Multiple Access Protocols

Multiple Access Protocols are methods that allow multiple devices to share a communication channel efficiently, ensuring data is transmitted without collision or interference. These protocols are crucial in networks where multiple devices need to communicate over a shared medium, such as Ethernet or wireless networks.

The protocols can be categorized into three main types: Channel Partitioning, Random Access, and Controlled Access.

1. Channel Partitioning Protocols

Channel partitioning divides the communication channel into distinct segments to allocate resources efficiently. These protocols are deterministic and ensure fair access but may lead to underutilization if devices do not fully use their allocated slots.

1.1 Time Division Multiple Access (TDMA)

The channel is divided into time slots, and each device is assigned a specific slot to transmit data. Devices transmit in a synchronized manner to avoid collisions.

• Advantages: Prevents collisions, predictable performance.
• Disadvantages: Wasted slots if a device has no data to send.

1.2 Frequency Division Multiple Access (FDMA)

The channel is divided into frequency bands, with each device assigned a unique band for communication.

• Advantages: Continuous transmission possible, no timing synchronization needed.
• Disadvantages: Requires complex hardware, inefficient with low traffic.

1.3 Code Division Multiple Access (CDMA)

Devices transmit over the entire frequency spectrum using unique codes to distinguish transmissions.

• Advantages: High utilization of bandwidth, resistant to interference.
• Disadvantages: Complex encoding and decoding, requires precise synchronization.

2. Random Access Protocols

In random access protocols, devices transmit without pre-coordination. Collisions may occur, and mechanisms are needed to detect and recover from these collisions.

2.1 ALOHA

Devices transmit whenever they have data to send. If a collision occurs, devices wait for a random time and retransmit.

• Variants: Pure ALOHA and Slotted ALOHA.
• Efficiency: Slotted ALOHA doubles the efficiency by limiting transmissions to discrete slots.

2.2 Carrier Sense Multiple Access (CSMA)

Devices sense the channel before transmitting to avoid collisions.

• Variants:
  • CSMA/CD (Collision Detection): Used in wired networks (e.g., Ethernet). Devices stop transmitting when a collision is detected.
  • CSMA/CA (Collision Avoidance): Used in wireless networks (e.g., Wi-Fi). Devices use acknowledgments and backoff timers to reduce collisions.
• Efficiency: Improves with lower load; performance degrades with high traffic.

3. Controlled Access Protocols

Controlled access protocols ensure orderly access to the channel by coordinating the transmission among devices.

3.1 Polling

A central controller polls devices sequentially to grant them the right to transmit.

• Advantages: No collisions, fair access.
• Disadvantages: Delay increases with the number of devices, failure of the controller disrupts communication.

3.2 Token Passing

A token circulates in the network, and only the device holding the token can transmit.

• Advantages: No collisions, deterministic behavior.
• Disadvantages: Token loss requires recovery mechanisms, complexity increases with network size.

Applications of Multiple Access Protocols

These protocols are essential in various network types:

• TDMA: Cellular networks, satellite communication.
• FDMA: Radio broadcasting, GSM networks.
• CDMA: 3G networks, military communication.
• CSMA/CA: Wi-Fi networks.
• Token Passing: Token Ring networks.