Chapter 5 — Session 5: CSMA/CD vs. CSMA/CA
This is a very testable Network+ comparison.
Your Chapter 5 notes boil it down to:
CSMA/CD = wired → detect collisions
CSMA/CA = wireless → avoid collisions
📋 Topics to Mark Off
⬜ Contention method
⬜ CSMA
⬜ CSMA/CD
⬜ Carrier Sense
⬜ Multiple Access
⬜ Collision Detection
⬜ Wired Ethernet
⬜ Listen before transmitting
⬜ Jam signal
⬜ Random backoff
⬜ Retransmission
⬜ CSMA/CA
⬜ Collision Avoidance
⬜ Wireless / Wi-Fi
⬜ Backoff countdown
⬜ ACK / Acknowledgment
⬜ Why Wi-Fi cannot detect collisions while transmitting
⬜ Wireless overhead
⬜ CSMA/CD vs. CSMA/CA
📝 Notebook Notes — Quick Review
🚦 Contention Method
A contention method is basically:
A set of rules devices use to decide who gets to transmit on a shared medium.
Think of several people trying to use the same microphone.
👤 👤 👤 👤 🎤
They need rules so everyone doesn't talk at once.
🧠 Memory
Contention = competing for a turn.
👂 CSMA
CSMA = Carrier Sense Multiple Access
Break it apart:
Carrier Sense
Listen first 👂
Multiple Access
Multiple devices share the medium 👥
So:
CSMA = Multiple devices listen before trying to use the shared medium.
🔌 CSMA/CD ⭐ EXAM GOLD
CSMA/CD =
Carrier Sense Multiple Access with Collision Detection
Used with:
🔌 Wired Ethernet
Your chapter's flow is:
👂 LISTEN
↓
Clear?
↓ YES
📤 SEND
↓
💥 Collision?
↓ YES
🚨 JAM SIGNAL
↓
🎲 RANDOM BACKOFF
↓
TRY AGAIN
👂 Step 1 — Listen
Before transmitting, the device checks:
Is anyone else transmitting?
If the wire is busy:
Wait.
If it's clear:
Send.
Your source describes the host checking for a signal on the wire before transmitting.
💥 Step 2 — Detect Collision
While transmitting, the wired host continues monitoring the medium.
If another device transmits at the same time:
💥 COLLISION
The signals interfere with each other.
🚨 Step 3 — Jam Signal
If a collision occurs:
A jam signal is sent.
This tells devices:
STOP — collision happened!
🧠 Memory
JAM = traffic jam 🚗💥🚗
🎲 Step 4 — Random Backoff
After the collision:
Devices wait for a random amount of time.
Then they try again.
Why random?
If both devices waited exactly the same amount:
Device A waits 2 sec
Device B waits 2 sec
↓
Both send again
↓
💥 AGAIN
Random delays reduce the chance they'll collide again.
🧠 Memory
Backoff = Back away, wait, try again.
⭐ CSMA/CD Memory Story
Imagine two cars entering one narrow road.
🚗 → ← 🚗 💥
They collide.
Both back up:
🚗 ← → 🚗
Then each waits a random amount of time before trying again.
That's:
CSMA/CD
📡 CSMA/CA ⭐ EXAM GOLD
CSMA/CA =
Carrier Sense Multiple Access with Collision Avoidance
Used with:
📡 Wireless
The big difference:
Wireless tries to:
AVOID the collision BEFORE it happens.
Your Chapter 5 flow is:
👂 LISTEN ↓ 🎲 BACKOFF COUNTDOWN ↓ 📤 SEND ↓ ✅ WAIT FOR ACK
🤔 Why Doesn't Wi-Fi Use Collision Detection?
This connects directly to the half-duplex wireless concept you just studied.
Your notes explain that a radio cannot reliably listen for a collision while it is transmitting.
So wireless cannot simply:
Send → listen while sending → detect collision.
Instead it tries to:
AVOID
collisions before transmitting.
🧠 Memory
Wireless can't hear while it's talking. 📻
Therefore:
CA = Collision Avoidance
🎲 Wireless Backoff
With CSMA/CA, a wireless station:
Listens.
Waits through a backoff countdown.
Transmits when allowed.
This helps multiple wireless devices share the channel.
Think:
📱 Device A → wait 3 💻 Device B → wait 7 📺 Device C → wait 5
A reaches zero first:
📱 transmits first.
✅ ACK
ACK = Acknowledgment
After the wireless frame is sent:
The sender expects an acknowledgment.
Think:
📱 ───── DATA ─────▶ 📡 📱 ◀──── ACK ────── 📡
ACK basically means:
"I got your frame." ✅
🧠 Memory
ACK = Acknowledged!
🚨 What If There's No ACK?
