Carrier-sense multiple access facts for kids

Carrier-sense multiple access (CSMA) is a smart rule for computers and devices to share the same communication path. Think of it like many people wanting to talk on the same radio channel. CSMA helps them avoid talking over each other.

When a device wants to send a message, it first "listens" to the path. It checks if anyone else is already sending data. This is called "carrier sensing." If the path is clear, the device sends its message. If the path is busy, the device waits until it becomes free. This way, many devices can take turns sending and receiving information on the same shared path.

There are different ways CSMA works. Some versions try to avoid messages crashing into each other (called "collisions"). Others can even detect if a collision happens and fix it.

How Devices Share the Path

Different types of CSMA use various methods to decide when to send data. These methods can be more "aggressive" or more "patient." An aggressive method might send data faster. But it also has a higher chance of messages crashing into each other.

1-Persistent CSMA

This method is quite "aggressive."

• When a device is ready to send, it checks if the path is free.
• If the path is free, it sends the message right away.
• If the path is busy, the device keeps listening. As soon as the path becomes free, it sends the message immediately.
• If two messages crash (a collision), the device waits a random short time. Then it tries the whole process again.
• This method is used in systems like Ethernet, which connects many computers in a local area network.

Non-Persistent CSMA

This method is more "patient."

• When a device is ready to send, it checks if the path is free.
• If the path is free, it sends the message right away.
• If the path is busy, the device does not keep listening. Instead, it waits a random amount of time and then tries the whole process again from the start.
• This approach helps avoid collisions. It makes the overall system work better, but it might take a little longer for a message to start sending.

P-Persistent CSMA

This method is a mix of the aggressive and patient types.

• When a device is ready to send, it checks if the path is free.
• If the path is free, it sends the message with a certain probability (a chance, like 50% or 70%).
• If it doesn't send (because of the probability), it waits for the next available time slot. Then it tries again with the same probability.
• If the path is busy, the device keeps listening. As soon as the path becomes free, it sends the message with that same probability.
• This method is used in Wi-Fi networks and other wireless systems.

O-Persistent CSMA

In this method, each device gets a specific turn to send.

• A main device decides the order for all other devices.
• When the communication path becomes free, devices wait for their assigned turn.
• The device that is first in line sends its message right away.
• Other devices listen and update their place in line.
• This method is used in systems like Controller Area Network (CAN), often found in cars.

Improving CSMA Protocols

Engineers have made changes to CSMA to make it work even better, especially for different types of networks.

CSMA with Collision Detection (CSMA/CD)

• This version improves CSMA by stopping a message as soon as a collision is detected.
• This saves time because the device doesn't keep sending a message that's already crashed.
• It then waits and tries again.
• CSMA/CD is used in Ethernet networks.

CSMA with Collision Avoidance (CSMA/CA)

• This version tries to prevent collisions from happening in the first place.
• If a device wants to send a message and the path is busy, it waits for a random amount of time.
• This random wait makes it less likely that two devices will start sending at the exact same time when the path becomes free.
• CSMA/CA is used in Wi-Fi networks.

CSMA with Collision Resolution (CSMA/CR)

• This method uses special priority rules in the message itself to avoid collisions.
• It is used in the Controller Area Network (CAN).

Virtual Time CSMA (VTCSMA)

• This method is designed for systems where messages need to be sent by a very specific time, like in real-time control systems.
• It uses two internal "clocks" to give messages different priorities based on their deadline. This helps avoid collisions when timing is critical.

See also

In Spanish: Carrier sense multiple access para niños

• Local collision
• Remote collision