The classifications of media access control (MAC) protocols primarily fall into three categories: contended, controlled, and contention-free access protocols.
These categories define how devices on a shared network medium, such as a wired Ethernet or wireless network, gain access to transmit data. Each method has its own advantages and disadvantages in terms of efficiency, complexity, and suitability for different network environments. Let's explore them in detail:
1. Contended Access Protocols
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Definition: Contended access protocols allow multiple devices to compete for access to the network medium. There's no central authority or pre-assigned time slots; devices transmit when they need to. Collisions can occur when multiple devices transmit simultaneously.
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Examples:
- Carrier Sense Multiple Access with Collision Detection (CSMA/CD): Used in wired Ethernet networks. Devices "listen" to the network before transmitting (carrier sense). If a collision is detected during transmission (collision detection), the devices stop transmitting and send a jamming signal, then retry after a random backoff period.
- Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA): Used in wireless networks (e.g., Wi-Fi). Devices listen before transmitting, and if the channel is idle, they send a Request to Send (RTS) signal. The receiving device responds with a Clear to Send (CTS) signal. This process aims to avoid collisions. If a collision is detected or no CTS is received, the device waits a random backoff time before retrying.
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Advantages:
- Simple to implement.
- Efficient under low network load.
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Disadvantages:
- Performance degrades significantly under high network load due to increased collisions and retransmissions.
- Not suitable for real-time applications or applications requiring guaranteed bandwidth.
2. Controlled Access Protocols
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Definition: Controlled access protocols regulate access to the network medium through a central authority or a predetermined schedule. This eliminates collisions but introduces overhead.
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Examples:
- Polling: A central controller polls each device, granting it permission to transmit if it has data to send.
- Token Passing: A special "token" is passed sequentially from one device to another. Only the device holding the token can transmit. (e.g., Token Ring)
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Advantages:
- Collision-free operation.
- Guaranteed bandwidth allocation (in some cases).
- Better performance under high network load compared to contended access.
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Disadvantages:
- More complex to implement than contended access.
- Introduces overhead due to polling or token passing.
- Single point of failure (in centralized polling systems).
- Inefficient under low network load, as devices may have to wait for their turn even if the network is otherwise idle.
3. Contention-Free Access Protocols
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Definition: Contention-free protocols guarantee collision-free communication without relying on token passing or strict centralized control like polling. They often utilize pre-assigned time slots or frequency bands.
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Examples:
- Time Division Multiple Access (TDMA): Each device is assigned a specific time slot in which it can transmit.
- Frequency Division Multiple Access (FDMA): Each device is assigned a specific frequency band in which it can transmit.
- Code Division Multiple Access (CDMA): Each device is assigned a unique code used to encode its data. Devices can transmit simultaneously, and the receiver can decode the desired signal based on the unique code.
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Advantages:
- Collision-free operation.
- Guaranteed bandwidth allocation.
- Predictable performance.
- Suitable for real-time applications and applications requiring guaranteed bandwidth.
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Disadvantages:
- More complex to implement than contended access.
- Can be inefficient if devices do not always have data to send during their assigned slots or bands.
- Requires careful planning and coordination to assign time slots or frequency bands.
In summary, media access control protocols are classified into contended, controlled, and contention-free methods, each providing distinct mechanisms for devices to share a network medium. The choice of which protocol to use depends heavily on the specific requirements of the network, including factors like network load, real-time requirements, and implementation complexity.
