--- title: "Ethernet with CSMA and CSMA/CD" subject: "Telecommunications" chapter: 8 key: "semester_8/telecommunications:ethernet-csma-cd" source: https://www.bctengineer.com/q/semester_8/telecommunications/ethernet-csma-cd site: BCT Engineering --- # Ethernet with CSMA and CSMA/CD Telecommunications · Chapter 8: Data Communication - Asked in: 2069 Bhadra (old course), 2+10 marks - Video: [CSMA/CD and CSMA/CA Explained](https://www.youtube.com/watch?v=iKn0GzF5-IU) — PowerCert Animated Videos - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/telecommunications/ethernet-csma-cd - Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-8 ## Question What is an Ethernet? Keeping in view of CSMA and CSMA/CD explain its working principle. ## Short answer Ethernet is a LAN technology (IEEE 802.3) using CSMA/CD. Stations sense the carrier; if idle, they transmit while listening for collisions. On collision, they send a 32-bit jam signal and wait a random backoff time (K×51.2 µs) before retrying. ## Answer #### Ethernet **Ethernet** is the most widely used LAN technology, standardised as **IEEE 802.3**. It was developed at Xerox PARC (Metcalfe, 1973) and later specified by DEC, Intel and Xerox (DIX). All stations share one channel (a bus, or a hub/switch in star wiring) and access it with **CSMA/CD (Carrier Sense Multiple Access with Collision Detection)**. Classic Ethernet runs at 10 Mbps with Manchester coding; Fast Ethernet (100 Mbps), Gigabit and 10 Gigabit Ethernet followed. | Type | Medium | Max segment | |---|---|---| | 10Base5 | Thick coax | 500 m | | 10Base2 | Thin coax | 185 m | | 10Base-T | UTP to hub | 100 m | | 10Base-F | Optical fibre | 2000 m | **Ethernet (802.3) frame** ```text +--------+---+----+----+-----+---------+-----+ |Preamble|SFD| DA | SA | Len | Data+Pad| FCS | | 7 | 1 | 6 | 6 | 2 | 46-1500 | 4 | +--------+---+----+----+-----+---------+-----+ (bytes) ``` - Preamble 10101010… for clock sync; SFD 10101011 marks frame start. - 48-bit destination and source MAC addresses. - Data 46–1500 bytes (padded), so frame size is 64–1518 bytes. - FCS: 32-bit CRC for error detection. #### CSMA (Carrier Sense Multiple Access) In CSMA a station **listens before talking**: it senses the channel and transmits only if the channel is idle. This reduces collisions compared with ALOHA, but collisions still happen because a signal takes time (propagation delay) to reach other stations; two stations may both sense idle and start together. Persistence methods when the channel is busy: - **1-persistent:** keep sensing; send as soon as it becomes idle (probability 1). Used by Ethernet. Many collisions if several are waiting. - **Non-persistent:** if busy, wait a random time and sense again. Fewer collisions, more delay. - **p-persistent:** (slotted channels) when idle, send with probability p, otherwise defer to the next slot. #### CSMA/CD – working principle of Ethernet CSMA/CD adds **collision detection**: a station keeps listening while transmitting, and if it detects a collision it stops at once instead of sending the whole damaged frame. 1. Station with a frame senses the channel (carrier sense). 2. If busy, wait until idle (1-persistent), then wait the inter-frame gap (9.6 µs). 3. Transmit the frame while monitoring the line. 4. If no collision during the whole frame: success. 5. If a collision is detected (signal on the line differs from sent signal / voltage too high): - stop sending the data and send a **32-bit jam signal** so all stations notice the collision; - increment the collision counter $n$; if $n = 16$ (16 failed attempts), give up and report an error; - otherwise wait a random **binary exponential backoff** time, then go to step 1. **Binary exponential backoff:** after the $n$-th collision choose a random integer $$ K \in \{0, 1, \dots, 2^{m} - 1\}, \quad m = \min(n, 10) $$ and wait $K \times$ slot time. Slot time = 512 bit times = **51.2 µs** at 10 Mbps. The waiting range doubles after each collision, so the load adapts. ```text Frame ready | Sense channel <------------------+ | idle | Transmit + listen | | | | no collision collision | | | | Success send jam, n = n+1 | n = 16 ? --yes--> abort| | no | wait K x 51.2 us ----------+ ``` **Minimum frame size:** a sender must still be transmitting when news of a collision returns from the far end, so frame time ≥ round-trip time ($2\tau$). For 10 Mbps and 2500 m (with 4 repeaters) the slot is 512 bits, hence the **64-byte minimum frame**. **Example:** stations A and C both find the bus idle and start at almost the same time. Their signals collide; both detect it, send jam and stop. After the first collision each picks K from {0, 1}; if A picks 0 and C picks 1, A sends at once and C, on sensing a busy channel, waits until A finishes. **Features:** simple, cheap, no central control, works well at light and moderate load; at heavy load collisions reduce efficiency. Modern switched full-duplex Ethernet has no shared medium, so CSMA/CD is not needed there. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why do collisions still occur in CSMA although stations listen first? A. Stations ignore the carrier B. Propagation delay: two stations may both sense idle and start together C. Frames are too short D. The channel is full duplex Answer: B. A signal takes time to reach others, so two stations can sense idle and transmit at nearly the same time. (Key point: CSMA collisions) 2. What does a 1-persistent station do when the channel is busy? A. Waits a random time and senses again B. Keeps sensing and sends as soon as it becomes idle C. Sends with probability p in next slot D. Gives up the frame Answer: B. 1-persistent (used by Ethernet) transmits with probability 1 as soon as the channel is idle. (Key point: 1-persistent) 3. After detecting a collision, what does a CSMA/CD station do? A. Finishes the frame and then checks B. Stops, sends a jam signal, and waits a random binary exponential backoff C. Retransmits immediately D. Switches to a different channel Answer: B. It stops, sends a 32-bit jam so all notice, then waits K x slot time with K chosen from a doubling range. (Key point: Collision handling) 4. Why does Ethernet need a 64-byte minimum frame? A. To make the preamble work B. The sender must still be transmitting when collision news returns, so frame time >= round-trip time C. To limit the FCS to 4 bytes D. To reduce the jam length Answer: B. Frame time must be at least 2 x propagation delay; for 10 Mbps this gives a 512-bit slot, i.e. 64 bytes. 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