Interference Models¶

Interference models reduce the effective bandwidth of a link when multiple nearby links are simultaneously active. This inter-link contention is applied on top of per-link fair sharing (where N flows on the same link each get bandwidth / N).

The combined formula for a single flow on a link is:

effective_bw = (base_bandwidth * interference_factor) / num_flows_on_link

The interference model is selected via the interference key in the scenario YAML or the --interference CLI flag.

None (`none`)¶

No inter-link interference. The interference factor is always 1.0.

Links operate at their full declared bandwidth regardless of concurrent transfers on other links.
Appropriate for wired networks or scenarios where spectrum contention is not relevant.

config:
  interference: none

Proximity (`proximity`)¶

A simple distance-based interference model. Links whose geometric midpoints fall within a configurable radius are considered to be contending.

If k active links (including the link being evaluated) have midpoints within the radius, the interference factor is 1/k.
Configure the radius with the interference_radius YAML key or the --interference-radius CLI flag (default: 15.0 meters).
Dynamic: the factor is recalculated whenever a transfer starts or completes.
Provides a quick approximation of spectrum contention without requiring WiFi-specific RF parameters.

config:
  interference: proximity
  interference_radius: 10.0

CSMA/CA Clique (`csma_clique`)¶

An 802.11-aware static model that builds a conflict graph based on carrier sensing rules and divides bandwidth by the worst-case clique size.

Uses RF propagation (path loss, carrier sensing threshold) to determine which links can sense each other's transmissions.
Links that can sense each other form edges in a conflict graph. The Bron-Kerbosch algorithm (exact for networks with 50 or fewer links; greedy approximation otherwise) finds the maximum clique containing each link.
At setup time, each link's bandwidth is set to:
```
link.bandwidth = PHY_rate / max_clique_size
```
After setup, the interference factor is always 1.0 -- contention is baked into the bandwidth value and does not change during simulation.
Conservative: represents the worst-case bound where all links in the largest clique transmit simultaneously.
Requires RF parameters (tx_power_dBm, freq_ghz, path_loss_exponent, cca_threshold_dBm, etc.) via the rf: YAML section.

config:
  interference: csma_clique
  rf:
    tx_power_dBm: 20
    freq_ghz: 5.0
    path_loss_exponent: 3.0

CSMA/CA Bianchi (`csma_bianchi`)¶

The most realistic WiFi interference model. It dynamically separates two distinct interference mechanisms:

1. Contention Domain (Conflict Graph Neighbors)¶

Links that appear as neighbors in the conflict graph operate under CSMA/CA: they cannot transmit simultaneously. Instead, they share airtime according to Bianchi's saturation throughput model.

Each of n contending links (including the link itself) gets a fraction eta(n) / n of the channel, where eta(n) is the Bianchi MAC efficiency for n stations.

Because CSMA prevents simultaneous transmission, contending links do not cause SINR degradation at each other's receivers.

2. Hidden Terminals (Non-Conflict-Graph Neighbors)¶

Active links that are not in the conflict graph of the evaluated link may transmit simultaneously, causing RF interference at the receiver. Their combined interference power degrades the SINR, which may force a lower MCS rate.

The SINR-based rate (R_SINR) is computed using only hidden terminal interference. The base rate (R_base) is the SNR-only PHY rate.

Combined Factor¶

factor = (R_SINR / R_base) * (eta(n) / n)

Where:

n = 1 + |active contending neighbors|
eta(n) = Bianchi MAC efficiency for n stations
R_SINR = MCS rate under SINR (hidden terminal degradation)
R_base = MCS rate under SNR only (no interference)

The factor is recalculated whenever a transfer starts or completes, making this a fully dynamic model.

config:
  interference: csma_bianchi
  rf:
    tx_power_dBm: 20
    freq_ghz: 5.0
    path_loss_exponent: 3.0
    noise_floor_dBm: -95
    cca_threshold_dBm: -82
    channel_width_mhz: 20
    wifi_standard: "ax"

Key insight

CSMA prevents simultaneous transmission within the contention domain, so contending links affect throughput via time-sharing, not via SINR degradation. Only hidden terminals -- links outside the conflict graph -- contribute to SINR reduction. This separation is what distinguishes csma_bianchi from simpler models.

Comparison¶

Feature	None	Proximity	CSMA Clique	CSMA Bianchi
Interference type	None	Distance-based	Carrier sensing	SINR + MAC
Dynamic	N/A	Yes	No (static)	Yes
WiFi-aware	No	No	Yes	Yes
RF parameters required	No	No	Yes	Yes
Accuracy	N/A	Low	Medium	High
Use case	Wired networks	Quick approximation	WiFi without SINR	Realistic WiFi

Worked Example: Proximity Interference¶

This example uses the interference_test.yaml scenario.

Topology¶

Four nodes with two parallel links, 5 units apart:

graph LR
    n0["n0 (0,0)"] -- "l01: BW=100 MB/s" --> n1["n1 (5,0)"]
    n2["n2 (0,5)"] -- "l23: BW=100 MB/s" --> n3["n3 (5,5)"]

Link l01 midpoint: (2.5, 0)
Link l23 midpoint: (2.5, 5)
Distance between midpoints: 5.0 units

Two DAG edges create simultaneous transfers: T0 -> T1 on l01 (100 MB) and T2 -> T3 on l23 (100 MB). Each task has compute_cost: 10 on nodes with compute_capacity: 1000, so task execution takes 0.01 s.

Without Interference (`none`)¶

Transfer	Bandwidth	Time	Total
T0 -> T1 on l01	100 MB/s	100 / 100 = 1.0 s	0.01 + 1.0 + 0.01 = 1.02 s
T2 -> T3 on l23	100 MB/s	100 / 100 = 1.0 s	0.01 + 1.0 + 0.01 = 1.02 s

Makespan: 1.02 s

With Proximity Interference (`radius=10`)¶

Since the midpoint distance (5.0) is less than the radius (10.0), both links contend. With k=2 active contending links, the interference factor is 1/k = 0.5.

Transfer	Effective BW	Time	Total
T0 -> T1 on l01	100 * 0.5 = 50 MB/s	100 / 50 = 2.0 s	0.01 + 2.0 + 0.01 = 2.02 s
T2 -> T3 on l23	100 * 0.5 = 50 MB/s	100 / 50 = 2.0 s	0.01 + 2.0 + 0.01 = 2.02 s

Makespan: 2.02 s -- transfers take 2x longer due to interference.

YAML Configuration¶

Proximity Model¶

config:
  interference: proximity
  interference_radius: 15.0   # meters (default)

WiFi Models (Clique or Bianchi)¶

config:
  interference: csma_bianchi   # or csma_clique
  rf:
    tx_power_dBm: 20
    freq_ghz: 5.0
    path_loss_exponent: 3.0
    noise_floor_dBm: -95
    cca_threshold_dBm: -82
    channel_width_mhz: 20
    wifi_standard: "ax"
    shadow_fading_sigma: 0.0
    rts_cts: false

CLI Overrides¶

ncsim --scenario my_scenario.yaml --output results/ \
      --interference proximity --interference-radius 10.0

ncsim --scenario my_scenario.yaml --output results/ \
      --interference csma_bianchi --tx-power 20 --freq 5.0 \
      --path-loss-exponent 3.0 --wifi-standard ax --rts-cts