Medium Access Control & LAN Technologies: CSMA, Token Passing, Ethernet, Wi-Fi

TAKEAWAYS:

• Medium Access Control (MAC) determines who transmits when in shared networks, using methods like CSMA (with/without collision detection) or token passing.
• Ethernet (IEEE 802.3) uses CSMA/CD for wired LANs, while Wi-Fi (802.11) uses CSMA/CA to avoid collisions via RTS/CTS handshakes.
• Token passing (802.5) guarantees fair access but is slower; CSMA is faster but prone to collisions.
• Ethernet frames include Preamble, SFD, Destination/MAC, Type, Data, FCS, with minimum frame size (64 bytes) to detect collisions.
• Wi-Fi channels (2.4 GHz/5 GHz) use spread spectrum (DSSS, OFDM) to reduce interference; 802.11n/ac add MIMO for speed.
• Real-world use: eSewa’s payment queues (CSMA-like contention), Daraz’s order routing (Ethernet switches), NTC’s fiber backbones (token-ring legacy).

1. Why Medium Access Control (MAC)?

LANs connect multiple devices to a shared medium (cable, airwaves). Without MAC, devices would collide constantly. MAC protocols define:

• Who gets to transmit?
• How do they detect/avoid collisions?
• What happens if two devices transmit at once?

2. Contention-Based MAC: CSMA (Carrier Sense Multiple Access)

Devices listen before transmitting (like raising your hand in class). Two variants:

A. CSMA/CD (Collision Detection) – Wired Ethernet (802.3)

How it works:

1. Carrier Sense: Device listens to the medium. If idle, it transmits.
2. Collision Detection: While transmitting, it monitors for collisions (two signals overlapping).
3. Collision Handling:
   • If a collision is detected, both devices send a jam signal (32-bit pattern).
   • Each waits a random backoff time (exponential backoff: 0–1, 0–3, 0–7 slots) before retrying.

Why exponential backoff? Avoids repeated collisions by making retries unpredictable.

Ethernet Frame Structure (with collision detection):

Worked Example: Collision in Daraz’s Order Processing

• Scenario: Two Daraz servers try to update the same inventory simultaneously.
• CSMA/CD in action:
  1. Server A sends "Update stock: Product X" (60-byte frame).
  2. Server B senses the medium is busy, waits.
  3. Server A finishes transmitting, but Server B starts just as A’s signal fades.
  4. Collision detected → Both send a jam signal.
  5. Both wait random backoff times (e.g., A waits 2 slots, B waits 5 slots) before retrying.

Advantages/Disadvantages:

B. CSMA/CA (Collision Avoidance) – Wi-Fi (802.11)

Wireless networks have the hidden node problem: Two devices may not hear each other but collide at the access point (AP).

How CSMA/CA avoids collisions:

1. Carrier Sense: Device checks if the medium is free.
2. Random Backoff: If free, it waits a random time before transmitting.
3. RTS/CTS Handshake (optional but used in dense networks):
   • Request to Send (RTS): Sender asks AP for permission.
   • Clear to Send (CTS): AP replies, silencing other devices.
   • Data Transmission: Sender transmits; others hear CTS and wait.

Wi-Fi Frame Exchange (with RTS/CTS):

sequenceDiagram
    participant Sender as Device A
    participant AP as Access Point
    participant Receiver as Device B
    participant Others as Other Devices

    Sender->>AP: RTS (Request to Send)
    AP->>Others: CTS (Clear to Send)
    Sender->>Receiver: Data Frame
    Receiver->>Sender: ACK

Worked Example: NTC’s Wi-Fi Hotspots

• Scenario: Multiple users in a café connect to NTC’s Wi-Fi.
• CSMA/CA in action:
  1. User 1’s laptop senses the channel is free and waits 3 slots.
  2. User 2’s phone senses the channel is free but waits 1 slot.
  3. User 2 transmits first (shorter backoff). User 1 hears the transmission and backs off.
  4. If User 1 had sent an RTS, the AP would reply with CTS, telling User 2 to wait.

