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CFP (contention free period)

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Contention-Free Period (CFP) is a mechanism used in wireless networks that allows devices to communicate with each other without contention. CFP is a feature of the IEEE 802.11 standard that provides a way for wireless devices to send and receive data in a more controlled and efficient manner. In this article, we will discuss what CFP is, how it works, and its benefits.

Background

In wireless networks, multiple devices can communicate with each other using the same channel. However, if multiple devices transmit data at the same time, a collision can occur, which results in data loss. To prevent collisions, devices use a mechanism called Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA).

CSMA/CA is a protocol that ensures that devices do not transmit data at the same time. Before a device transmits data, it first listens to the channel to ensure that it is not in use. If the channel is free, the device transmits its data. If the channel is busy, the device waits for a random period before trying again.

Although CSMA/CA is effective in reducing collisions, it can still result in contention, which can lead to delays and reduced network performance. To address this issue, the IEEE 802.11 standard introduced CFP.

What is CFP?

CFP is a mechanism that provides a contention-free period during which devices can communicate with each other without contention. During the CFP, devices are not allowed to transmit data unless they have been granted access by the Access Point (AP).

CFP is divided into two parts: the Contention-Free Interval (CFI) and the Contention-Free End (CFE). The CFI is the period during which devices can transmit data without contention, while the CFE is the end of the CFP and the start of the contention period.

The CFP is controlled by the AP, which assigns time slots to devices that want to transmit data. Devices that are granted access to the CFP can transmit data without the risk of collision. This improves network efficiency and reduces delays.

How does CFP work?

To use CFP, devices must first indicate to the AP that they want to transmit data during the CFP. This is done using the Contention-Free Request (CF-REQ) mechanism. The CF-REQ informs the AP of the device's desire to transmit data during the CFP.

The AP then assigns time slots to devices that have requested access to the CFP. These time slots are called Contention-Free Period Time (CFPT) slots. The AP informs the devices of their assigned time slots using the Contention-Free Period Announcement (CF-End) mechanism.

Once the CFP starts, devices that have been granted access to the CFP can transmit data without the risk of collision. The AP monitors the CFP to ensure that devices are following the rules. If a device violates the CFP rules, the AP can revoke its access to the CFP.

When the CFP ends, the AP sends a CF-End+CF-Poll frame to each device that has transmitted data during the CFP. The CF-Poll frame is used to request any remaining data from the device.

Benefits of CFP

CFP provides several benefits to wireless networks, including:

  1. Reduced contention: CFP reduces contention by providing a controlled period during which devices can transmit data without the risk of collision.
  2. Improved network efficiency: By reducing contention, CFP improves network efficiency and reduces delays.
  3. Guaranteed access: Devices that are granted access to the CFP are guaranteed access to the channel during that period.
  4. Increased network capacity: CFP can increase the network capacity by reducing collisions and delays.
  5. Better Quality of Service (QoS): CFP can provide better QoS by ensuring that devices that require low latency or high bandwidth have access to the channel during the CFP.
  6. Support for time-sensitive applications: CFP can support time-sensitive applications such as voice and video by providing a guaranteed bandwidth and reducing delay.
  7. Power saving: Devices that are not granted access to the CFP can enter a low-power mode, which reduces energy consumption.

Overall, CFP is an important feature of wireless networks that improves network efficiency, reduces contention, and provides guaranteed access to the channel. By reducing delays and increasing network capacity, CFP can support time-sensitive applications and provide better QoS.

Conclusion

In this article, we have discussed Contention-Free Period (CFP), a mechanism used in wireless networks to provide a controlled period during which devices can communicate with each other without contention. CFP is an important feature of the IEEE 802.11 standard that provides several benefits to wireless networks, including reduced contention, improved network efficiency, and guaranteed access. CFP can also support time-sensitive applications and provide better QoS. Overall, CFP is an essential mechanism for improving the performance and reliability of wireless networks.