2.1 Medium Access Control Protocols for Wireless Sensor Networks

MAC PROTOCOL

Communication among wireless sensor nodes is usually achieved by means of a unique channel. It is the characteristic of this channel that only a single node can transmit a message at any given time. Therefore, shared access of the channel requires the establishment of a MAC protocol among the sensor nodes. The objective of the MAC protocol is to regulate access to the shared wireless medium such that the performance requirements of the underlying application are satisfied

the MAC protocol functionalities are provided by the lower sublayer of the data link layer (DLL).

MAC functions:

1. The assembly of data into a frame for transmission by appending a header field containing addressing information and a trailer field for error detection.

2. The disassembly of a received frame to extract addressing and error control information to perform address recognition and error detection and recovery

3. The regulation of access to the shared transmission medium in a way commensurate with the performance requirements of the supported application

FUNDAMENTAL OF MAC

One major difficulty in designing effective MAC protocols for shared access media arises from the spatial distribution of the communicating nodes.

the nodes must exchange some amount of coordinating information before getting knowledge about the access which recursively requires use of the communication channel itself.

This increases the complexity of the access control protocol and consequently, the overhead required to regulate access among the competing nodes.

spatial distribution does not allow a given node on the network to know the instantaneous status of other nodes on the network.

Two main factors, the intelligence of the decision made by the access protocol and the overhead involved, influence the aggregate behavior of a distributed multiple- access protocol.

Most of the proposed distributed multiple-access protocols for WSNs operate somewhere along a spectrum of information ranging from a minimum amount of information to perfect information.

Furthermore, the information can be predetermined, dynamic global, or local.

Predetermined information is known to all communicating nodes.

Dynamic global information is acquired by different nodes during protocol operation.

Predetermined and dynamic global information may result in efficient, potentially perfect coordination

among the nodes. However, there usually is a high price to pay in terms of wasted

channel capacity.

Local information is known to individual nodes. The use of local information has potential to reduce the overhead required to coordinate the competing nodes, but may result in poor overall performance of the protocol.

performance requirements of MAC protocols

issues such as delay, throughput, robustness, scalability, stability, and fairness have dominated the design of MAC protocols

1. Delay
   Delay refers to the amount of time spent by a data packet in the MAC layer before it is transmitted successfully.
   For time-sensitive applications, MAC protocols must ensure delay-bound guarantees to meet Quality of Service (QoS) requirements.
   Delay guarantees can be probabilistic, with expected values, variance and confidence intervals, or deterministic, ensuring predictable number of state transitions between message arrival and transmission.
   
   
2. Throughput
   Throughput in a communication system refers to the rate at which messages are serviced, often measured in messages or bits per second. In wireless environments, it represents the portion of channel capacity used for data transmission. Initially, throughput increases as system load rises. However, beyond a certain threshold, further load increments may not raise throughput and can even cause it to decline. An essential goal of MAC protocols is to optimize channel throughput while minimizing message delay.
   
   
3. Robustness
   Robustness in communication protocols encompasses reliability, availability, and dependability, indicating the protocol's resilience to errors and misinformation. It involves various strategies such as error confinement, detection, masking, reconfiguration, and restart. Achieving robustness in dynamic networks like WSNs is challenging due to varying failure models of links and nodes. It requires addressing multidimensional concerns simultaneously to ensure uninterrupted and dependable communication despite adverse conditions.
   
   
4. Scalability
   Scalability in communication systems refers to maintaining performance regardless of network size or node count. In Wireless Sensor Networks (WSNs), scalability is crucial, especially with potentially large node counts. Achieving scalability is challenging, particularly in dynamic wireless environments. Strategies involve avoiding reliance on global network states and localizing interactions through hierarchical structures and information aggregation. For example, clustering sensor nodes facilitates scalable medium access protocols, while aggregating sensor data enables efficient traffic pattern utilization, crucial for scaling MAC protocols in large-scale WSNs.
   
