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CSMA/CD Flow Diagram and Comparison with ALOHA

CSMA/CD Flow Diagram and Comparison with ALOHA

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Nov 3, 2026

Carrier Sense Multiple Access with Collision Detection, CSMA/CD is the classic Ethernet mechanism for shared-bus networks. A station transmits only when the medium appears idle, monitors for collisions during transmission, and upon collision sends a jam signal and backs off before retrying. Binary exponential backoff is what prevents repeated collisions.

Key behavioral elements you must include in your flow diagram:

  • Carrier sensing
  • Collision detection
  • Jam signal
  • Backoff and retransmission

Below is a standard decision-flow you can draw directly from.

Interpretation: after a collision, every transmitting station backs off for a random time, so the probability of a later “winner” increases as contention continues (but the contention window grows, reducing repeated collisions).

CSMA/CD & Collision Detection (overview)

Explaining the CSMA/CD flow diagram (what each node means)

  1. Frame ready to send → carrier sensing

    • The station checks the shared medium before transmitting. If the channel is busy, it waits and keeps sensing. This reduces collisions by not transmitting on top of an ongoing transmission. Carrier sense.

  2. Medium idle → transmit

    • The station begins sending the entire frame.

  3. During transmission → collision detection

    • Because stations share the medium, simultaneous transmissions can interfere. CSMA/CD assumes the network hardware can detect such interference before the station finishes sending the frame. Collision detection.

  4. Collision found → jam signal

    • The station sends a jam so that other transmitting stations can reliably detect the collision too, ensuring consistent “everyone knows it failed” behavior. Jam signal.

  5. Backoff with exponential growth

    • After jamming, the station waits a random number of time slots. The slot selection is from a window whose size typically doubles with each collision attempt. This is the “binary exponential backoff” part. Backoff window.

  6. Retry or drop

    • If the station has not exceeded the maximum number of attempts, it returns to carrier sensing. Otherwise, it drops the frame (failure).

How to draw it quickly on paper

Use two loops: (1) a carrier-sense loop for “medium busy,” and (2) a collision-retry loop for “collision detected → jam → backoff → sense again.” Then add the terminal “drop frame” when attempts exceed the limit.

Collision detection requires timing constraints

In real Ethernet, collision detection only works correctly when the propagation delay and frame timing satisfy the standard constraints (e.g., minimum frame time). If a station couldn’t detect collisions early enough, CSMA/CD would fail its core assumption.

Compare CSMA/CD with ALOHA

Both protocols attempt to share a medium among multiple stations, but they differ fundamentally in when they sense access and how time is structured.

ALOHA:

  • Uses random access without carrier sensing.
  • Pure ALOHA allows starts at any time.
  • Slotted ALOHA restricts starts to slot boundaries.

CSMA/CD:

  • Performs carrier sense before transmitting.
  • Detects collisions while sending (works for certain wired shared-media setups).
  • Uses exponential backoff to adapt to congestion.

Throughput comparison (classic analytical results)

Let GG be the average offered load (average number of transmission attempts per packet time).

  • Pure ALOHA throughput: Spure(G)=Ge−2GS_{\text{pure}}(G) = G e^{-2G}

  • Slotted ALOHA throughput: Sslotted(G)=Ge−GS_{\text{slotted}}(G) = G e^{-G}

At the optimum loads, the maximum throughputs are approximately:

  • Pure ALOHA: about 1/(2e)≈0.1841/(2e) \approx 0.184
  • Slotted ALOHA: about 1/e≈0.3681/e \approx 0.368

For CSMA/CD, throughput is typically better than ALOHA for wired CSMA/CD contexts because carrier sensing reduces collisions and collision detection prevents “wasting full transmissions” undetected. However, CSMA/CD performance depends strongly on propagation delay relative to transmission time, and it is not a direct closed-form Ge−kGG e^{-kG} result like ALOHA’s idealized model.

Idealized maximum throughput comparison (classic ALOHA results)

Pure and slotted ALOHA have well-known ideal maxima; CSMA/CD is generally higher under its assumptions, but depends on propagation vs transmission timing.

11) Sense channel
22) If idle: transmit
33) Detect collision while transmitting
44) If collision: jam + backoff (binary exponential)
55) Retry via sensing (until limit)

Collision handling timeline: CSMA/CD vs ALOHA

ALOHA start

t0

Station transmits immediately (pure) or at next slot (slotted)."

Collision may occur

t0..t1

Overlapping transmissions destroy packets; sender typically waits and retries later."

Sense → transmit

CSMA/CD start (wired shared medium)

Station senses idle, then transmits while monitoring for collision."

Detect → jam → backoff

During transmit

Upon collision, jam to signal others, backoff exponentially, then retry."

Quick conceptual Q&A

Knowledge Check

Question 1 of 4
Q1Single choice

In CSMA/CD, what does a station do immediately after detecting a collision?

Explore Related Topics

1

Delay Comparison: Pure ALOHA vs Slotted ALOHA at Low Load

2

Which Access Method Listens Before Transmitting?

3

Purpose of the Jam Signal in CSMA/CD

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