Classification of MAC protocols

The Media Access Control (MAC) is the first protocol layer above the Physical Layer in ad hoc. The primary task of any MAC protocol is to control the access of the nodes to a shared medium. It plays an important role in coordinating access to the shared communication medium. To achieve this phase various strategies have evolved, leading to a diverse range of MAC protocols. This classification examines the different approaches employed by these protocols to manage, access and ensure efficient data transmission across the network.

MAC (Medium Access Control) Layer

The Media Access Control (MAC) layer is an important sublayer within the Data Link Layer of the OSI model. It's responsible for controlling how devices on a shared network gain access to and utilize the communication medium. Coordinating access to the shared medium and preventing collisions enables multiple devices to share the network bandwidth effectively.

Key Responsibilities:

• Medium Access Control
  • Carrier Sense Multiple Access (CSMA)
  • CSMA/CD (Carrier Sense Multiple Access with Collision Detection)
  • Token Passing
• Addressing
• Frame Formatting

Classification of MAC Protocols

MAC protocols can be classified into four main categories: contention-based, reservation-based, polling-based, and hybrid. Contention-based protocols, such as CSMA/CD and CSMA/CA, allow nodes to compete for access to the medium. Reservation-based protocols, like TDMA and FDMA, allocate specific time slots or frequencies to each node. Polling-based protocols rely on a central node to grant permission to transmit. Finally, hybrid protocols combine elements of these approaches for improved performance.

These are as follows.

1. Contention-Based Protocols Without Reservation/Scheduling

Multiple devices attempt to access the channel simultaneously without coordination. When collisions occur, devices back off and try again. Devices transmit when they have data, relying on techniques like carrier sensing or random backoffs to reduce collisions. It is simple to implement, and adaptive to changing network conditions.

Key Points:

• Bandwidth is not reserved.
• No guarantees.
• Sender-initiated protocols: The transmission of packets are initiated by the sender node.
• Single-channel sender initiated
• Multiple-channel sender initiated protocols.
• Receiver-initiated protocols: The connection is initiated by the receiver node.

2. Contention-Based Protocols With Reservation Mechanisms

A central controller periodically polls devices to grant them exclusive access to the channel, ensuring each one of them gets a turn. A master or base station queries each device in turn and only the polled device is allowed to transmit.

Key Points:

• Bandwidth is reserved for transmission.
• Guarantees can be given.
• Synchronous protocols: In synchronous transmission, data is sent in continuous streams or blocks without start/stop bits for each character. Sender and receiver must be synchronized with a common clock signal. It is more complex to implement compared to asynchronous transmission.
• Asynchronous protocols: In asynchronous transmission, data is sent character-by-character, with start and stop bits added to each character for synchronization. Easier and less expensive to implement. It is less efficient due to the overhead of start/stop bits. Relative time information is used to achieve effecting reservations.

3. Contention-Based Protocols with Scheduling Mechanisms

The network allocates specific time/frequency/code resources to each device, so transmissions do not overlap. Each station is assigned a unique time slot, frequency band, or spreading code, ensuring non-interfering transmissions. It provides deterministic access, predictable QoS, and efficient utilization under steady traffic conditions.

• Polling schemes in Bluetooth piconets (master polls slaves), industrial networks using a central controller.
• Ideal for networks needing strict timing guarantees, such as industrial control systems or sensor networks requiring reliable data collection.
• Cellular networks, satellite communications, and environments where guaranteed bandwidth and predictable latency are critical.

4. Other Hybrid Protocols

Combine features of contention-based and scheduled approaches (or other methods) to balance flexibility, efficiency, and reliability. The part of the bandwidth or time is allocated deterministically (scheduled slots), while the remainder is accessed using contention or polling. It is adaptable to varying traffic conditions, can provide QoS guarantees and handle heavy traffic efficiently.

• IEEE 802.15.4’s beacon-enabled mode (mix of CSMA/CA and a guaranteed time slot), Wi-Fi with HCCA (Hybrid Coordination Function Controlled Channel Access).
• More complex to implement and configure than pure approaches.
• Networks with mixed traffic profiles (e.g., both delay-sensitive and best-effort data), scenarios where some devices require guaranteed service while others transmit sporadically.