   
5. Stability
   Stability in a communication system refers to its ability to handle traffic load fluctuations over extended periods. A stable MAC protocol should manage instantaneous loads exceeding sustained levels, provided the long-term load stays within channel capacity. Typically, MAC protocol scalability is assessed in terms of delay or throughput. A delay-stable MAC protocol ensures bounded message waiting times, maintaining a limited backlog in the transmission queue. For throughput stability, the protocol sustains throughput levels despite increasing load. Maintaining stability in time-varying large-scale WSNs is challenging, but adaptation to traffic fluctuations through careful bursty traffic scheduling is a potential solution
   
   
6. Fairness
   A MAC protocol is deemed fair when it evenly distributes channel capacity among competing nodes while maintaining network throughput. Fairness ensures equitable QoS, preventing certain nodes from being favored over others. However, achieving fairness becomes complex when nodes have varied traffic patterns and QoS requirements. To address this, nodes may be assigned weights reflecting their resource share, achieving proportional fairness based on these weights. Proportional fairness ensures no node's service rate drops below its fair share when another node's allocation increases.
   In wireless networks, fair resource allocation is challenging due to the need for global information to coordinate medium access. Time-varying wireless links further complicate calculating each node's fair share, even with centralized resource allocation methods.
   
   
7. Energy Efficiency
   Wireless sensor nodes, powered by small-capacity batteries and often deployed in unattended environments, face severe energy constraints that impact their lifetime. Energy conservation is crucial to prolong node lifespan, with low-power electronics integration being a fundamental step. However, efficient operation of processing and communication capabilities is equally vital to maximize energy gains. This necessitates the design of energy-aware communication protocols, especially at the MAC layer.
   Several sources contribute to energy inefficiency in MAC protocols. Collisions, occurring when multiple nodes transmit simultaneously, increase energy consumption due to packet retransmissions. Idle listening, where nodes monitor silent channels, also wastes energy. Overhearing, receiving packets not intended for the node, can dissipate significant power. Control packet overhead, necessary for channel access regulation, can further drain energy if excessive. Additionally, frequent mode transitions, (frequent switching) like between sleep and active modes, lead to significant energy consumption.
   Energy-efficient link-layer protocols mitigate these issues by controlling radio activity to minimize energy waste because of mentioned causes.. Comprehensive energy management schemes, addressing not only radio usage but also other sources of energy consumption, further enhance energy savings.

Common Protocols
The MAC method chosen significantly impacts the performance of a Wireless Sensor Network (WSN). Various strategies have been proposed to address the shared medium access problem, aiming to balance resource allocation quality and associated overhead. These strategies fall into three major categories: fixed assignment, demand assignment, and random assignment.

1. FIXED ASSIGNMENT
   Fixed assignment strategies allocate resources (e.g., time slots, frequencies) to nodes in a predetermined manner.
   Nodes are assigned specific resources for communication, regardless of demand or network conditions.
   Each node uses its allocated resources exclusively without competing with other nodes.
   This approach simplifies coordination but may lead to inefficiencies under varying traffic loads or dynamic network conditions.
   Three common protocols in this category include Frequency-Division Multiple Access (FDMA), Time-Division Multiple Access (TDMA), and Code-Division Multiple Access (CDMA).
   
   A> FDMA (Frequency-Division Multiple Access):
   Divides the available bandwidth into subchannels, allocating different carrier frequencies to communicating nodes.
   Nodes communicate on separate frequency bands to avoid interference, requiring frequency synchronization.
   Communication occurs when the receiver tunes to the transmitter's assigned frequency.
   B> TDMA (Time-Division Multiple Access):
   Divides the radio frequency into time slots, assigning unique slots to each node.
   Nodes take turns transmitting and receiving in a round-robin fashion within their allocated time slots.
   Only one node uses the channel at any given time during a time slot.
   C>CDMA (Code-Division Multiple Access):
   It Utilizes spread spectrum technology
   spreading the signal over a wider bandwidth than necessary for the data rate.
   Signals are combined with noise-like signals (chipping codes) for transmission, making it difficult to detect or intercept.
   CDMA systems may use frequency hopping (FHSS) or direct sequence (DSSS) techniques.
   Frequency Hopping Spread Spectrum (FHSS): Modulates data with a narrowband carrier signal that hops between frequencies in a predictable pattern.
   Direct Sequence Spread Spectrum (DSSS): Divides data into chunks and combines it with a higher-rate chipping code, increasing resistance to interference.
   
