Controlled Access Protocol
Controlled Access Protocol is a type of media access control (MAC) protocol used to manage how multiple devices share a communication channel. In contrast to random access protocols like ALOHA or CSMA, where devices can transmit whenever the medium is free, controlled access protocols ensure that only one device can transmit at a time. This is done using a structured mechanism, which eliminates the chances of collisions, but it requires coordination among the devices.
Key Characteristics:
- Centralized or Distributed Control: Controlled access protocols involve some form of control, either through a central authority or a distributed mechanism, that determines which device has the right to transmit.
- Eliminates Collisions: Since only one device can transmit at a time, collisions (where two devices send data simultaneously, causing corruption) are avoided.
- Coordination Needed: Devices either request permission to transmit or take turns in an orderly fashion. This requires more overhead and management than random access protocols.
Types of Controlled Access Protocols:
There are three main types of controlled access protocols:
1. Reservation Protocol
In reservation-based access, devices reserve the communication channel before transmitting data. This approach ensures that only one device is using the medium at a given time.
How it Works:
- The medium is divided into time slots or periods.
- Devices that wish to transmit data first reserve the medium during a reservation period. They request the use of the medium by sending a reservation request.
- Once the medium is reserved for a specific device, it can transmit its data without risk of collisions.
Example:
- In TDMA (Time Division Multiple Access), time is divided into slots, and each device is assigned a specific time slot for transmission. The device can only transmit during its allocated slot, eliminating collisions.
2. Polling Protocol
In polling, a central controller (also called the master or polling device) manages which device is allowed to transmit. The controller “polls” each device to see if it has data to transmit.
How it Works:
- The central controller polls each device in a round-robin fashion or based on a priority scheme.
- If a device has data to send, it transmits when it is polled.
- Polling is a mechanism where a central device (the controller or master) periodically asks each connected device if it needs to communicate, such as whether it has data ready to be transmitted over the network.
- If it has no data to send, the controller moves to the next device.
Example:
- Token Bus or Token Ring protocols use polling mechanisms to give each device the opportunity to transmit in an orderly fashion.
- USB uses polling between the host and devices connected to the bus. The host periodically checks each device to see if it has data to send or receive.
3. Token Passing Protocol
In token passing, a special data packet called a token circulates through the network. Only the device that holds the token can transmit. Once the device is done transmitting, it passes the token to the next device.
How it Works:
- A token (a small data packet) is passed from one device to another in a logical or physical ring.
- When a device receives the token, it can transmit its data.
- After finishing transmission, the device passes the token to the next device in the sequence.
- If a device has no data to send, it passes the token immediately to the next device.
Example:
- Token Ring (IEEE 802.5) is a network protocol where devices are connected in a ring topology, and a token is passed between them to control access to the communication channel.
- Token Bus (IEEE 802.4) also uses the token passing mechanism, but in a bus topology.
Advantages of Controlled Access Protocols:
- No Collisions: Since only one device is allowed to transmit at a time, there is no risk of data collisions. This makes controlled access protocols more efficient in networks with high traffic.
- Deterministic: Controlled access provides deterministic behavior, meaning each device knows when it will be able to transmit, reducing the uncertainty present in random access protocols.
Disadvantages of Controlled Access Protocols:
- Overhead: The control mechanisms (such as reservation, polling, or token passing) introduce additional overhead, which can slow down the network when traffic is low.
- Centralized Control (Polling): In systems where polling is used, the central controller can become a bottleneck or point of failure if it malfunctions.
- Latency: Devices may need to wait for their turn (polling or token passing), which introduces some delay (latency) compared to Random Access Protocol where devices can transmit as soon as the medium is free.
Comparison of Controlled Access Protocols with Random Access Protocols:
| Feature | Controlled Access Protocols | Random Access Protocols (e.g., CSMA, ALOHA) |
|---|---|---|
| Collision Risk | No collisions (devices take turns) | High risk of collisions (devices compete to transmit) |
| Efficiency | High in high-traffic networks | Low efficiency in high-traffic networks due to collisions |
| Access Control | Centralized or decentralized control (polling, token passing) | Decentralized (devices transmit whenever the medium is idle) |
| Overhead | Higher overhead (due to coordination mechanisms) | Lower overhead |
| Latency | Predictable, but potentially higher | Variable, depends on traffic and collision occurrence |
| Use Cases | Networks requiring deterministic access (e.g., industrial control ne tworks) | General-purpose LAN and wireless networks |
Examples of Controlled Access Protocols in Use:
- Token Ring and Token Bus are classic examples of controlled access protocols used in LANs. While not as commonly used today, these protocols were popular in certain types of networks, particularly where deterministic access was required.
- Polling is used in systems like USB, where the host periodically checks connected devices to see if they have data to send or receive.
Conclusion:
Controlled Access Protocols provide a reliable way to manage access to a shared communication medium, especially in networks where high traffic makes collisions costly. By ensuring that only one device can transmit at a time, controlled access protocols avoid the inefficiencies and unpredictability of random access protocols, making them ideal for high-traffic or deterministic network environments.
References
Information
- date: 2024.10.07
- time: 06:58
This version includes your requested structure and formatting, with appropriate linking and display text. Let me know if there’s anything else you’d like to adjust!