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Interference Verification

The run_interference_verification.py script validates the csma_bianchi WiFi interference model by comparing simulation results against analytical predictions computed directly from the WiFi RF functions. It runs nine experiments covering single-link PHY rates, parallel-link contention, hidden terminal SINR degradation, multi-phase rate transitions, bandwidth sharing, and combined effects.

Running the Script

python run_interference_verification.py

Output directory: /tmp/ncsim_interference_verification (configurable via the OUTDIR variable in the script).

Default RF Configuration

All experiments use the same default RFConfig:

Parameter Value
TX power 20 dBm
Frequency 5 GHz
Path loss exponent 3.0
Noise floor -95 dBm
CCA threshold -82 dBm
Channel width 20 MHz
WiFi standard 802.11ax
Shadow fading 0 (disabled)
RTS/CTS disabled

Data size per transfer: 10 MB. Tolerance for all checks: 1%.

Varies the distance between a single transmitter-receiver pair (n0 to n1) with no other links present.

  • Purpose: Verify that the PHY rate matches the analytical SNR-to-MCS prediction when there is zero interference.
  • Distances tested: 1m, 3m, 5m, 8m, 12m, 16m, 20m, 25m, 30m, 36m, 42m, 50m, 58m, 66m, 75m, 85m, 95m, 105m, 120m, 140m.
  • Analytical prediction: rate_mbps_to_MBps(snr_to_rate_mbps(snr_dB(received_power_dBm(tx, d, rf), noise)))
  • Verification: Simulated data rate (computed as data_size / duration) matches the predicted rate within 1%.

At distances beyond the minimum MCS threshold (SNR < 5 dB), the PHY rate drops to zero and the simulator uses a 0.001 MB/s fallback.

Two parallel 30m horizontal links separated by a variable vertical distance.

Link A: n0(0,0) -----> n1(30,0)

                 separation (varies)

Link B: n2(0,sep) ---> n3(30,sep)
  • Purpose: Test both contention (within carrier sensing range) and hidden terminal (outside carrier sensing range) effects.
  • Separations tested: 5m, 10m, 15m, 20m, 30m, 40m, 50m, 60m, 65m, 70m, 75m, 80m, 100m, 140m, 200m.

Key Behaviors

Regime Condition Behavior
Contention Separation <= carrier sensing range Both links in conflict graph. Share airtime via Bianchi MAC: each gets base_rate * eta(2) / 2.
Hidden terminal Separation > carrier sensing range Links outside conflict graph. Transmit simultaneously, causing SINR degradation at receivers.
No interference Separation very large Interferer power negligible. Both links approach solo PHY rate.
  • Verification: Both links' effective rates match analytical predictions (Bianchi contention factor for in-conflict, SINR-based rate for hidden terminal) within 1%.

Symmetry

By construction, both links in this experiment always see identical interference geometry. The script verifies that both links produce the same rate.

Experiment 3: Two Transmitters to Same Receiver

Two transmitting nodes send to a shared receiver at the origin, with transmitters placed at 90 degrees apart at equal distance.

           n1(d, 0)
            \
             \  link l10
              v
               n0(0, 0)  <-- shared receiver
              ^
             /  link l20
            /
           n2(0, d)
  • Purpose: Test SINR calculation when multiple transmitters target the same receiver.
  • Distances tested: 5m to 130m.
  • Key insight: When both links are in the conflict graph, they share airtime via Bianchi. When they are hidden terminals, both transmitters cause equal-power interference at the shared receiver, resulting in very low SINR (the model clamps the interference factor to MIN_FACTOR).
  • Verification: Both links' rates match predictions within 1%.

Three parallel 30m links stacked vertically at equal spacing.

Link A: n0(0, 0)     -----> n1(30, 0)
Link B: n2(0, sep)   -----> n3(30, sep)
Link C: n4(0, 2*sep) -----> n5(30, 2*sep)
  • Purpose: Validate multi-interferer SINR, combined Bianchi + SINR factors, and symmetry between outer links A and C.
  • Separations tested: 10m, 20m, 35m, 40m, 50m, 60m, 70m, 75m, 80m, 100m, 150m.

Three Regimes

Regime Condition Description
all_conf All three pairs in conflict graph 3-way Bianchi contention. All links get base_rate * eta(3) / 3.
mixed AB and BC in conflict, but AC not Center link B contends with both; outer links A and C also see each other as hidden terminals. Multi-phase completion.
all_hid No conflict graph edges All interference is SINR-based. Multiple interferers with different distances. Multi-phase completion as links finish.
  • Verification: Predicted multi-phase effective rates match simulation within 1%. Outer links A and C always have symmetric (equal) rates.

Experiment 5: Staggered Transfer Start

Two parallel 30m links at 100m separation (hidden terminals), with Transfer A starting later than Transfer B due to a high compute cost on T0.

  • Purpose: Test dynamic recalculation of SINR when transfers start and complete at different times.
  • Key phases:
    1. Phase 1: B transmits solo at base rate for delta seconds.
    2. Phase 2: Both A and B active; both degrade to SINR rate.
    3. Phase 3: After B completes, A returns to solo base rate.
  • Delay costs tested: 10000, 30000, 50000, 80000 (producing different stagger durations).
  • Verification: Both transfer durations match the multi-phase analytical prediction within 1%. A and B always have equal total durations (by symmetry of phases).