   
   
2. DEMAND ASSIGNMENT
   Demand assignment strategies dynamically allocate resources based on nodes' communication needs.
   Nodes request resources as needed, and the MAC protocol assigns resources accordingly, aiming to optimize utilization and reduce contention.
   This approach offers flexibility and adaptability to changing network conditions but may incur higher overhead due to resource negotiation and management.
   Unlike fixed-assignment schemes, demand assignment protocols consider only nodes ready to transmit, ignoring idle nodes. The channel is allocated to the selected node for a specified duration, which can vary from a fixed time slot to the time required to transmit a data packet.
   Demand assignment protocols require a mechanism to arbitrate channel access among contending nodes.A logical control channel may be used for nodes to request access to the communication medium.
   Demand assignment protocols can be centralized or distributed.Centralized schemes, like polling, involve a master control device querying slave nodes. Distributed schemes, like token- and reservation-based protocols, distribute control among network nodes.
   A> POLLING
   In a polling scheme, a master control device sequentially queries each slave node to check if it has data to transmit. If a polled node has data, it notifies the controller, which then allocates the channel for transmission. The ready node utilizes the full data rate to transmit its traffic. If a node has no data to transmit, it declines the controller's request, and the controller moves on to query the next node.
   The main advantage of polling is that it ensures all nodes have equal access to the channel. Additionally, high-priority nodes can be polled more frequently to give them preference. However, polling incurs substantial overhead due to the large number of messages generated by the controller to query nodes. Moreover, the efficiency of the polling scheme relies on the reliability of the controller, which can be a drawback in case of controller failures or disruptions.
   B> RESERVATION
   In a reservation-based scheme, time slots are allocated for carrying reservation messages, termed minislots, which are typically smaller than data packets. When a station has data to send, it requests a data slot by sending a reservation message to the master during a reservation minislot.
   In some reservation-based schemes, such as fixed-priority-oriented demand assignment, each station is assigned its own minislot. In others, like packet demand assignment multiple access, stations contend for access to a minislot using distributed packet-based contention schemes like slotted ALOHA. Upon receiving reservation requests, the master computes a transmission schedule and announces it to the slaves.
   In a reservation-based scheme, collision avoidance can be achieved if each station has its own reservation minislot. Additionally, if reservation requests include a priority field, the master can prioritize urgent data over delay-insensitive data. Packet collisions only occur when stations contend for the minislot, which utilizes a small fraction of total bandwidth. This ensures efficient utilization of the majority of the bandwidth assigned to data packets.
   
   

RANDOM
Random assignment protocols allocate channel resources to nodes using probabilistic methods, without predetermined schedules or reservations. Instead, nodes contend for access to the channel, and collisions are resolved using backoff mechanisms or random access protocols.
These strategies do not assign any predictable or scheduled time for any node to transmit. All backlogged nodes must contend to access the transmission medium.
Collision occurs when more than one node attempts to transmit simultaneously. To deal with collisions, the protocol must include a mechanism to detect collisions and a scheme to schedule colliding packets for subsequent re-transmissions.
Random assignment protocols offer simplicity and efficiency, especially in low to moderate load scenarios. However, under heavy traffic or congestion, the performance of random assignment protocols may degrade due to increased collisions and contention for channel access.
Random access protocols were initially devised for long radio links and satellite communications. The ALOHA protocol, commonly known as pure ALOHA, was among the earliest media access protocols. In pure ALOHA, nodes transmit data whenever they have information to send.
Efforts to enhance the efficiency and performance of pure ALOHA gave rise to various improved schemes, CSMA, CSMA/CD, CSMA/CA.