If the sender doesn't receive the expected acknowledgment:
It may assume:
The frame did not arrive successfully.
Then:
It may need to retransmit.
⭐ CSMA/CA Memory Story
Imagine people using walkie-talkies.
You:
👂 Listen
⏳ Wait
📻 Talk
Wait for:
"Roger!" ✅
That "Roger" is like the:
ACK
🆚 CSMA/CD vs. CSMA/CA
CSMA/CD | CSMA/CA | |
|---|---|---|
Used with | 🔌 Wired | 📡 Wireless |
Goal | Detect collisions | Avoid collisions |
Listen first | ✅ | ✅ |
Jam signal | ✅ | ❌ |
Random/backoff behavior | After collision | Before sending |
ACK | Not the key mechanism here | ⭐ Yes |
Memory | D = Detect | A = Avoid |
🧠 BEST MEMORY TRICK
Look only at the last letter:
CD
💥 Collision DETECTION
CA
🚧 Collision AVOIDANCE
Then:
🔌 D = Detect = wired
📡 A = Avoid = wireless
💻 Knowt Notes — Deep Understanding
Why Did Wired Ethernet Need CSMA/CD?
Older shared Ethernet environments, especially hubs, put multiple devices in the same collision domain.
Imagine:
PC-A ─┐
PC-B ─┼── HUB
PC-C ─┘
Everybody shares the medium.
If A and B transmit simultaneously:
A ───▶ 💥 ◀─── B
Their signals collide.
CSMA/CD gives them a way to recover:
Detect → Jam → Backoff → Retry
Your chapter explains that heavy collisions lead to delay, lower throughput, and congestion.
🔀 What About Modern Switches?
Remember from earlier:
Each switch port is its own collision domain.
Modern full-duplex switched Ethernet essentially removes the traditional shared-medium collision problem.
But for Network+:
⭐ Still memorize
CSMA/CD = wired Ethernet collision detection
because CompTIA can absolutely test the concept.
📡 Why Wireless Is Different
Wireless is shared air.
Multiple devices may all hear the same AP:
📱 \ 💻 ))) 📡 ((( 📺 / 🎮
But a wireless radio cannot reliably transmit and detect another transmission at the same time.
So instead of:
Detect after collision
it uses:
Avoid before collision
Then uses an:
ACK
to confirm success.
📉 Wireless Overhead
Your chapter also points out something important:
Wireless's listen/wait/ACK process introduces:
Overhead
That means the actual useful throughput can be much lower than the advertised link rate.
Your notes give the example that an advertised 54 Mbps 802.11g connection may deliver roughly 25–28 Mbps, with throughput dropping further as more devices share the air.
🧠 Why?
Because the wireless channel isn't only carrying your useful data.
There is also:
Waiting
Backoff
ACKs
Management overhead
Other devices competing for airtime
⚠ Exam Traps
🚨 Trap #1
CSMA/CD = wireless
❌ Wrong.
CD = wired
🚨 Trap #2
CSMA/CA = wired Ethernet
❌ Wrong.
CA = wireless
🚨 Trap #3
Wireless detects collisions while transmitting.
❌ Wrong according to your chapter.
Wireless cannot reliably listen while transmitting.
That's WHY it uses avoidance.
🚨 Trap #4
A jam signal belongs to CSMA/CA.
❌ Wrong.
Jam = CSMA/CD
🚨 Trap #5
ACK belongs to the CSMA/CD collision process.
❌ Not the important association.
ACK = CSMA/CA
🚨 Trap #6
Backoff means the exact same thing in both.
Be careful.
For your exam notes:
CD → collision occurs → random backoff
CA → backoff countdown before transmitting
🎴 Multiple-Choice Flashcards
⭐ 1 — EXAM GOLD
What does CSMA/CD stand for?
A. Carrier Signal Multiple Access / Collision Delay
B. Carrier Sense Multiple Access with Collision Detection
C. Carrier Sense Managed Access / Collision Detection
D. Collision Sense Multiple Access / Carrier Detection
✅ Answer: B
🧠 CD = Collision Detection.
⭐ 2 — EXAM GOLD
CSMA/CD is traditionally associated with:
A. Wi-Fi
B. Wired Ethernet
C. Bluetooth only
D. DNS
✅ Answer: B
🧠 Wired = Detect.
⭐ 3
Before transmitting with CSMA/CD, a host first:
A. Sends a jam signal
B. Listens to determine whether the medium is clear
C. Requests an IP address
D. Sends an ACK
✅ Answer: B
🧠 Carrier Sense = listen first. 👂
⭐ 4 — EXAM GOLD
What happens after a collision is detected in CSMA/CD?
A. An ACK is immediately returned
B. A jam signal is sent
C. The device changes its MAC address
D. The router drops the VLAN
✅ Answer: B
🧠 Collision → JAM. 🚗💥
5
What happens after the jam signal?
