SIP Trunk vs PRI
PRI gave you fixed TDM channels. SIP trunks reuse Erlang B for concurrency but add IP media bandwidth, SBC limits, and codec negotiation.
Channel count for both PRI and SIP is still an Erlang B problem from busy-hour traffic and Grade of Service. SIP additionally requires per-call IP bandwidth (G.711/G.729/Opus), SBC session capacity, and QoS class sizing that PRI timeslots never exposed.
SIP Trunk Sizing Calculator
Converts BHCA/CPS and hold time into concurrent SIP channels plus codec-aware IP bandwidth for ITSP and SBC planning.
Best for
- ▸ITSP / enterprise SIP interconnect orders
- ▸SBC concurrent session BOM
- ▸WAN voice QoS class sizing with codecs
E1/T1 Timeslot Planner
Maps PCM trunks into signaling and bearer timeslots for legacy TDM/PRI deployments still in interconnect or migration paths.
Best for
- ▸Remaining PRI / SS7 TDM spans
- ▸Hybrid migration where SIP replaces subsets of timeslots
- ▸Clear channel vs robbed-bit planning
How to choose
If you are ordering IP voice capacity, size channels with the SIP trunk calculator (Erlang B + codec Mbps). If you still operate or migrate PRI/E1/T1 plant, use the timeslot planner for physical channel maps, then re-run SIP sizing for the cutover target. Do not assume 23/30 PRI channels equal the same SIP order without re-checking busy-hour Erlangs and codec bandwidth.
FAQ
Does one PRI equal one SIP trunk?
No. A PRI is a physical span with a fixed channel count (e.g. 23B+D or 30B+D). A SIP trunk is a logical interconnect whose channel cap is contractual. Always re-dimension from traffic, not from “replace 1:1.”
Why do SIP quotes mention concurrent calls and Mbps?
Concurrent calls are the Erlang capacity. Mbps is media (and sometimes signaling) bandwidth after codec and IP headers — irrelevant on TDM PRI but mandatory on IP WANs.
Which calculator should I open first?
Start with Erlang B or the SIP trunk sizer for concurrency, then VoIP bandwidth for QoS. Use E1/T1 planner only when physical TDM inventory still matters.