A. PURE ALOHA
Simple Random Access Protocol:
ALOHA facilitates access to a shared transmission medium among uncoordinated users.
It was initially developed for ground-based packet broadcasting networks and and was used to connect remote users to mainframe computers.
Asynchronous Channel Access:
Channel access in pure ALOHA is asynchronous and independent of ongoing activity on the transmission medium.
Nodes are allowed to transmit data whenever they are ready, without coordination.
Transmission and Acknowledgment:
After transmission, a node listens for a period equal to the longest round-trip propagation time in the network. If an acknowledgment is received within this time frame, the transmission is considered successful. Acknowledgment is determined by examining the correctness of the received data through error checksum.
Retransmission Mechanism:
In the absence of acknowledgment within the specified time, the transmitting node assumes a failure. The data may then be retransmitted. If the number of transmission attempts exceeds a predefined threshold without acknowledgment, the node refrains from further retransmissions and reports an fatal error.
Decentralized and Easy to Manage:
ALOHA requires no central control, allowing for easy addition and removal of nodes.
It is simple to implement and operate.
Drawback: Performance Degradation with Increased Collisions:
One major drawback of ALOHA is its degraded performance under heavy loads. As the number of collisions increases with higher traffic, network performance deteriorates rapidly. This limitation makes ALOHA less effective in scenarios with significant network congestion.

B. SLOTTED ALOHA
Slotted ALOHA was proposed as an enhancement to improve the performance of the original ALOHA protocol.
Synchronization and Slot Division:
In slotted ALOHA, all communication nodes are synchronized and all packets have the same length. The communication channel is divided into uniform time slots, each having the same duration as the transmission time of a data packet. Transmission is allowed only at the beginning of a time slot, meaning that communication can occur only at slot boundaries.
Collision Handling:
With transmission restricted to slot boundaries, collisions can only occur at the start of a slot. Colliding packets overlap entirely in time.
Improved Channel Utilization:
By limiting channel access to slot boundaries, the length of collision intervals is significantly reduced.
This reduction leads to increased utilization of the communication channel compared to pure ALOHA.
Drawback
Despite the performance improvement offered by slotted ALOHA, both ALOHA and pure ALOHA protocols remain inefficient under moderate to heavy load conditions. Additionally, in networks where propagation delay is much shorter than the transmission time of a data packet, nodes can quickly detect ongoing transmissions

Enhancements
1. CSMA (Carrier Sense Multiple Access)
protocols operate in both continuous time, known as unslotted CSMA, and discrete time, called slotted CSMA. These protocols can be further categorized into nonpersistent CSMA and persistent CSMA, depending on their strategies for channel access and contention resolution.
Nonpersistent CSMA:
When a node is ready to transmit, it first senses the carrier to check if the channel is idle. If the channel is clear, the node transmits its packet immediately and waits for an acknowledgment. The acknowledgment timeout value considers round-trip propagation delay and acknowledgment transmission time. In case of no acknowledgment, the node assumes packet loss due to collision or noise and schedules retransmission. If the channel is busy, the node backs off for a random period before attempting to sense the channel again. This process repeats until successful transmission. Advantages include minimized interference between transmissions, but drawbacks include potential channel idleness during back-off periods, reducing overall throughput. 