Multiple flows with different data sizes share a single 30m link (l01).

  • Purpose: Verify that concurrent flows on the same link fairly share bandwidth, and that when faster flows complete, remaining flows see increased bandwidth (multi-phase fair sharing).
  • Test cases:
    • 3-flow: data sizes 5, 10, 15 MB
    • 3-flow-eq: data sizes 5, 5, 15 MB (two finish simultaneously)
    • 4-flow: data sizes 3, 6, 9, 12 MB
  • Verification: Each flow's duration matches the multi-phase prediction from predict_fair_share_durations() within 1%.

Experiment 7: N-Way Bianchi Scaling

N parallel 30m links at 5m vertical separation (all within carrier sensing range, forming a complete conflict graph).

  • Purpose: Verify that Bianchi efficiency scales correctly from N=2 through N=8 contending stations.
  • Analytical prediction: Each link gets base_rate * eta(N) / N.
  • Verification: All N links have the same rate, matching the Bianchi prediction within 1%.
N eta(N) Per-link share: eta(N)/N
2 ~0.94 ~0.47
3 ~0.88 ~0.29
4 ~0.82 ~0.21
5 ~0.77 ~0.15
8 ~0.65 ~0.08

Five parallel 30m links at 100m vertical separation (all hidden terminals to each other) with data sizes 2, 4, 6, 8, and 10 MB.

  • Purpose: Test data_remaining tracking through 4+ recalculation phases, SINR recomputation with a shrinking active link set, and correct completion ordering with asymmetric geometry (links at different vertical positions see different aggregate interference).
  • Verification: Each link's total duration matches the cascading multi-phase prediction from predict_hidden_cascade_durations() within 1%.

Experiment 9: Combined Bandwidth Sharing + Hidden Terminal Interference

Two flows share link l01 while a third flow on link l23 (100m away) acts as a hidden terminal.

  • Purpose: Test the interaction between per-link fair sharing and inter-link SINR interference, including multi-phase transitions as flows complete.
  • Test cases:
    • case1: l01 flows 5/10 MB, l23 flow 8 MB
    • case2: l01 flows 5/10 MB, l23 flow 3 MB (l23 finishes earliest)
    • case3: l01 flows 3/15 MB, l23 flow 20 MB (l01 flow finishes first)
  • Verification: All three flow durations match the multi-phase analytical prediction within 1%.

Tolerance

All experiments use a 1% tolerance for matching simulation output to analytical predictions. The comparison functions are:

  • rates_match(sim, pred) -- checks |sim - pred| / pred <= 0.01
  • durations_match(sim, pred) -- same relative tolerance on transfer durations

Interpreting Results

Each experiment prints a detailed comparison table with predicted vs. actual values and a per-row OK / FAIL indicator.

Result Meaning
PASS All rows in the experiment match within tolerance. The interference model implementation is correct for this scenario class.
FAIL At least one row does not match. Indicates a bug in the interference model, SINR calculation, Bianchi efficiency, or multi-phase rate tracking.

The final summary at the end of the script reports the pass/fail status for all nine experiments:

  Summary
  ============================
  Experiment 1 (Link Length vs Rate):            PASS
  Experiment 2 (Parallel Link Separation):       PASS
  Experiment 3 (Two TX to Same RX):              PASS
  Experiment 4 (Three Parallel Links):           PASS
  Experiment 5 (Staggered Transfer Start):       PASS
  Experiment 6 (Per-Link Bandwidth Sharing):     PASS
  Experiment 7 (N-Way Bianchi Scaling):          PASS
  Experiment 8 (Five-Link Hidden Cascade):       PASS
  Experiment 9 (Combined Sharing+Interference):  PASS

  Overall: ALL PASS

If any experiment fails

A failure indicates a regression in the interference model. Check the printed comparison table to identify which specific distance or separation value failed, then investigate the corresponding analytical helper and the csma_bianchi implementation in ncsim/models/interference.py and ncsim/models/wifi.py.

Key Imports

The script imports the following from ncsim.models.wifi for analytical predictions:

from ncsim.models.wifi import (
    RFConfig,
    received_power_dBm,
    snr_dB,
    sinr_dB,
    snr_to_rate_mbps,
    rate_mbps_to_MBps,
    carrier_sensing_range,
    bianchi_efficiency,
    path_loss_dB,
    MCS_TABLE_AX,
)
Function Purpose
RFConfig Dataclass holding all RF parameters (TX power, frequency, path loss exponent, etc.)
received_power_dBm Compute received power at a given distance: P_tx - PL(d)
snr_dB Signal-to-noise ratio: P_rx - noise_floor
sinr_dB Signal-to-interference-plus-noise ratio (linear domain aggregation)
snr_to_rate_mbps MCS table lookup: highest rate whose minimum SNR requirement is met
rate_mbps_to_MBps Unit conversion from Mbps to MB/s (ncsim's internal unit)
carrier_sensing_range Maximum distance at which CCA detects a transmission
bianchi_efficiency Bianchi MAC saturation throughput efficiency for N contending stations
path_loss_dB Log-distance path loss model
MCS_TABLE_AX 802.11ax MCS rate table (min_snr, rate_mbps) pairs