 1748 / CXC4011060 · VoLTE Performance VP | admission on measured utilization (MSR), headroom for fading/mobility | ACTIVATED |
The thresholds form a corridor: dlAdmDifferentiationThr=800 (low-ARP GBR refused) → dlAdmOverloadThr=850 (hard wall; MOM default 950 — this operator stops earlier); UL mirrors both; MOM rule Overload ≥ Diff. Pre-emption is armed (arpBasedPreEmptionState=ACTIVATED) with two refinements: preemptInactTimerMin=15 s (default 4 — don't evict the briefly silent) and recalcPreemptableUeTimer=1 s. Rejected high-ARP requests cascade into pre-emption of admitted low-ARP bearers. Costs land in named counters: pmErabEstabFailGbrDlEnb/UlEnb, pmErabEstabAttRejAdmMsr…Arp (per-ARP bins), and victims as abnormal releases — when pre-emption is armed, some drop rate is policy, not fault.
Call setup (~2 s ladder) races mobility's continuous clock (A3/A5/B1/B2). LTE Delayed Mobility for Robust Call Setup (FAJ 121 5850 / CXC4012835) is selective deafness during setup, per direction, via the RobustCallSetup MO: intraLteMobilityDelayEnabled (discard A3/A5, block blind IF-HO), geranUtranMobilityDelayEnabled (discard B1/B2, block blind RwR to GSM), endcSetupDelayEnabled (discard EN-DC B1; FD recommends reportIntervalB1 ≥ MS_480). Side-effect catalog: ANR's PCI-conflict discovery and every coverage-triggered feature are delayed too — regression scope is the mobility KPIs.
Robust SRVCC Handover at Call Setup (FAJ 121 4855 / CXC4012240, VoLTE VP): when QCI-1 establishes and srvccDelayTimer>0, a B2-triggered SRVCC preparation is cached (newest kept), released three ways — A1 (coverage back: discard), A3/A5 (LTE solution wins), or timer expiry (proceed, late enough to survive). On this metro grid both features sit DEACTIVATED with srvccDelayTimer=0 — geography is a parameter; corridor/rural networks flip it.
After radio link failure the UE sends RRCConnectionReestablishmentRequest (TS 36.331 §5.3.7) carrying C-RNTI + PCI of the failed cell + shortMAC-I — a fingerprint, valid for a seconds-long window. The basic feature (FAJ 121 3019, unlicensed) answers “yes” only where the context lives; calls die at borders onto foreign eNodeBs. Multi-Target (FAJ 121 3023 / CXC4011366, ACTIVATED here) extends it: cross-eNodeB context fetch over X2, basic inter-vendor support, and the EN-DC case with a clean Secondary Node Release first. FD requirement worth framing: emergency ARP must be uniform network-wide or X2→S1 handovers fail later — multi-node features audit your network for you, rudely. Every success subtracts directly from pmErabRelAbnormalEnbActQci[1].
Idle UEs wake only at occasions computed from identity + defaultPagingCycle=128 (1.28 s) and nB=T (TS 36.304); one paging message carries ≤ maxNoOfPagingRecords=7; stale pages die at pagingDiscardTimer=3 — late delivery is worse than none, the MME re-pages fresh. A lost page is the stealthiest accessibility killer: the UE never attempts, so nothing steps in the radio counters — only the paging counters see it. Priority Paging (FAJ 121 3081 / CXC4011711, ACTIVATED) lets MME-marked voice pages (S1AP paging priority) jump the queue: it creates no capacity, it decides who gets the capacity that exists. 41 golden rows govern the Paging MO — the least casual tuning surface in the module: idle-mode dials move whole tracking areas.
Layered and absolute: never barred (acBarringForEmergency=false, presence MANUAL); recognized by the Rel-8 emergency establishment cause above every ordinary class; privileged at admission via Emergency Call Prioritization (FAJ 121 1039 — unlicensed, hence no FeatureState object in the kget: expected) protecting initial access, E-RAB establishment, and incoming handover, with the PA seats and paArpOverrideHpa lane; emergency attach/PDN per TS 23.401, PSAP via TS 23.167; fallbacks defined across every boundary (CSFB emergency; Limited Service Mode FAJ 121 1743; LTE↔NR standalone). Emergency configuration is audited as compliance with evidence, not optimization.
Each scenario follows the professional loop: KPI symptom → counter evidence → root cause → parameter/feature action (exact MO.attribute) → verification plan → rollback. All configuration named is real; the traffic narratives are the engineering situations these features exist for.
pmErabEstabAttAddedQci[1] itself fell — fewer attempts ever reached the eNodeB. SIB2 captures show Load-Based barring active (factors recalculated from the 95/4 staging); the cell withdrew from IFLB per the FD.EUtranCellFDD.acBarringSkipForMmtelVoice=true + FAJ 121 4329 activation on event cells only (precondition: barring presence active).pmErabEstabFailGbrDlEnb + admission rejects.pmRrcConnEstabFailDynUeAdmCtrlMoVoice ≈ 0 while plain DUAC rejects climb steeply — the voice lane worked. Ticketed IMEIs are Rel-10/11.mo-VoiceCall-v1280; their calls arrived as mo-Data and were triaged with the crowd.pmErabEstabFailGbrDlEnb steps only in that hour; measured (MSR) utilization peaks ~87% — above dlAdmOverloadThr=850.AdmissionControl.dlAdmOverloadThr 850→900 on that cell, knowing it spends the fading/mobility headroom Dynamic GBR exists to protect.pmErabRelAbnormalEnbActQci[1] (drops must not rise); next-cell-unlock change window per MOM.preemptInactTimerMin=15 (no eviction of briefly-silent users), recalcPreemptableUeTimer=1.EUtranCellFDD.srvccDelayTimer = a few seconds; ③ if traces also show A5 storms: FAJ 121 5850 with intraLteMobilityDelayEnabled=true only — keep the IRAT escape live.pagingDiscardTimer=3; discards uniformly distributed — voice dying with notifications.Diagnosis before medication: name the KPI, walk its tree (Module 1, lesson 1-5), find the counter that moved, read the config that governs it, and only then touch a parameter — on a trial, with a two-sided verification plan and a one-line rollback. Features whose golden state is Deactivated are arguments to understand, not defaults to flip.