Persistent CSMA: 
1-Persistent CSMA: A node continuously listens to the channel until it becomes idle. Once idle, the node transmits immediately. Ensures the channel is never left idle if a node is ready to transmit. Suitable for scenarios where immediate transmission is critical. p-Persistent CSMA: The p-persistent algorithm represents a compromise between the nonpersistent and 1-persistent schemes. Nodes transmit with a probability p when the channel is sensed idle. If not transmitted, the node waits for a specific time period before attempting transmission again. The value of p influences protocol stability and throughput. High p values increase collision likelihood under heavy traffic, while low p values may cause unnecessary waiting under light traffic. 2. CSMA/CD CSMA/CD (Carrier Sense Multiple Access with Collision Detection) is an enhancement of CSMA protocols designed to reduce collision intervals and improve network efficiency. Here's a breakdown of its operation and challenges: Operation: In CSMA/CD, nodes listen to the channel before transmitting to detect ongoing transmissions. If the channel is idle, the node starts transmitting its data while continuing to monitor the channel. If a collision is detected while transmitting, the node immediately aborts its transmission, minimizing wasted bandwidth. After a collision, contending nodes wait for a random period before attempting retransmission. The waiting time after collision is determined by a probabilistic algorithm, such as the truncated binary exponential back-off algorithm. Drawbacks: Provisioning sensor nodes with collision detection capabilities poses challenges due to limited storage, processing power, and energy resources. Wireless transceivers are typically half-duplex, requiring additional circuitry for collision detection.To detect collision, the sensor node must therefore be capable of ‘‘listening’’ while ‘‘talking.’’ Cost and complexity constraints in WSNs limit the feasibility of implementing CSMA/CD. Detecting collisions in a wireless environment is challenging due to signal attenuation, time-varying channel properties, and rapid signal power decrease over distance. In wired mediums, collision detection at the transmitter can infer collisions at the receiver due to consistent signal-to-noise ratios. However, wireless environments lack this unambiguous inference capability. Overall, while CSMA/CD offers advantages in reducing collision intervals, its implementation in wireless environments like WSNs is hindered by technical challenges and cost considerations. CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) addresses the hidden- and exposed-node problems inherent in wireless environments. The hidden- and exposed-node problems result indirectly from the time-varying properties of the wireless channel, which are caused by physical phenomena such as noise, fading, attenuation, and path loss. Here's a breakdown of its operation and the methods used to mitigate these issues: Hidden-Node Problem: Definition: A hidden node is within the range of the destination node but out of range of the transmitting node. Scenario: Node A wants to transmit to Node B, and Node C also wants to transmit to Node B. Nodes A and C are out of each other's range but within Node B's range. Issue: Both Node A and Node C transmit simultaneously, causing collisions at Node B, resulting in packet loss. Exposed-Node Problem: Definition: An exposed node is within the range of the sender but out of the range of the destination. Scenario: Node B wants to transmit to Node A, and Node C wants to transmit to Node D. Node D is within Node B's range but out of Node A's range. Issue: Node C senses the channel as busy due to Node B's transmission, unnecessarily delaying its own transmission. Approaches to Mitigate Hidden- and Exposed-Node Problems: Busy-Tone Approach: Uses separate data and control channels. Receiver emits a busy tone on the control channel while receiving data, indicating to other nodes that it's busy. Nodes must listen to the control channel before transmitting to avoid collisions. Solves both hidden- and exposed-node problems but requires duplex operation, increasing node complexity and cost. RTS/CTS Handshake: Nodes perform a handshake before transmitting to avoid collisions. Node A sends a Request-to-Send (RTS) packet to Node B. Node B responds with a Clear-to-Send (CTS) packet, indicating it's ready to receive. Node A then transmits its data packet. Used widely but doesn't completely solve hidden-node problem. Limitations of RTS/CTS Handshake: Scenario 1: Collisions can occur if Node D starts transmitting while Node B is sending the CTS packet. Scenario 2: Collisions can occur if Node C starts transmitting while Node B is sending the CTS packet, causing interference with Node A's transmission. While CSMA/CA with RTS/CTS handshake greatly reduces collisions and increases bandwidth utilization, it doesn't completely eliminate hidden- and exposed-node problems. However, it's widely used in wireless networks to mitigate these issues and improve network performance. SENSOR-MAC CASE STUDY The Sensor-MAC (S-MAC) protocol aims to reduce energy waste in wireless sensor networks by addressing issues like collisions, idle listening, control overhead, and overhearing. While it prioritizes energy efficiency and network stability, it may sacrifice some performance in per-hop fairness and latency.S-MAC uses multiple techniques to reduce energy

consumption, control overhead, and latency, in order to improve application-level

performance.

