Wireless medium access issues and MAC protocols

What is Wireless Medium Access

• Wireless medium access in the Internet of Things (IoT) refers to the methods and protocols used to manage
• how multiple IoT devices communicate over shared wireless communication channels.
• Efficient medium access control (MAC) is critical in IoT networks to ensure reliable data transmission,
• minimize collisions and interference, and optimize the use of limited wireless spectrum.
• Given the often dense and heterogeneous nature of IoT environments,
• effective MAC mechanisms are essential to maintaining network performance.

What are the Challenges in Wireless Access Medium?

• In wireless networks, the communication medium is the radio spectrum, which is a shared and finite resource.
• Efficient medium access control (MAC) is critical to ensure that devices can
• communicate effectively without excessive collisions, interference, or delays.
• Here are some key concepts and issues related to wireless medium access:

Collisions

• Collisions happen when several devices try to send data simultaneously.
• simultaneously on the same frequency channel, causing their signals to interfere with each other.
• Collisions result in corrupted data packets, requiring retransmissions, which lead to reduced network throughput and increased latency.
• Collision avoidance protocols like Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) are used to reduce the likelihood of collisions.

Hidden Node Problem

• This problem arises when two devices that are out of each other's transmission range attempt to send data to a common receiver
• simultaneously, causing a collision at the receiver.
• The hidden node problem can lead to frequent collisions and reduced network performance.
• Techniques such as Request to Send/Clear to Send (RTS/CTS) are used to mitigate this issue by coordinating access to the medium.

Exposed Node Problem

• This occurs when a device refrains from transmitting because it senses another transmission,
• even though its transmission would not cause interference at the intended receiver.
• The exposed node problem leads to underutilization of the available bandwidth, reducing network efficiency.
• Solutions: Adjustments in the MAC protocol to better differentiate
• between actual interference and non-interfering transmissions can help alleviate this problem.

Interfaces

• Interference occurs when signals from different devices overlap in the frequency spectrum, degrading the quality of the received signal.
• Interference can cause data corruption, increased error rates, and reduced network performance.
• Channel allocation strategies, frequency hopping, and advanced signal processing techniques are employed to minimize interference.

Bandwidth Allocation

• Efficiently allocating the limited bandwidth among multiple users is a critical challenge in wireless networks.
• Poor bandwidth allocation can lead to network congestion, unfair access, and reduced overall network performance.
• Dynamic bandwidth allocation techniques and Quality of Service (QoS) mechanisms are used to ensure fair and efficient distribution of bandwidth.

MAC Protocol

Understanding Medium Access / Multiple Access Protocols

• Medium Access Protocols (MAPs) and Multiple Access Protocols (MAPs) are
• designed to control how data is transmitted and received over a shared communication channel,
• ensuring that multiple devices can communicate efficiently and without interference.
• These protocols are essential in preventing collisions and managing the medium in a way that maximizes network performance and reliability.
• The Medium Access Control (MAC) protocol in the Internet of Things (IoT) refers to a set of rules and mechanisms that govern
• how IoT devices access and share the wireless communication medium.
• The MAC protocol is a critical component of the data link layer in the OSI model and is
• responsible for coordinating the transmission of data packets to minimize collisions, reduce interference, and
• ensure efficient and reliable communication between numerous devices in an IoT network.

• Random Access Protocols {Aloha and CSMA }
• Control Access Protocols { Polling and Token Passing }
• Channelize Protocols {FDMA and TDMA }

Random Access Protocols

• Random Access Protocols are a subset of MAC (Media Access Control) protocols used in IoT networks to manage
• how several devices use a common communication channel.
• These protocols allow devices to transmit data whenever they have data to send, rather than following a predetermined schedule.
• Random Access Protocols are crucial in the IoT landscape due to their flexibility and simplicity.
• They allow devices to access the communication medium in an uncoordinated manner,
• making them suitable for networks with sporadic and bursty traffic patterns.

