Multiple_Access_protocols

1. Pure ALOHA

Concept:

• Developed in the 1970s for early wireless networks.
• Completely decentralized—no synchronization required.
• Stations transmit whenever they have data.
• If a collision occurs, they wait for a random backoff time before retransmitting.

How It Works:

1. Transmission:
   • A station sends data immediately when ready.
2. Collision Detection:
   • If two stations transmit at the same time, their packets collide and are destroyed.
3. Retransmission:
   • After a collision, stations wait for a random delay before trying again.

Efficiency:

• Maximum throughput: ~18.4% (due to high collision probability).
• Vulnerable Period: Two packets can collide if sent within 2 × packet transmission time (2T).

Advantages:

• Simple to implement.
• No need for synchronization.

Disadvantages:

• Low efficiency due to high collision rate.
• Poor performance in high-traffic networks.

Use Case:

• Early satellite and wireless networks.

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2. Slotted ALOHA

Concept:

• An improvement over Pure ALOHA.
• Time is divided into fixed-length slots (equal to packet transmission time).
• Stations can only transmit at the start of a slot (synchronized).

How It Works:

1. Slot Synchronization:
   • All stations agree on slot boundaries.
2. Transmission:
   • A station transmits only at the beginning of a slot.
3. Collision Handling:
   • If two stations transmit in the same slot, they collide and retry after a random delay.

Efficiency:

• Maximum throughput: ~36.8% (twice as efficient as Pure ALOHA).
• Vulnerable Period: Only 1 × slot time (T) (better than Pure ALOHA).

Advantages:

• Higher efficiency than Pure ALOHA.
• Still simple to implement.

Disadvantages:

• Requires time synchronization.
• Still suffers from collisions in high-load scenarios.

Use Case:

• Early satellite communications, RFID systems.

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3. CSMA/CD (Carrier Sense Multiple Access with Collision Detection)

Concept:

• Used in Ethernet (wired networks).
• Stations sense the channel before transmitting.
• If a collision is detected, transmission is aborted immediately.

How It Works:

1. Carrier Sensing:
   • A station listens to the medium before sending.
   • If idle, it transmits; if busy, it waits.
2. Collision Detection:
   • If two stations transmit simultaneously, a collision occurs.
   • Both detect the collision and stop transmission.
3. Backoff & Retransmission:
   • Stations wait for a random time (binary exponential backoff) before retrying.

Efficiency:

• Works well in low to moderate traffic.
• Breaks down in high-traffic or large networks (due to increased collisions).

Advantages:

• Reduces collisions compared to ALOHA.
• Widely used in wired Ethernet.

Disadvantages:

• Not suitable for wireless networks (collision detection is hard in wireless).
• Performance degrades with network size.

Use Case:

• Traditional Ethernet (IEEE 802.3).

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4. CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance)

Concept:

• Used in Wi-Fi (wireless networks).
• Since collision detection is difficult in wireless, CSMA/CA avoids collisions rather than detecting them.
• Uses RTS/CTS (Request-to-Send / Clear-to-Send) for reservation.

How It Works:

1. Carrier Sensing:
   • Station checks if the channel is idle.
2. Random Backoff (IFS - Interframe Space):
   • Waits for a DIFS (DCF Interframe Space) period.
   • If idle, it picks a random backoff timer before transmitting.
3. Virtual Sensing (NAV - Network Allocation Vector):
   • Uses RTS/CTS to reserve the channel.
   • Other stations defer transmission based on NAV.
4. Acknowledgment (ACK):
   • Receiver sends an ACK after successful reception.

Efficiency:

• Better suited for wireless networks than CSMA/CD.
• Slower than CSMA/CD due to overhead (RTS/CTS/ACK).

Advantages:

• Avoids hidden terminal problem (using RTS/CTS).
• Works well in wireless environments.

Disadvantages:

• Higher overhead (RTS/CTS/ACK).
• Slower than CSMA/CD.

Use Case:

• Wi-Fi (IEEE 802.11).

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5. Comparison of Multiple Access Protocols

Protocol	Type	Key Feature	Efficiency	Used in
Pure ALOHA	Random Access	No synchronization, immediate transmission	~18.4%	Early wireless networks
Slotted ALOHA	Random Access	Time-slotted transmission	~36.8%	Satellite comms, RFID
CSMA/CD	Contention-Based	Detects collisions, aborts transmission	High (wired)	Ethernet (802.3)
CSMA/CA	Contention-Based	Avoids collisions (RTS/CTS)	Moderate (wireless)	Wi-Fi (802.11)