Assumptions:

1. Large number of sensor nodes with limited resources (storage, communication, processing).
2. Nodes form an ad hoc, self-organized, and self-managed network. Data transmission follows a store-and-forward approach.
3. Applications alternate between long idle periods and bursty active periods, with tolerance for increased latency to extend network lifetime. Typical applications include surveillance, habitat monitoring, and infrastructure protection, where sensors remain inactive until triggered by an event. Where The frequency at which these events occur is typically orders of magnitude slower than the time it takes to transmit a message across the network toward the base station.

Techniques used in SMAC:

1. Periodic Listen and Sleep Operations
   Nodes periodically transition between active and sleep states.
   During the sleep state, the node's radio is completely turned off to conserve energy.
   Nodes become active and turn on their radios only when there is traffic or communication activity in the network.
   Each node in the network sets a wake-up timer, determining the duration of the sleep period.
   Once the timer expires, the node wakes up and enters the listen state to check for any communication needs.
   This cycle, comprising sleep and listen states, constitutes a frame in the S-MAC protocol.
   The duty cycle of a node is defined as the ratio of the listening interval to the frame length.
   While nodes have the flexibility to independently select their listening intervals, the protocol assumes uniform values for simplicity.
   Although nodes are capable of scheduling their own sleep and listen intervals, coordinating schedules among neighboring nodes is advantageous.
   Coordinated scheduling helps minimize the control overhead required for communication between neighboring nodes.
   
   
2. Schedule selection and coordination
   In S-MAC, neighboring nodes coordinate their sleep and listen schedules to ensure synchronized operation, minimizing energy waste and optimizing communication efficiency.
   Each node independently selects a schedule for sleep and listen periods.
   To select a schedule, a node listens to the channel for a fixed duration, at least as long as the synchronization period.
   If no schedule is heard from neighboring nodes during this period, the node chooses its own schedule and broadcasts a SYNC packet to announce it.
   Physical carrier sensing is performed before broadcasting the SYNC packet to reduce the chance of collisions.
   If a node receives a schedule from a neighbor before announcing its own, it adopts the received schedule and waits until the next synchronization period to announce it to other nodes.
   If a node receives a different schedule after announcing its own, it may discard its own schedule and adopt the new one if it has no other neighbors with a shared schedule.
   If the node has neighbors already using its schedule, it adopts both schedules and adjusts its waking times accordingly.
   Carrying multiple schedules allows border nodes to broadcast only one SYNC packet, simplifying coordination.
   However, border nodes consume more energy as they spend less time in sleep mode.
   
   
3. Schedule Synchronization
   In S-MAC, neighboring nodes periodically synchronize their schedules to prevent long-term clock drift. Schedule updating is achieved through the transmission of SYNC packets. To ensure that nodes can receive both SYNC packets and data packets, the listen interval is divided into two subintervals: one for SYNC packet transmission and the other for data packet transmission.
   This division allows nodes to receive synchronization information as well as data from neighboring nodes effectively.
   Access to the channel during these subintervals is managed using a multislotted contention window.
   In the SYNC packet subinterval, contending stations randomly select a time slot, perform carrier sensing, and transmit their SYNC packets if the channel is idle.
   During the data packet subinterval, the RTS/CTS handshake is used to secure exclusive access to the channel for data transmission.
   This access procedure guarantees that neighboring nodes can receive both synchronization and data packets effectively.
   