Common Types of Random Access Protocols

Carrier Sense Multiple Access (CSMA)

• CSMA/CD (Collision Detection): Used in wired networks like Ethernet. Devices listen to the medium before transmitting.
• If a collision is detected, they stop, wait for a random time, and retry.
• CSMA/CA (Collision Avoidance): Used in wireless networks like Wi-Fi. Devices listen to the medium and wait if it is busy.
• They use techniques like Request to Send/Clear to Send (RTS/CTS) to further reduce collision chances.

ALOHA

• Pure ALOHA: Devices transmit whenever they have data. If a collision occurs, the device waits a random time before retransmitting.
• Slotted ALOHA: Time is segmented into slots, and devices are allowed to transmit only at the beginning of each slot.
• the beginning of a slot, which reduces the probability of collisions compared to Pure ALOHA.

Variants of CSMA

• CSMA with Collision Detection (CSMA/CD): Devices detect collisions during transmission and take corrective action.
• CSMA/CA (Collision Avoidance): Used in wireless networks like Wi-Fi. Devices listen to the medium and wait if it is busy.

Control Access Protocols

• Controlled Access Protocols are another subset of MAC (Media Access Control) protocols used in IoT networks.
• Unlike Random Access Protocols, where devices independently decide when to transmit data,
• Controlled Access Protocols regulate access to the communication medium through a coordinated approach.
• This coordination can be centralized or distributed, ensuring organized and collision-free communication.
• Controlled Access Protocols are crucial for applications where reliable, predictable, and efficient communication is required.

Common types of Control Access Protocols

Polling

• Polling is a controlled access method where a central controller,
• known as the master, sequentially checks or "polls" each device, known as slaves, to see if they need to transmit data.

How Polling Works

• Central Controller: There is a master device responsible for controlling access to the medium.
• Sequential Polling: The master device sends a poll request to each slave device in turn.
• Response Time: Each polled device responds if it has data to send, transmitting its data during its allocated turn.
• Empty Polls: If a device has no data to send, it can either send a negative acknowledgment or simply not respond, and the master moves to the next device.
• Repeat Cycle: The process repeats, continuously polling each device in sequence.

Applications of Polling

• Bluetooth Networks
• Industrial Control Systems

Token Passing

• Token Passing is a controlled access method where a special data packet called a token circulates around the network.
• Only the device that holds the token can transmit data.

How Token Passing Works

• Token Circulation: A token, a special frame, is passed from one device to another in a predetermined order.
• Transmission Rights: A device can only transmit data if it holds the token.
• Passing the Token: After transmitting data, the device passes the token to the next device in the sequence.
• Idle State: If a device has no data to send, it simply passes the token to the next device.

Channelization Protocols

• Channelization protocols are a subset of MAC (Media Access Control) protocols used in IoT networks to manage
• how multiple devices share and access the communication medium by dividing it into separate channels.
• These channels can be based on time, frequency, or code, and they help in
• organizing and optimizing the use of the communication medium to avoid collisions and ensure efficient data transmission.
• Efficient Use of Resources: By dividing the medium, these protocols maximize the use of available bandwidth and reduce collisions.

Common Types of Channelization Protocols

Time Division Multiple Access (TDMA)

• Every device is allocated a particular time slot for data transmission.
• This method ensures that multiple devices can share the same frequency channel without interference by transmitting at different times.

How TDMA Works

Time Slot Assignment

• The communication medium is divided into frames, each frame containing several time slots.
• Each device is assigned one or more specific time slots within each frame.

Synchronized Transmission

• Devices must synchronize their clocks to ensure they transmit data only during their assigned time slots.
• This synchronization can be achieved using a master clock or synchronization signals.

Frequency Division Multiple Access (FDMA)

• FDMA is a channelization protocol that divides the communication medium into separate frequency bands.
• This method allows multiple devices to transmit simultaneously over different frequencies without interference.
• The communication medium is divided into multiple frequency bands.
• Each device is assigned a distinct frequency band for its communication.
• Devices can transmit data simultaneously as long as they use different frequency bands.
• There is no need for synchronization in the time domain, as frequency separation ensures no interference.

Conclusion