Media Access Control (MAC) utilizes three primary methods for managing how devices access and transmit data on a shared network medium: contended access, controlled access, and contention-free access.
Contended Access
- How it works: In contended access, devices "compete" for the opportunity to transmit. If a device wants to send data, it checks if the channel is free. If it is, the device transmits. However, there's a risk of collision, where two or more devices transmit simultaneously.
- Collision Handling: Mechanisms like Carrier Sense Multiple Access with Collision Detection (CSMA/CD) or Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) are used to detect and resolve collisions. CSMA/CD is used in Ethernet, while CSMA/CA is used in WiFi.
- Examples: Ethernet (using CSMA/CD) and Wi-Fi (using CSMA/CA) are common examples of networks employing contended access.
- Advantages: Simple to implement and works well under light to moderate network loads.
- Disadvantages: Performance degrades under heavy network loads due to increased collisions.
Controlled Access
- How it works: Controlled access methods avoid collisions by regulating which device can transmit at a given time. This is typically achieved through polling or token passing.
- Polling: A central controller polls each device to see if it has data to transmit.
- Token Passing: A special "token" is passed from device to device, and only the device holding the token can transmit.
- Examples: Token Ring networks and some older network architectures utilize controlled access.
- Advantages: Eliminates collisions and provides more predictable performance, especially under heavy loads.
- Disadvantages: More complex to implement and can introduce latency due to the polling or token-passing process. The central controller or token can become a single point of failure.
Contention-Free Access
- How it works: Contention-free access ensures that each device has a dedicated time slot or frequency channel for transmission, preventing collisions altogether. This is often implemented through Time Division Multiple Access (TDMA) or Frequency Division Multiple Access (FDMA).
- Examples: Fiber Distributed Data Interface (FDDI) and some wireless communication systems use contention-free access. Cellular networks also utilize contention-free methods to manage access among users.
- Advantages: Guarantees collision-free transmission and provides deterministic performance.
- Disadvantages: Requires more complex synchronization and scheduling, and can be less efficient if some devices consistently have less data to transmit. Can be more expensive to implement than contention-based methods.
In summary, the three methods of media access control provide different strategies for managing access to the network medium, each with its own set of advantages and disadvantages depending on the network environment and application requirements.
