- Wireless Overview - The MAC level
- Table of Contents
- 1. Introduction
- Medium Access Control Protocols for Cognitive Radio Ad Hoc Networks: A Survey
When batteries are depleted, nodes must be replaced or recharged. As an illustration, consider the application presented in [ 3 ], where a node transmits every 80 milliseconds and the hardware platform uses Another example application is leakage in an industrial plant with hazardous chemicals. People must evacuate, but a sensor network may be deployed by dropping nodes from a plane.
In this case there is no control over the network topology and no way to recharge batteries either. An additional complication is that individual monitoring applications have widely different requirements in throughput, delay, network topology, and so forth. Regarding physical topology, the bridge monitoring and the chemical leak monitoring are applications using nodes possibly located in random positions. In contrast, if the situation is patient monitoring in a medical facility, the network may need a specific layout in order to avoid interference with medical equipment.
Regarding delay, human health monitoring may have a tighter delay requirement than the other two mentioned applications since vital signs of the patient may indicate the need of immediate treatment. Since different applications have different requirements, WSNs will employ a family of communication standards, each member designed to optimize the critical parameter s. Since the terminology for wireless sensor networks is often used with different meanings in the literature, a single, common set of definitions is necessary to prevent confusion.
The MAC layer main functions are frame delimiting and recognition, addressing, transfer of data from upper layers, error protection generally using frame check sequences , and arbitration of access to one channel shared by all nodes [ 4 ]. Additionally, protocols must be scalable according to the network size and should adapt to changes in the network such as addition of new nodes, death of existing nodes, and transient noise on the wireless channel [ 5 ].
Node state where the radio is turned off [ 6 ]. Data unit containing information from a MAC layer protocol and possibly from upper layers [ 4 ]. Data unit with information from a network layer protocol and possibly from upper layers [ 4 ]. Event where two or more frames are received at the same time, damaging the resulting signal.
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- Medium access control.
All information is lost [ 5 ]. To receive a packet whose destination is any other node [ 6 ]. Overhearing results in wasted energy. Another source of wasting energy occurs when a node has its radio on, listening to the medium while there are no transmissions [ 6 ]. To transmit a message when the destination is not ready for receiving it. Energy for sending the message is wasted [ 5 ]. All frames containing protocol information and not application data. Energy for transmitting and receiving these frames is considered to be wasted [ 6 ].
Wireless Overview - The MAC level
Phenomenon present in some analog modulation schemes, such as frequency modulation FM. Two signals with different amplitudes arrive at a receiver and go through the passband filter at the same time. The lower amplitude signal is greatly attenuated at the demodulator output, so the stronger signal is successfully received [ 7 ].
Sending a message to all nodes in the network [ 5 ]. Most clocks in networking equipment use quartz oscillators, which change with age, temperature, magnetic fields, and mechanical vibration. As the oscillator changes, the time presented by the clock also changes and this is called clock drift [ 8 ].
Standards for wireless communications exist for different applications: cellular telephony, satellite communications, broadcast radio, local area networks, and so forth. Three well-known standards for wireless data communication have been proposed for use in WSNs, each with certain advantages. However, WSNs do not have widely accepted standard communication protocols in any of the layers in the OSI model sense.
The following subsections describe standardized protocols which may match WSN requirements. The protocols provide wireless data transmission with appropriate data rates for a wide range of applications, they can be implemented in battery-powered devices, and they do not require complicated planning and setup. Several commercial products use these wireless standards, which could be an advantage for WSNs in cost and ease of implementation.
Table of Contents
The purpose of this section is to familiarize the reader with the standards, show their advantages and disadvantages, and discuss their use in WSNs. The MAC protocol has two modes [ 9 ]. Mode with no central device controlling the communication. Carrier sensing: a node senses the medium. If it is idle, the node transmits the data frame. If the medium is busy, the node waits until it becomes idle again, waits for a random time and transmits. Upon frame reception, the receiver node answers with an ACK acknowledgment control frame. If a collision occurs, transmitting nodes wait a random time and try again later.
