How much of a 100 Mb/s port do two 9-2LE merging units use, why a protection IED receives both streams with VLAN filtering, and what a GOOSE avalanche adds. Numbers from the SCL NetStudio sample substation.
| Process bus | Switch PB1, 100 Mb/s device ports, 1 Gb/s uplink |
| SV streams | MU_Q01 and MU_T1, 9-2LE: 80 samples per cycle, 1 ASDU per frame, 8 channels, 50 Hz; frames of 131 and 130 bytes, VLAN 11 |
| Subscribers | P132_Q01 subscribes to MU_Q01 only; P87T_T1 to MU_T1 only |
| GOOSE | Trip GOOSE of 184–202 bytes, MinTime 4 ms, MaxTime 1000 ms |
| Multicast filtering | By VLAN (every port in VLAN 11 receives every SV stream) |
Open SCL NetStudio ▸ Bandwidth with the sample project loaded.
Each frame occupies its own bytes plus 8 bytes of preamble and 12 bytes of inter-frame gap on the wire.
With filtering by VLAN, the switch sends every frame of VLAN 11 to every port that is a member of VLAN 11. P132_Q01 therefore receives MU_T1's stream as well as its own.
With per-stream filtering (static multicast entries or MMRP) the port would carry only MU_Q01: 4.8 %. On a larger process bus with ten merging units in one VLAN, VLAN-only filtering would put about 48 Mb/s on every IED port.
| Check | Requirement | Actual | Status |
|---|---|---|---|
| SV stream load | ≈ 5 Mb/s for 9-2LE at 50 Hz | 4.83 / 4.80 Mb/s | ✓ PASS |
| Busiest device port (burst) | < 50 % steady, < 80 % burst | 9.6 % | ✓ PASS |
| Uplinks | < 50 % | < 0.1 % of 1 Gb/s | ✓ PASS |
Key insight: On the process bus, size ports for every SV stream they can receive, not just the ones the IED subscribes to — the multicast filtering method decides which that is.
Every input in this example is editable in the live calculator — no signup needed.
Open SCL NetStudio (IEC 61850 network design) calculator →Every Ethernet frame is preceded by an 8-byte preamble and start delimiter and followed by a 12-byte inter-frame gap; the wire is busy for that time too.
Metering streams at 256 samples per cycle usually pack 8 ASDUs per frame: 1 600 frames/s at 50 Hz but much larger frames — enter the stream with its smpRate and nofASDU and the tool recalculates.