Wireless Protocols

CSMA/CD

Q: Why does CSMA/CD enforce a minimum 64-byte Ethernet frame?

The 64-byte minimum frame exists so that a transmitting station is still sending when the most distant collision can possibly reach it. The slot time of classic Ethernet is 512 bit times, which equals 51.2 microseconds at 10 Mbps and corresponds to 64 bytes. A station must occupy the medium for at least one slot time so that a collision at the far end of the collision domain propagates back to the sender before it finishes the frame; otherwise the sender would assume success and never retransmit. Because 512 bit times bounds the round-trip propagation, it also caps the maximum collision domain diameter (about 2,500 meters with repeaters for 10BASE5). Frames shorter than 64 bytes are padded so the rule always holds.

Q: How does binary exponential backoff choose the retransmission delay in CSMA/CD?

After the n-th consecutive collision, a station picks a random integer r uniformly from the range 0 to (2 raised to k) minus 1, where k = min(n, 10), then waits r slot times of 512 bit times each before retransmitting. The first collision draws from 0 or 1 slot, the second from 0 to 3, the third from 0 to 7, and so on, doubling the contention window up to 1023 slots at the tenth attempt. The window stops growing after attempt 10 but the station keeps trying until attempt 16, after which it reports an excessive-collision error and discards the frame. This randomized doubling statistically spreads contending stations apart so the medium settles under load.

/C-S-M-A C-D/ (Carrier Sense Multiple Access with Collision Detection)

Short for Carrier Sense Multiple Access with Collision Detection, the media access control discipline that let early shared-medium Ethernet stations cooperate on one cable. Each station performs carrier sensing and defers while the medium is busy; if two stations begin almost simultaneously and their signals overlap, both detect the collision by comparing transmitted and received energy, send a 32-bit jam burst to ensure every node sees the event, then reschedule using binary exponential backoff. Standardized in IEEE 802.3, CSMA/CD imposed the 512 bit-time slot and the 64-byte minimum frame that still define the Ethernet frame format, even though switched full-duplex links have made collisions, and the algorithm itself, effectively obsolete.

Category: Wireless Protocols

Slot Time: 512 bit times (51.2 μs @ 10 Mbps)

Standard: IEEE 802.3 (half-duplex)

How Collision Detection Governs Shared Ethernet

CSMA/CD answers a basic question for any bus or hub-based network: how do many stations share a single conductor without a central scheduler? The carrier-sense part requires each station to listen first and transmit only when the medium is idle, which avoids most contention. The collision-detection part handles the residual case where two stations both sense an idle medium within the propagation window and start at nearly the same instant. On 10BASE-T and 10BASE5, a transmitter continuously compares the signal it drives onto the wire against the signal it reads back; a mismatch in voltage or a doubling of energy indicates that another station is talking over it.

When a collision is detected, the station does not simply stop. It transmits a 32-bit jam sequence to guarantee that the collision lasts long enough for all stations, including the most distant one, to recognize it. Both colliding stations then enter the backoff procedure, drawing a random number of slot times to wait before retrying. This randomization is what prevents the two stations from colliding again on the very next attempt, and the exponential growth of the contention window keeps a heavily loaded segment stable rather than collapsing into perpetual collisions.

The entire scheme is timing-bound. A station must keep transmitting for at least one round-trip propagation time across the collision domain so that a late collision can still reach it before it considers the frame delivered. That round-trip bound, fixed at 512 bit times, simultaneously sets the slot time, the 64-byte minimum frame, and the maximum cable span. Exceed the span and you create the late-collision and frame-fragment errors that plagued over-extended legacy segments.

Slot Time, Backoff, and Efficiency

Slot time (round-trip bound):
T_slot = 512 bit times = 512 / R (51.2 μs at R = 10 Mbps)

Binary exponential backoff delay:
T_wait = r × T_slot, r ∈ [0, 2^k − 1], k = min(n, 10)

Maximum collision-domain diameter (propagation ceiling):
D_max ≈ (T_slot × v_prop) / 2 ≈ 5,100 m (cable propagation alone; ~2,500 m for real 10BASE5 once repeater and transceiver delays consume the slot)

Channel efficiency under load (1-persistent):
η ≈ 1 / (1 + a), a = T_prop / T_frame

Where R = data rate, n = collision count for this frame (giving up after 16), v_prop ≈ 2×10⁸ m/s in copper, a = ratio of propagation time to frame transmission time. Small a (long frames, short cable) yields high efficiency.

CSMA/CD Versus Other Access Methods

Access Method	Medium	Collision Handling	Coordination	Typical Use
CSMA/CD	Shared half-duplex wire	Detect after start, jam + backoff	Distributed, no master	Classic 10/100 Ethernet hubs
CSMA/CA	Shared RF (cannot detect own collisions)	Avoid via RTS/CTS + random backoff	Distributed, NAV-based	Wi-Fi (IEEE 802.11)
Pure ALOHA	Shared radio channel	None; retransmit on no ACK	None	Early packet radio (~18% max)
TDMA	Shared channel, time slots	None; slots prevent overlap	Central scheduler	GSM, satellite uplinks
Switched full-duplex	Dedicated point-to-point	Collisions impossible	None needed	Modern Gigabit Ethernet

Common Questions

Frequently Asked Questions

Why does CSMA/CD enforce a minimum 64-byte Ethernet frame?

A sender must still be transmitting when the worst-case collision can reach it. The 512 bit-time slot (51.2 μs at 10 Mbps, equal to 64 bytes) bounds the round-trip across the collision domain, so any collision propagates back before the frame ends. This same bound caps the maximum cable span (about 2,500 m for 10BASE5). Frames shorter than 64 bytes are padded so the rule always holds.

How does binary exponential backoff choose the retransmission delay in CSMA/CD?

After the n-th collision a station waits r slot times, with r drawn uniformly from 0 to 2^k−1 where k = min(n, 10). The window doubles each collision: 0 to 1, then 0 to 3, then 0 to 7, up to 0 to 1023 at attempt 10. It stops growing there but retries continue until attempt 16, after which the frame is dropped with an excessive-collision error. The randomization spreads contending stations apart.

Why is CSMA/CD disabled on modern switched full-duplex links?

CSMA/CD only matters on a shared half-duplex medium where two stations can collide. A switch port on a dedicated full-duplex pair or fiber gives each station separate transmit and receive paths, so collisions are physically impossible. IEEE 802.3 explicitly disables the CSMA/CD MAC in full-duplex mode, which is why Gigabit and 10 Gigabit Ethernet run collision-free while still keeping the 64-byte frame format CSMA/CD defined.

Related Terms

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