Primary Rate Interface (23B+D / 30B+D) for Legacy PBXs, Voice Trunks & Clean Migrations
PRI (Primary Rate Interface) is the classic digital voice trunk between your PBX and the PSTN—delivered over T1 (DS1, 23B+D) in North America or E1 (30B+D) internationally.
It’s still the right tool when you need deterministic, SLA-backed voice for legacy systems, elevators, life-safety lines, or when you’re migrating to SIP in controlled phases.
Related transport: T1 → /t1 • Lit Fiber (EPL/EVPL/E-LAN) → /lit-fiber
Migration target: SIP Trunking → /sip-trunking • POTS Replacement → /pots
Catalog pages: Connectivity → /connectivity
🎯 Outcomes (Why PRI with SolveForce)
- SLA-backed voice capacity — 23 or 30 bearer channels with predictable signaling and timing.
- PBX compatibility — plug-and-play with legacy PBXs and key systems; supported signaling variants.
- E911/NG911 readiness — DID mapping, location records, and test-call validation.
- Clean migration path — PRI today, SIP gateway tomorrow, at your pace (keep numbers & routing).
- Evidence-first ops — baselines, alarms, and call metrics exported to SIEM/SOAR.
🧭 Scope (What We Deliver)
- Circuit types — T1/DS1 (23B+D) North America; E1 (30B+D) RoW. Fractional options where needed.
- Handoffs — RJ48C at smartjack/CSU-DSU; PBX side via PRI interface card; optional PRI↔SIP media gateway.
- Numbering — DID blocks, CNAM, LNP (port your numbers), hunt groups.
- 911 — E911/NG911 location provisioning and test-call procedures.
- Redundancy — dual PRIs (same or diverse carriers), or PRI + SIP failover.
🧱 Technical Notes (Spelled out)
- Signaling/Profiles — NI-2 (common in NA), 5ESS, DMS-100, Euro-ISDN (E1), Q.921/Q.931.
- Framing/line coding (T1) — ESF/B8ZS (typical); timing via network clock.
- Channelization — 23 (T1) or 30 (E1) voice/data B-channels + 1 D-channel for signaling.
- Clocking — PBX set to network (line) clock to avoid slips; verify with loopback/BERT during turn-up.
- Fax/T.38 — via PRI native or through PRI↔SIP gateway policies if you’re mixing SIP.
Need more than 23/30 channels? Use multiple PRIs or migrate to SIP Trunking for elastic capacity. → /sip-trunking
📐 SLO Guardrails (Typical PRI Targets)
| KPI | Target (Typical) | Notes |
|---|---|---|
| Bearer channels | 23 (T1) / 30 (E1) | Per circuit |
| Availability | 99.9–99.99% | With diversity |
| MTTR | ≤ 4–8 hours | Carrier SLA |
| Call setup time (PSTN edge) | ≤ 1–3 s | Signaling path dependent |
| Errored seconds (ES/SES) | Carrier thresholds enforced | Monitored via CSU/DSU |
| 911 provisioning accuracy | = 100% of DIDs | Test calls & address validation |
SLO breaches open tickets and trigger NOC escalation & evidence export to SIEM. → /circuit-monitoring • /noc • /siem-soar
🧰 Design Patterns (Pick your fit)
A) Classic PBX with Dual PRIs
- Two PRIs (or PRI + SIP) for failover; DID blocks split; clocking verified; 911 tested.
→ Use SIP gateway alongside for gradual migration. /sip-trunking
B) PRI ↔ SIP Gateway (Migration Bridge)
- Keep existing PBX; add PRI-to-SIP media gateway; move sites/users to SIP in waves; preserve DIDs.
- Policy: TLS/SRTP on the SIP side; PRI stays deterministic to PBX.
C) Elevators / Life-Safety / Analog Plant
- PRI with channelized FXS/FXO cards or gateway to support analog endpoints during POTS retirement.
→ See /pots
D) Rural Sites / Deterministic Voice
- T1 PRI where fiber/coax absent; pair with SD-WAN for data and out-of-band management. /sd-wan
🔒 Security, 911 & Compliance
- Toll-fraud controls — class-of-service, dialing restrictions, international policy, and call thresholds.
- SIP side hardening (if using a gateway) — TLS/SRTP, SBC in front of public SIP, topology hiding.
- E911/NG911 — DID→address mapping, test calls, ELIN/ERL policies for multi-building campuses.
- Logging & retention — CDRs, PRI alarms (LOS, LOF, AIS), error seconds to SIEM; SOAR can auto-open incidents.
→ /siem-soar
🔌 Interop & Handoffs
- Demarc — smartjack/CSU-DSU; RJ48C to your PBX or voice gateway.
- PBX settings — switch type (NI-2/5ESS/DMS-100/Euro-ISDN), framing/line coding, clock source, overlap vs en-bloc dialing.
- Media gateway (for PRI↔SIP) — DTMF mode, fax policy, codec allowlist (G.711/G.729), T.38 as required.
📊 Observability & NOC
- Trunk utilization (busy hour Erlangs), ASR/ACD, CDR, error seconds, Q.921/Q.931 events, alarms (LOS/AIS/RDI).
- Dashboards & monthly reports; carrier escalation tree held by NOC.
→ /circuit-monitoring • /noc
💵 Commercials (What drives cost)
- Circuit count (single/dual), loop length, local exchange footprint.
- DID blocks, CNAM, LNP port-in fees; 911/USF surcharges.
- Term 12/24/36 mo; NRC for install, MRC per PRI; gateway CapEx/Opex if bridging to SIP.
🧪 Turn-Up & Acceptance (What we test)
1) Provisioning — smartjack/CSU-DSU, framing/line coding, clocking, switch type.
2) Loopbacks/BERT — verify BER, slips, and timing stability.
3) Signaling — Q.931 call setup/teardown; inbound/outbound DID tests.
4) 911 — E911/NG911 test calls with PSAP confirmation and documentation.
5) Failover — dual PRI or PRI↔SIP cutover tests; record results as evidence.
Artifacts (screen shots, BERT results, call logs, PSAP confirmations) are stored and exported to SIEM for audits. → /siem-soar
✅ Pre-Engagement Checklist
- 🔧 PBX make/model, current trunk config, gateway availability.
- ☎️ Channels required (current & forecast), DID plan (main, DIDs, toll-free), hunt groups.
- 🆔 911 addresses per DID/site; multi-tenant/campus ELIN/ERL strategy.
- 🧭 Signaling profile (NI-2/5ESS/DMS-100/Euro-ISDN), framing/line coding, clocking (line).
- 🔄 Migration intent (stay PRI, dual, or move to SIP); SBC/SIP provider on deck.
- 📊 Monitoring destination (SIEM), reporting cadence, escalation contacts.
🔄 Where PRI Fits (Recursive View)
1) Grammar — a voice underlay in /connectivity (often on /t1).
2) Syntax — used by legacy PBXs and gateway bridges to modern SIP.
3) Semantics — /cybersecurity enforces toll-fraud, 911, and evidence logging.
4) Pragmatics — /solveforce-ai anticipates trunk saturation and recommends migration steps.
5) Foundation — definitions & mapping kept coherent via /primacy-of-language.
6) Map — indexed across /solveforce-codex & /knowledge-hub.
📞 Order PRI / Plan a SIP Migration the Right Way
Related pages:
/t1 • /sip-trunking • /pots • /lit-fiber • /connectivity • /circuit-monitoring • /noc • /cybersecurity • /knowledge-hub