CRA (Contention resolution ALOHA)

Contention Resolution ALOHA (CRA) is a random access protocol used for data transmission in wireless networks. It was developed as a modification of the ALOHA protocol by Roberts in the early 1970s. CRA is a contention-based protocol, meaning that multiple devices can transmit data simultaneously. However, in the presence of collisions, CRA has a mechanism to resolve contention and reduce collisions.

In the CRA protocol, the wireless devices transmit data packets randomly in the available time slots. The packets transmitted by different devices can collide in the channel, causing a loss of data. To overcome this issue, the CRA protocol introduces a contention resolution mechanism. In this mechanism, the devices that detect a collision in the channel initiate a backoff procedure. This procedure randomly selects a time slot for retransmission, thus reducing the probability of another collision.

The CRA protocol can be described in three phases:

1. Transmission phase: In this phase, a device that has data to transmit selects a random time slot to transmit its packet. If the channel is idle during that time slot, the packet is transmitted successfully. However, if there is a collision with other packets transmitted during the same time slot, the device will detect it and initiate a backoff procedure.
2. Backoff phase: In this phase, the device that detected a collision selects a random waiting time before retransmitting its packet. The waiting time is selected from a range of waiting times that increase exponentially with the number of collisions. The waiting time is selected randomly from the range to avoid synchronization between the devices. After the waiting time has elapsed, the device returns to the transmission phase to attempt to retransmit the packet.
3. Contention resolution phase: In this phase, the devices that have initiated the backoff procedure contend for the next available time slot. The contention resolution can be done using different techniques, such as binary exponential backoff or persistent contention. In binary exponential backoff, the devices randomly select a time slot from a range of available time slots that increase exponentially with the number of collisions. In persistent contention, the devices wait for a fixed time after the backoff phase before attempting to retransmit the packet.

The CRA protocol has several advantages and disadvantages. One advantage is that it allows for efficient use of the channel by allowing multiple devices to transmit simultaneously. Another advantage is that it provides a mechanism for resolving contention and reducing collisions. This mechanism reduces the loss of data and improves the overall network throughput. Moreover, the CRA protocol is simple and easy to implement, making it suitable for low-cost and low-power devices.

However, the CRA protocol also has some disadvantages. One major disadvantage is that it suffers from the hidden terminal problem. The hidden terminal problem occurs when two devices are out of range of each other but are in range of a third device. In this case, the third device may transmit data and cause a collision with one of the out-of-range devices, leading to a loss of data. Another disadvantage of the CRA protocol is that it has a higher latency than other protocols, as devices must wait for a random time before retransmitting their packets. This latency can increase as the number of collisions increases, reducing the overall network throughput.

In conclusion, the CRA protocol is a contention-based protocol used for data transmission in wireless networks. It allows multiple devices to transmit data simultaneously and provides a mechanism for resolving contention and reducing collisions. Although it has some disadvantages, the CRA protocol is a simple and efficient protocol suitable for low-cost and low-power devices.

The CRA protocol is widely used in many applications, including satellite communication, radio frequency identification (RFID), and wireless sensor networks. In satellite communication, the CRA protocol is used to transmit data from the satellite to the ground station. In RFID applications, the CRA protocol is used to allow multiple RFID tags to transmit data simultaneously. In wireless sensor networks, the CRA protocol is used to allow multiple sensors to transmit data to the base station.