A. Every host sends immediately
B. Devices perform a random backoff before retrying
C. The AP sends an ACK
D. The default gateway changes
✅ Answer: B
🧠 Back away → random wait → retry.
⭐ 6 — EXAM GOLD
What does CSMA/CA stand for?
A. Carrier Sense Multiple Access with Collision Acknowledgment
B. Carrier Sense Multiple Access with Collision Avoidance
C. Collision Sense Multiple Access with Carrier Avoidance
D. Carrier Signal Managed Access with Collision Alert
✅ Answer: B
🧠 CA = Collision Avoidance.
⭐ 7 — EXAM GOLD
CSMA/CA is primarily associated with:
A. Wired hubs
B. Fiber Ethernet
C. Wireless networks
D. Routers only
✅ Answer: C
🧠 Wireless = Avoid.
⭐ 8
Why does wireless use collision avoidance instead of collision detection?
A. Wireless has no shared medium
B. APs use IP instead of MAC
C. A wireless radio cannot reliably listen for a collision while transmitting
D. Wireless never experiences contention
✅ Answer: C
🧠 Can't hear while talking. 📻
⭐ 9
Which sequence BEST represents CSMA/CA?
A. Send → collide → jam → backoff
B. Listen → backoff → send → ACK
C. ACK → send → jam → listen
D. Route → send → ARP → ACK
✅ Answer: B
🧠 Wireless: Listen → Wait → Send → ACK.
⭐ 10 — EXAM GOLD
What does ACK mean?
A. Access Collision Key
B. Acknowledgment
C. Automatic Carrier Keepalive
D. Address Control Key
✅ Answer: B
🧠 ACK = "I got it!" ✅
11
A wireless station sends a frame but never receives the expected ACK. What is the likely conclusion?
A. The frame was definitely delivered
B. The frame may not have been successfully received
C. The router changed its IP
D. A wired collision occurred
✅ Answer: B
🧠 No ACK = no confirmation.
⭐ 12 — Scenario
A technician sees devices detect a collision, send a jam signal, and wait random periods before trying again.
Which technology is being described?
A. CSMA/CA
B. CSMA/CD
C. DNSSEC
D. Round robin
✅ Answer: B
🧠 Jam signal = CD clue.
⭐ 13 — Scenario
A client listens to a wireless channel, waits for a backoff timer, transmits, and then waits for confirmation.
Which technology?
A. CSMA/CD
B. CSMA/CA
C. STP
D. ARP
✅ Answer: B
🧠 Backoff-before-send + ACK = CA.
🚨 14 — EXAM TRAP
Which pairing is correct?
A. CSMA/CD → Wireless
B. CSMA/CA → Wired Ethernet
C. CSMA/CD → Detect, CSMA/CA → Avoid
D. CSMA/CD → ACK, CSMA/CA → Jam
✅ Answer: C
⭐ 15
Heavy collisions on a shared Ethernet segment can cause:
A. Higher throughput
B. Delay, low throughput, and congestion
C. Faster DNS resolution
D. Improved wireless coverage
✅ Answer: B
Your source explicitly lists those effects.
🧠 Memory Board
🔌 WIRED CSMA/CD CARRIER SENSE 👂 Listen MULTIPLE ACCESS 👥 Shared medium COLLISION DETECTION 💥 Detect collision 🚨 JAM 🎲 RANDOM BACKOFF 🔁 RETRY 📡 WIRELESS CSMA/CA 👂 Listen ⏳ Backoff countdown 📤 Send ✅ ACK CD = DETECT CA = AVOID
🎯 Network+ Summary
There is one comparison I want you to know instantly:
🔌 CSMA/CD = Wired = Collision Detection
📡 CSMA/CA = Wireless = Collision Avoidance
For CD:
Listen → Send → Collision → Jam → Random Backoff → Retry
For CA:
Listen → Backoff → Send → ACK
And the WHY matters:
Wireless cannot reliably listen for collisions while transmitting, so it must try to avoid collisions instead.
⭐ Network+ Exam Priorities
Memorize these cold:
CSMA = Carrier Sense Multiple Access
Carrier Sense = Listen first
Multiple Access = Multiple devices share medium
CSMA/CD = Wired
CD = Collision Detection
Collision → Jam Signal
Jam → Random Backoff
CSMA/CA = Wireless
CA = Collision Avoidance
Wireless → Backoff before transmission
Wireless → ACK
ACK = Acknowledgment
Why CA? → Wireless radio can't reliably listen while transmitting
Heavy collisions → Delay + low throughput + congestion
And for this session, when you color-code the topics again, give me your 🟢/🟡/🔴 list and I’ll build the next adaptive website around exactly what isn’t clicking.