Random Access Protocol

06 Sep 2022 . network .

Aloha

  1. Any station can transmit data to a channel at any time.
  2. It does not require any carrier sensing.
  3. Collision and data frames may be lost during the transmission of data through multiple stations.
  4. Acknowledgment of the frames exists in Aloha. Hence, there is no collision detection.
  5. It requires retransmission of data after some random amount of time.

Pure Aloha

  • Each station transmits data to a channel without checking whether the channel is idle or not, the chances of collision may occur, and the data frame can be lost.
  • When any station transmits the data frame to a channel, the pure Aloha waits for the receiver’s acknowledgment. If it does not acknowledge the receiver end within the specified time, the station may assume the frame has been lost or destroyed.
  • The station waits for a random amount of time and retransmits the frame until all the data are successfully transmitted to the receiver.

Slotted Aloha

  • The shared channel is divided into a fixed time interval called slots. If a station wants to send a frame to a shared channel, the frame can only be sent at the beginning of the slot, and only one frame is allowed to be sent to each slot.
  • If the stations are unable to send data to the beginning of the slot, the station will have to wait until the beginning of the next slot.
  • However, the possibility of a collision remains when two or more station try to send a frame at the beginning of a time slot.

CSMA (Carrier Sense Multiple Access/载波监听多路复用)

Sense the traffic on a channel before transmitting the data. If the channel is idle, the station can send data to the channel. Otherwise, it must wait until the channel becomes idle. Hence, it reduces the chances of a collision on a transmission medium.

1-Persistent

  • First sense the shared channel and if the channel is idle, it immediately sends the data.
  • Else it must wait and keep track of the status of the channel to be idle and broadcast the frame unconditionally as soon as the channel is idle.

Non-Persistent

  • Sense the channel, and if the channel is idle, it immediately sends the data.
  • Otherwise, the station must wait for a random time (not continuously), and when the channel is found to be idle, it transmits the frames.

P-Persistent

  • Senses the channel, and if the channel is idle, it sends a frame with a P probability and Q(Q = 1-P probability) probability not send.
  • If the data is not transmitted, it waits for a random time and sends the frame with P probability the next time slot.

CSMA/ CD

  • It senses of the shared channel is busy for broadcasting. It sends data when channel is idle and interrupts the broadcast until the channel is free.
  • Upon collision detection, the transmission is stopped and a jam signal is sent by the stations and then the station waits for a random time context before retransmission.

CSMA/ CA

  • It senses whether the channel and if the channel is idle, it sends a RTS (Request To Send Request Is Sent). The RTS includes the address of the sender, the address of the receiver and the time when the next data will be sent.
  • The receiver returns CTS (Clear To Send) and the sender sends data after receiving CTS.
  • The receiver will check whether the packet data is correct with CRC and returns ACK packet if it is correct. When the sender does not receive ACK packet, it will be considered that the packet is lost during transmission and resend the packet all the time.