The channel access protocol plays a decisive role in the performance of the network and is one of the research focuses of WSNs technology. When the channel access protocol of a single channel increases, the network performance obviously deteriorates. Using multi-channel mechanism can improve network throughput, reduce propagation delay, reduce the probability of collision, and more easily support network QoS (Quality of Service) guarantee. The multi-channel protocol should mainly solve two problems: channel allocation and access control. Channel allocation is to allocate corresponding channels to different communication nodes. Access control is to determine the timing of node access to the channel, and to solve competition and avoid conflicts. problem. This article introduces three kinds of multi-channel channel access protocols: multi-channel CSMA (Carrier Sense MulTIple Access), FAMAC (Frequency Assignment based mulTI-channel MulTIple Access Control), DCA-PC, where DCA-PC is the focus of this article.
Multi-channel MAC layer protocol based on CSMA and FAMAC
â— Multi-channel CSMA
It is a channel access protocol based on carrier sensing. The design goal is to reduce the impact of hidden terminal problems and reduce the collision of data packets by using multiple channels. It uses quasi-channel reservation technology to distribute multiple channels through distributed carrier monitoring.
The principle of the quasi-channel reservation technology is that when a node sends a message, it preferentially selects the channel used last time; if the channel is busy, it randomly selects an idle channel to send data through carrier monitoring.
Multi-channel CSMA divides the available channels into N non-overlapping channels. Generally speaking, N is smaller than the number of nodes in the wireless sensor network. The bandwidth of each sub-channel is 1 / N of the entire channel bandwidth.
â— FAMAC protocol
FAMAC is a multi-channel access protocol based on frequency allocation, setting a control channel and multiple data channels. During implementation, all nodes use a fixed frequency as a control channel to complete the interaction of RTS (Request To Send) and CTS (Clear To Send). The node in the idle state resides on the control channel. In addition, each node is assigned a different frequency as its data channel value. During channel selection, the RTS sent by the sender to the receiver carries its own frequency information. After receiving the RTS, the receiver records the sender's frequency, returns the CTS, switches the radio to the sender's frequency, and receives the data.
The above two protocols are designed for multi-channel, which better solves the problems of access control and channel selection; the influence of hidden terminals and exposed terminals is also well controlled. Because the nodes of WSNs usually operate in a harsh and even dangerous remote environment inaccessible to people, energy cannot be replaced and it is difficult to supplement. Therefore, the constraints of energy constraints require that the MAC protocol should first focus on energy efficiency and save energy as much as possible, and the above two protocols have not given sufficient consideration to this. From the viewpoint of saving energy consumption, the DCA-PC protocol explores the possibility of integrating two mechanisms of power control and multi-channel access in the design of the MAC protocol.
Multi-channel protocol DCA-PC with power control
The DCA-PC protocol first combines the concept of power control with multi-channel access in the MAC layer design of WSNs, saving energy consumption, reducing mutual interference when neighboring nodes share channels, and improving channel reuse.
â— Channel model
The bandwidth is divided into a control channel and n data channels D1, D2, ..., Dn. The control channel is used to control the transmission of messages. The goal is to allocate data channels reasonably to the nodes to avoid conflicts; the data channel is used to transmit data messages and ACK (Acknowledgement) messages.
From the perspective of avoiding conflicts and improving the success rate of channel reservations, the maximum power of control message transmission is used; in order to reduce energy consumption and improve channel reuse, the transmission power of data packets is divided into several levels of different sizes. The CTS handshake can calculate the minimum power necessary for communication between the two parties. The data level used for data packet transmission is closest to the minimum transmission power.
In order to achieve dynamic channel allocation and power control, each node must save 3 arrays. Take node A as an example to illustrate:
Power [i] Power list: The power level that node A should use when sending data packets to node i. (——It can be calculated by the principle of power control.)
CUL (Channel Usage List) [i] Channel usage status list: A list of used channels learned by node A. CUL [i] has 4 fields:
——CUL [i] .host: record the host of the neighbor node of node A.
——CUL [i] .ch: record the data channel occupied by CUL [i] .host.
——CUL [i] .rel-TIme: indicates the time to release the CUL [i] .ch data channel.
——CUL [i] .int records whether the signal sent by CUL [i] .host will be heard by node A (CUL [i] .int value is 1 or 0).
FCL free channel list: A list of available channels when a node sends data, which can be calculated according to CUL.
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