   
4. Adaptive Listening
   Adaptive listening is an aggressive technique employed by the S-MAC protocol to mitigate the increased latency that may occur due to the periodic listen and sleep scheme.
   Adaptive listening allows nodes to dynamically adjust their wake-up schedules based on the activity they overhear in the network, enabling more efficient packet forwarding and reducing latency in data transmission.
   When a node overhears the exchange of a Clear to Send (CTS) or Request to Send (RTS) packet between a neighboring node and another node during its listen period, it assumes that it may be the next hop along the routing path of the overheard packet.The overhearing node ignores its own wake-up schedule and schedules an extra listening period around the time the transmission of the packet terminates.
   The overhearing node determines the time necessary to complete the transmission of the packet from the duration field of the overheard CTS or RTS packet.
   Immediately upon receiving the data packet, the overhearing node issues an RTS packet to initiate an RTS/CTS handshake with the neighboring node involved in the transmission.
   If the neighboring node is awake, the packet forwarding process proceeds immediately between the two nodes, reducing latency.If the overhearing node does not receive an RTS packet during adaptive listening, it returns to its sleep state until the next scheduled listen interval.
   
   
5. Access Control and Data Exchange
   S-MAC employs a CSMA/CA-based procedure to regulate access to the communication channel among contending sensor nodes, aiming to mitigate the hidden and exposed terminal problems. Virtual Carrier Sensing with NAV:
   S-MAC utilizes the Network Allocation Vector (NAV) to perform virtual carrier sensing. The NAV variable contains the remaining time until the end of the current packet transmission.
   Initially, the NAV value is set to the duration field of the transmitted packet. It decrements over time until it reaches zero, indicating the end of the transmission.
   Nodes cannot initiate their own transmission until the NAV value reaches zero, ensuring that the channel remains clear.
   Physical Carrier Sensing:
   Nodes perform physical carrier sensing by listening to the channel to detect ongoing transmissions.
   Carrier sensing is randomized within a contention window to avoid collisions and starvation.
   A node is permitted to transmit only when both virtual and physical carrier sensing indicate that the channel is free.
   Overhearing Avoidance:
   To prevent energy waste due to packet overhearing, nodes enter sleep mode after overhearing the exchange of an RTS or CTS packet between two other nodes.
   The node sets its NAV with the duration field value from the RTS or CTS packet and enters sleep mode until the NAV reaches zero.
   Since data packets are usually larger than control packets, this overhearing avoidance process can lead to significant energy savings.
   Transmission Procedure:
   When a node wishes to transmit a message, it first senses the channel. If the channel is busy, the node goes to sleep and waits for it to become free again.
   If the channel is idle, the node sends an RTS packet and waits for a CTS packet from the receiver.
   Upon receiving the CTS packet, the node sends its data packet. The transaction concludes when the node receives an acknowledgment from the receiver.
   During this data exchange, nodes temporarily adjust their sleep schedules and do not resume their regular sleep cycles until the data transmission is complete.
   Broadcast packets, such as SYNC packets, do not require the RTS and CTS packet exchange. They can be transmitted directly without this handshake process.
   
   
6. Message Passing
   In S-MAC, the concept of message passing is introduced to enhance application-level performance. message passing in S-MAC optimizes the transmission process, reduces control overhead, and enhances energy efficiency, making it beneficial for applications where low-latency requirements are not stringent.
   Message Division:
   Messages are divided into small fragments to facilitate efficient transmission.
   These fragments constitute meaningful units of data that a node can process.
   Single Burst Transmission:
   Fragments of a message are transmitted in a single burst, reducing control overhead and improving efficiency.
   Only one RTS/CTS exchange occurs between the sending and receiving nodes to reserve the medium for the entire message transfer.
   Fragment Transmission:
   Each fragment carries in its duration field the time required to transmit all subsequent fragments and their corresponding acknowledgments.
   Upon transmitting a fragment, the sender waits for an acknowledgment from the receiver.
   If the acknowledgment is received, the sender proceeds with transmitting the next fragment.
   If the acknowledgment is not received, the sender extends the transmission time to include the next fragment and its acknowledgment, and then immediately retransmits the unacknowledged fragment.
   Energy Savings:
   Nodes that are in sleep mode can only become aware of transmission extensions if they hear extended fragments or their corresponding acknowledgments.
   Nodes that only heard the initial RTS and CTS packet exchange remain unaware of the transmission extension.
   This approach can lead to significant energy savings, making S-MAC suitable for applications where fairness is not critical, and increased latency is tolerable.