The MAC protocols in vehicular networks are of three kinds – contention based, contention-free and hybrid. In this paper, a survey on the realizable methods in hybrid MAC protocols is provided, primarily to improve the performance of safety and non-safety message delivery. Challenges dealt with 802.11p MAC protocol are also highlighted. Hybrid MAC acquires the best characteristics of two channel access schemes, thereby improving the performance when traffic load increases. 260 THE MEDIUM ACCESS CONTROL SUBLAYER CHAP.3.1.1 TDMA with FDD Dynamic Packet Reservation Multiple Access. The MAC protocols covered in this article include Random Access protocols. All signals are treated in the same way in IEEE 802.15.4 MAC protocol, which is the main limitation of IEEE 802.15.4 MAC’s usage in the WBAN scenario. Sensor nodes use CSMA/CA channel access mechanism in CAP and the CFP is divided into guaranteed timeslots (GTS) on a reservation-based approach. A hybrid MAC protocol ensures that all vehicles obtain a fair channel access for transmission of their messages.Whereas the lower sub-layer is used to handle and reduce the collision or multiple access on a channel. The upper layer has the responsibility to flow control and the error control in the data link layer, and hence it is termed as logical of data link control. It divides the layer into parts such as data link control and the multiple access resolution/protocol. Therefore.The data link layer is used in a computer network to transmit the data between two devices or nodes. Random access protocols such as CSMA/CA have proven not to be an efficient solution.
![]() In that case, multiple stations access the channel and simultaneously transmits the data over the channel. Suppose there is no dedicated path to communicate or transfer the data between two devices. What is a multiple access protocol?When a sender and receiver have a dedicated link to transmit data packets, the data link control is enough to handle the channel. Data Link ControlA data link control is a reliable channel for transmitting data over a dedicated link using various techniques such as framing, error control and flow control of data packets in the computer network. Mechanism For Dynamic Tdma With Random Access Reservation Full Of StudentsIn random access protocol, one or more stations cannot depend on another station nor any station control another station. Random Access ProtocolIn this protocol, all the station has the equal priority to send the data over a channel. Therefore it is the responsibility of a teacher (multiple access protocol) to manage the students and make them one answer.Following are the types of multiple access protocol that is subdivided into the different process as: A. All the students respond at the same time due to which data is overlap or data lost. When a teacher asks a question, all the students (small channels) in the class start answering the question at the same time (transferring the data simultaneously). Hence, the multiple access protocol is required to reduce the collision and avoid crosstalk between the channels.For example, suppose that there is a classroom full of students. Using this method, any station can transmit data across a network simultaneously when a data frameset is available for transmission. And hence, it does not receive by the receiver end.Following are the different methods of random-access protocols for broadcasting frames on the channel.It is designed for wireless LAN (Local Area Network) but can also be used in a shared medium to transmit data. Due to the collision, the data frame packets may be lost or changed. However, if more than one station sends the data over a channel, there may be a collision or data conflict. It requires retransmission of data after some random amount of time.Whenever data is available for sending over a channel at stations, we use Pure Aloha. Hence, there is no collision detection. Acknowledgment of the frames exists in Aloha. Collision and data frames may be lost during the transmission of data through multiple stations. It does not require any carrier sensing. Microsoft publisher for apple macThe total vulnerable time of pure Aloha is 2 * Tfr. Therefore, it retransmits the frame until all the data are successfully transmitted to the receiver. And the station may assume the frame has been lost or destroyed. If it does not acknowledge the receiver end within the specified time, the station waits for a random amount of time, called the backoff time (Tb). When any station transmits the data frame to a channel, the pure Aloha waits for the receiver's acknowledgment. Only two frames, frame 1.1 and frame 2.2, are successfully transmitted to the receiver end. Some frames collide because most stations send their frames at the same time. Successful transmission of data frame is S = G * e ^ - 2 G.As we can see in the figure above, there are four stations for accessing a shared channel and transmitting data frames. In slotted Aloha, the shared channel is divided into a fixed time interval called slots. Both frames are completely finished, and both stations must retransmit the data frame.The slotted Aloha is designed to overcome the pure Aloha's efficiency because pure Aloha has a very high possibility of frame hitting. If the new frame's first bit enters the channel before finishing the last bit of the second frame. Whenever two frames fall on a shared channel simultaneously, collisions can occur, and both will suffer damage. It means that if the channel is idle, the station can send data to the channel. The total vulnerable time required in slotted Aloha is Tfr.It is a carrier sense multiple access based on media access protocol to sense the traffic on a channel (idle or busy) before transmitting the data. The probability of successfully transmitting the data frame in the slotted Aloha is S = G * e ^ - 2 G. Maximum throughput occurs in the slotted Aloha when G = 1 that is 37%. However, the possibility of a collision remains when trying to send a frame at the beginning of two or more station time slot. And 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 slot for the next time. If it is found that the channel is inactive, each station waits for its turn to retransmit the data. If the data is not transmitted, it waits for a ( q = 1-p probability) random time and resumes the frame with the next time slot.O- Persistent: It is an O-persistent method that defines the superiority of the station before the transmission of the frame on the shared channel. The P-Persistent mode defines that each node senses the channel, and if the channel is inactive, it sends a frame with a P probability. 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: It is the combination of 1-Persistent and Non-persistent modes. 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: It is the access mode of CSMA that defines before transmitting the data, each node must sense the channel, and if the channel is inactive, it immediately sends the data. Hence, it reduces the chances of a collision on a transmission medium.1-Persistent: In the 1-Persistent mode of CSMA that defines each node, first sense the shared channel and if the channel is idle, it immediately sends the data. After that, it waits for a random time before sending a frame to a channel. If any collision is detected in the CSMA/CD, the station sends a jam/ stop signal to the shared channel to terminate data transmission. If the frame is successfully received, the station sends another frame. Therefore, it first senses the shared channel before broadcasting the frames, and if the channel is idle, it transmits a frame to check whether the transmission was successful. The CSMA/CD protocol works with a medium access control layer. Detects the collision of the frame when a sender receives an acknowledgment signal.Following are the methods used in the CSMA/ CA to avoid the collision:Interframe space: In this method, the station waits for the channel to become idle, and if it gets the channel is idle, it does not immediately send the data. But if it gets two signals (its own and one more in which the collision of frames), a collision of the frame occurs in the shared channel. If the station receives only a single (own) acknowledgments, that means the data frame has been successfully transmitted to the receiver. When a data frame is sent to a channel, it receives an acknowledgment to check whether the channel is clear. It is a protocol that works with a medium access control layer. In the controlled access method, each station interacts and decides to send a data frame by a particular station approved by all other stations. Controlled Access ProtocolIt is a method of reducing data frame collision on a shared channel. If the channel is still busy, it does not restart the entire process, except that it restarts the timer only to send data packets when the channel is inactive.Acknowledgment: In the acknowledgment method, the sender station sends the data frame to the shared channel if the acknowledgment is not received ahead of time. When the station/ sender is ready to transmit the data frame, it chooses a random slot number of slots as wait time. However, the IFS time is often used to define the priority of the station.Contention window: In the Contention window, the total time is divided into different slots.
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