Carrier Sense Multiple Access with Collision Detection (CSMA/CD)

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1. Principle of CSMA (Carrier Sense Multiple Access)

ALOHA protocols suffer low throughput because stations transmit blindly without checking the channel ("talk before listening").

CSMA introduces the rule: "Listen Before You Talk" (LBT):

• A station with a frame to transmit first senses the physical medium (carrier).
• If the medium is busy, the station defers transmission.
• If the medium is idle, the station transmits.

CSMA Persistence Strategies:

1. 1-Persistent CSMA:
   • If idle: Transmit immediately (with probability 1).
   • If busy: Continue listening continuously until the channel becomes idle, then transmit immediately.
   • Issue: If two stations are waiting for a busy channel to clear, both will transmit at the exact instant it clears, causing a guaranteed collision!
2. Non-Persistent CSMA:
   • If idle: Transmit immediately.
   • If busy: Wait a random period of time before sensing the channel again.
   • Advantage: Reduces collisions; Drawback: Channel may sit idle while stations are in backoff, increasing latency.
3. p-Persistent CSMA (Used in Slotted Channels):
   • If idle: Transmit with probability $p$; defer to the next slot with probability $(1 - p)$.

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2. Collision Detection: Adding "CD" to CSMA

Even with carrier sensing, collisions still occur due to Propagation Delay ($T_p$):

• Station A senses the channel idle at time $t_0$ and transmits.
• Station B, located at the far end of the cable, senses the channel at time $t_0 + \Delta t$ before Station A's signal has had time to propagate to B.
• Station B also senses the line idle and transmits $\rightarrow$ Collision occurs in transit!

In standard CSMA, stations continue transmitting the rest of the frame even after a collision, wasting channel time.
CSMA/CD adds Collision Detection: "Listen While Talking":

• The transmitting station continuously monitors the electrical voltage on the cable while transmitting.
• If the observed voltage exceeds a threshold ($V > V_{\text{normal}}$), a collision is detected.
• The station aborts transmission immediately, saving valuable channel bandwidth!

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3. Mathematical Condition for Collision Detection

A transmitting station must be able to detect a collision before it finishes transmitting its frame.

In the worst-case scenario, the collision occurs just before the signal reaches the furthest station (taking time $T_p$). The collision signal takes another $T_p$ to travel back to the original sender:

For the transmitter to still be transmitting when the collision signal returns:

*(This is the fundamental reason why classic $10\text{ Mbps}$ Ethernet over $2.5\text{ km}$ has a strict minimum frame size of 64 bytes (512 bits)!)*

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4. The Jam Signal and Exponential Backoff

When a station detects a collision:

1. Transmits a Jam Signal: A 32-to-48 bit pattern (e.g., alternating 1010...) broadcast onto the wire to ensure all other stations detect the collision and abort.
2. Invokes Truncated Binary Exponential Backoff (BEB):
   • Let $k$ be the collision attempt count for this frame ($1 \leq k \leq 16$).
   • The station chooses a random integer $R$ uniformly from the range:

• The station waits a backoff time:

• If after 16 consecutive collision attempts ($k = 16$) the frame still fails, the station gives up and reports an unrecoverable transmission error to the upper layer.

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5. Flowchart of CSMA/CD Operation

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