IoT Channel Contention: ALOHA vs CSMA/CA

Shared Radio Channel

Device count 18

Offered load per device 0.30 pkt/s

Packet airtime 80 ms

Access Method

Channel State

Throughput

0.0 pkt/s

Collision rate

Channel busy

Offered load G

0.00

Sent

Collided

Every device shares one radio channel — there is no per-pair link. Blue = transmitting cleanly, red = transmitting inside a collision, amber = deferring (CSMA backoff). Two frames that overlap in time are both destroyed, exactly like a real shared-medium radio collision.

How it works

All devices here transmit on one shared channel — think LoRa in a single frequency band, or a crowded 802.15.4 channel. There is no routing, no mesh, no gateway hops: the only question is whether two frames ever overlap in time. If they do, both are unrecoverable (a collision), exactly as on a real shared radio medium.

offered load   G = N · λ · T   (Erlangs)
  N = device count, λ = attempts/s per device, T = packet airtime

pure ALOHA:     S = G · e^(−2G)      max S = 1/(2e) ≈ 18.4% at G = 0.5
slotted ALOHA:  S = G · e^(−G)       max S = 1/e   ≈ 36.8% at G = 1.0

Pure ALOHA — a device transmits the instant it has data, never checking the channel first. Its frame is vulnerable for a full 2T window (its own airtime plus any frame that started up to T seconds earlier), which is why throughput collapses as load rises even though the channel sits mostly idle.

CSMA/CA (carrier-sense, listen-before-talk) — a device first senses the channel; if it hears another transmission in progress it does not transmit, instead waiting a random backoff interval and re-sensing. This shrinks the vulnerable window down to roughly the sensing/propagation delay, which is why the same offered load produces far fewer collisions and much higher usable throughput — the mechanism behind Wi-Fi's CSMA/CA and Zigbee's CSMA-based MAC layer.

Device count / offered load / airtime — together set G, the dimensionless load on the channel.
Pure ALOHA vs CSMA/CA — switches whether devices sense the channel and back off before transmitting.
Push G past ~0.5 under ALOHA and watch throughput fall even as more devices try to talk — the classic ALOHA collapse curve; CSMA/CA keeps climbing far longer at the same load.

Idle Backoff (CSMA) Transmitting Collision

Channel idle…

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