How Do CSMA/CD and Full-Duplex Manage Ethernet Framing?

Ethernet networks rely on structured data frames to deliver information reliably across local area networks. Depending on whether devices share a single communication medium or connect through dedicated switch ports, Ethernet manages these frames using either Carrier-Sense Multiple Access with Collision Detection or switched full-duplex links.

The Structure of an Ethernet Frame

An Ethernet frame packages raw data into a standardized format so network interfaces can recognize where a transmission begins, where it is going, and whether it arrived intact. A standard IEEE 802.3 frame begins with a 7-byte preamble and a 1-byte Start Frame Delimiter, which synchronize the receiving clock.

Following synchronization, the frame includes a 6-byte destination MAC address and a 6-byte source MAC address. A 2-byte EtherType or length field identifies the encapsulated protocol or payload size. The payload itself carries between 46 and 1500 bytes of data, followed by a 4-byte Frame Check Sequence that uses a cyclic redundancy check to detect transmission errors.

Half-Duplex Framing with CSMA/CD

In early Ethernet networks built with coaxial cables or passive hubs, all devices shared a single collision domain. Because only one device could transmit at a time without corrupting signals, Ethernet used Carrier-Sense Multiple Access with Collision Detection (CSMA/CD) to coordinate access.

Before sending a frame, a network interface listens to the medium to verify that no other device is transmitting. If the channel is idle, the interface begins sending its frame while continuing to monitor the wire. If two devices transmit simultaneously, their electrical signals interfere, causing a collision.

When a collision is detected, both transmitters immediately stop sending data and transmit a short jam signal so all other devices on the segment recognize the collision. Each colliding station then waits for a random backoff period determined by the truncated binary exponential backoff algorithm before attempting to retransmit. After \(c\) collisions, a station chooses a random integer \(k\) from the range \(0 \le k < 2^{\min(c, 10)}\) and waits for \(k\) slot times.

To ensure collisions are detected before a station finishes transmitting, CSMA/CD enforces a minimum frame size of 64 bytes. This guarantees that the round-trip propagation delay across the maximum cable length is shorter than the time required to send the smallest valid frame.

Switched Full-Duplex Framing

Modern Ethernet networks replace shared hubs with network switches, giving each device a dedicated point-to-point link to a switch port. On these links, separate transmit and receive wire pairs allow a device and a switch to send and receive frames simultaneously.

Because each link is isolated from other ports and uses independent paths for sending and receiving, collisions cannot occur. As a result, CSMA/CD is disabled in full-duplex mode, eliminating backoff delays and allowing both ends of the link to operate at full line rate.

Switches manage framing by reading the destination MAC address of each incoming frame and forwarding it only to the port associated with that address. If the switch receives frames faster than a port can transmit them, it stores the frames temporarily in internal memory buffers. When buffers approach capacity, full-duplex links can use IEEE 802.3x pause frames to ask the sender to temporarily halt transmission, preventing frame loss without relying on collision detection.