Collision Domain Calculator

Count collision domains in a mixed switch and hub topology, with CSMA/CD slot time and cable diameter limits.

Inputs

Switch portsports

Hubs or half-duplex segmentssegments

Stations per shared segmentstations

Shared segment speed

Collision Domains

Stations Sharing Bandwidth

CSMA/CD Slot Time

51.200µs

Propagation Budget

4,992m

Bandwidth per Shared Station

1.250Mbps

Assessment

Contended — 16 station(s) share 2 half-duplex segment(s) at 1.3 Mbps each

Step by step

Values used
Switch ports = 24 ports; Hubs or half-duplex segments = 2 segments; Stations per shared segment = 8 stations; Shared segment speed = 10 Mbps
Collision Domain
Each full-duplex switch port is its own collision domain; each hub or half-duplex segment is one shared domain whose bandwidth divides among its stations.
CSMA/CD timing
slot time = 512 ÷ line rate; cable budget = (slot time ÷ 2) × 0.65c.
Collision Domains
= 24
Stations Sharing Bandwidth
= 16
CSMA/CD Slot Time
= 51.200 µs
Propagation Budget
= 4,992 m
Bandwidth per Shared Station
= 1.250 Mbps
Assessment
= Contended — 16 station(s) share 2 half-duplex segment(s) at 1.3 Mbps each

How it works

CSMA/CD requires a sender to still be transmitting when a collision returns, which is why the minimum frame is 64 bytes and the slot time 512 bit times. That fixes a round-trip propagation budget and therefore a maximum network diameter, before repeater and transceiver latency is subtracted. Half-duplex Ethernet is rare but not gone — a duplex mismatch turns a switch port back into a collision domain, and the resulting late collisions and FCS errors look like a failing cable rather than a configuration fault.

Formulas

Collision Domain

Each full-duplex switch port is its own collision domain; each hub or half-duplex segment is one shared domain whose bandwidth divides among its stations.

slot time: 512 bit times — the 64-byte minimum frame, which is what makes collisions detectable
propagation budget: Half the slot time × 1.95 × 10^8 m/s signal velocity in copper
512 bit times: 51.2 µs at 10 Mbps, 5.12 µs at 100 Mbps

CSMA/CD timing

slot time = 512 ÷ line rate; cable budget = (slot time ÷ 2) × 0.65c.

Frequently Asked Questions

How is Collision Domain calculated?

Each full-duplex switch port is its own collision domain; each hub or half-duplex segment is one shared domain whose bandwidth divides among its stations. CSMA/CD requires a sender to still be transmitting when a collision returns, which is why the minimum frame is 64 bytes and the slot time 512 bit times. That fixes a round-trip propagation budget and therefore a maximum network diameter, before repeater and transceiver latency is subtracted.

Why does Collision Domain matter?

Half-duplex Ethernet is rare but not gone — a duplex mismatch turns a switch port back into a collision domain, and the resulting late collisions and FCS errors look like a failing cable rather than a configuration fault.

What values do I need to enter?

This calculator takes 4 inputs: Switch ports, Hubs or half-duplex segments, Stations per shared segment, Shared segment speed. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.

Why do I still see collision counters on a modern switch?

A non-zero late-collision counter on a gigabit port almost always means a duplex mismatch: one side auto-negotiated full duplex, the other is hard-coded to half. Fix the pair, do not replace the cable.

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