Advantages/Disadvantages:

3. Token-Passing MAC (802.5 – Token Ring)

Instead of contention, devices take turns using a token:

• Only the device holding the token can transmit.
• After transmitting, it passes the token to the next device.

How Token Ring Works:

1. A token frame circulates the ring.
2. A device wanting to transmit:
   • Waits for the token.
   • Converts the token to a data frame and transmits.
   • The destination copies the data and sends an ACK.
   • The sender removes the frame and releases a new token.
3. If no one transmits, the token keeps circulating.

Token Frame Structure:

Real-World Example: Ncell’s Legacy Token-Ring Networks

• Scenario: In the 1990s, Ncell used token-ring LANs in some office networks.
• Why? Guaranteed access for critical calls (no collisions).
• Downside: Slower than Ethernet for bursty traffic (e.g., file transfers).

Advantages/Disadvantages:

4. LAN Technologies: Ethernet vs. Wi-Fi

5. Ethernet Standards (Wired LANs)

Key Takeaway:

• Hubs create collision domains (all ports share the same segment).
• Switches eliminate collisions by forwarding frames to specific ports.

6. Wi-Fi Technologies (802.11)

How Wi-Fi Channels Work:

• 2.4 GHz: 11 channels (overlapping in most regions).
• 5 GHz: 23 non-overlapping channels (less interference).
• 6 GHz (802.11be): 59 channels (for ultra-fast Wi-Fi 6E).

7. Real-World Applications

A. eSewa’s Payment Queues (CSMA-like Contention)

• Problem: Multiple users submit payments simultaneously.
• Solution: eSewa’s backend uses priority queues (similar to CSMA’s backoff).
  • High-priority transactions (e.g., bill payments) get shorter backoff times.
  • Low-priority transactions (e.g., top-up) wait longer.

B. Daraz’s Order Routing (Ethernet Switches)

• Problem: Orders from Kathmandu and Pokhara must reach the warehouse without collisions.
• Solution: Daraz uses Layer 2 switches to forward frames directly to the warehouse’s MAC address.
  • No broadcast storms (unlike hubs).
  • VLANs separate order processing from inventory updates.

C. NTC’s Fiber Backbone (Token-Ring Legacy)

• Scenario: NTC’s old SDH (Synchronous Digital Hierarchy) rings used token-like synchronization.
• Why? Guaranteed bandwidth for voice/data (critical for telecom).

D. Pathao’s Rider Tracking (Wi-Fi + CSMA/CA)

• Problem: Riders’ phones must upload GPS data without collisions.
• Solution: Pathao’s servers use 802.11ac Wi-Fi with:
  • RTS/CTS to avoid collisions in dense areas (e.g., Thamel).
  • MU-MIMO to serve multiple riders simultaneously.

8. Exam Tip: How This Unit is Tested

1. Definitions & Comparisons (3–5 marks)

   • Differentiate CSMA/CD vs. CSMA/CA vs. Token Passing.
   • Explain why Ethernet uses CSMA/CD but Wi-Fi uses CSMA/CA.

2. Scenario-Based Questions (5–7 marks)

   • "Two devices in a Wi-Fi network collide. How does CSMA/CA resolve this?"
   • "Why does a 60-byte Ethernet frame cause a collision?" (Hint: Minimum frame size is 64B.)

3. Diagrams (4–6 marks)

   • Draw an Ethernet frame and label its fields.
   • Sketch a token-ring network with devices A, B, C.
   • Show RTS/CTS handshake in Wi-Fi.

4. Real-World Applications (3–5 marks)

   • "How does Daraz use Ethernet switches to avoid collisions?"
   • "Why does NTC prefer fiber over Wi-Fi for backhaul?"

Common Mistakes to Avoid:

• Forgetting exponential backoff in CSMA/CD.
• Confusing CSMA/CA’s RTS/CTS with TCP’s 3-way handshake.
• Ignoring the minimum frame size (64B) in collision detection.
• Mixing up 802.3 (Ethernet) and 802.11 (Wi-Fi) standards.

Final Note: MAC protocols are the traffic cops of LANs. Master CSMA/CD, CSMA/CA, and token passing, and you’ll ace the practical and theoretical parts of this unit!