Virtual carrier sensing: a node with a frame to transmit senses the medium. If it is idle, the node sends a control frame called RTS request to send , which contains the intended receiver address and the time required to send the information transmission delay. If the destination node agrees to communicate, it will answer with a CTS clear to send control frame which also contains the delay. All nodes hearing RTS or CTS should refrain from transmission until the transmission delay has elapsed and the medium is idle again.
The receiver must respond with an ACK for each data frame received. A special node, the access point AP , polls every node and controls the communication process. An AP periodically broadcasts a beacon control frame with parameters and invitations to join the network [ 9 ].
Advantages of IEEE Data rates are high for wireless end user transmission and radio ranges can be hundreds of meters. Also, as IEEE Disadvantages include the large overhead in control and data packets. Another possibility is using UDP which employs less overhead, 8 bytes for the header. Perhaps the most important problem for using Even though the standard has power saving mechanisms, according to Ferrari et al. In this standard, the physical layer uses 2. However, some Bluetooth devices have meter range [ 12 ].
An These networks are referred to as piconets and several piconets may communicate using a bridge node, forming a scatternet. In one time slot, the master will poll a single slave, inquiring if it has something to send. If the slave has data to transmit, it sends it to the master in the next time slot [ 13 ]. A master node must periodically transmit, even if there is no data to be exchanged, to keep slaves synchronized. Slaves cannot communicate directly; the information must go through the master node.
Figure 1 illustrates an example scatternet [ 14 ]. Figure 1 Bluetooth scatternet. Shaded circles are master nodes in two different piconets, A and B.
White circles show slaves belonging to each piconet. Active slaves are labeled according to the network they belong to; inactive slaves have label I.
AB is a slave belonging to both piconets. The figure shows coverage areas of both master nodes. An advantage to using Disadvantages of WSNs literature calls this organization cluster based, and the master node is referred to as cluster head [ 15 ]. Another problem arises in applications with random deployment because it is not always possible to ensure that all slave nodes are within range of the master. Additionally, the periodic transmissions used for synchronization waste energy at both the transmitter and the receivers.
The IEEE defined physical and MAC layer characteristics for establishing connectivity between devices with low-power consumption, low cost, and low data rate. Frequency bands are 2. The 2. Typical radio range according to the standard is 10 meters. Maximum packet size is bytes with payload of bytes. The One FFD can be a network coordinator, a router, or a gateway which connects the network to other networks. FFDs can communicate with any other device [ 17 ]. RFDs have low-power consumption and low complexity. Figure 2 presents two possible topologies using Figure 2 a Star topology with one full function device and 4 reduced function devices white circles.
A node checks the medium; if busy, it waits for a random period of time before trying to transmit. If idle, the node transmits. Beacon mode employs two periods: active divided in 16 time slots and inactive devices enter a low-power mode , as presented in Figure 3 [ 17 ]. At the beginning of the active period, the coordinator sends beacon frames with information regarding the period duration so the duty cycle can vary. A node waits for a random time, then checks the medium and if the channel is clear, transmits.
Medium Access Control Protocols for Cognitive Radio Ad Hoc Networks: A Survey
If the channel is busy, the device waits again. The first waiting time before checking the medium can be very small to minimize idle listening in low traffic. The node can sleep immediately after receiving an acknowledgement. Figure 3 Beacon mode structure. The coordinator cannot interact with the PAN during the inactive period and may sleep [ 17 ].
The major advantage to using Disadvantages to using However, like MICA2 uses the same microcontroller and memory but the radio works at The HART Communication Foundation extended the wired HART protocol for communication requirements in industrial plants, specifically compensating for electrically noisy environments and real-time delay constraints.
The protocol defines functionality in the physical, MAC, network, transport and application layers [ 20 ]. A blacklist feature blocks occupied channels, so hopping can take place at most in 16 different frequencies.