Ethernet frames are primarily transmitted using a method called Carrier Sense, Multiple Access with Collision Detection (CSMA/CD), particularly in older or shared Ethernet environments. This mechanism helps manage access to the shared network medium to ensure data can be sent reliably.
The CSMA/CD Mechanism
To transmit frames from one device to another, Ethernet utilizes Carrier Sense, Multiple Access with Collision Detection (CSMA/CD). This is a set of rules that devices follow before and during transmission.
Let's break down the components of CSMA/CD:
- Carrier Sense (CS): Before transmitting, a device listens to the network medium (like the cable) to see if any other device is currently transmitting.
- According to Fluke Networks, "Carrier Sense is the detection of the network and any signals being transmitted so a device knows when it's clear to send." This is the "listen before talk" rule.
- If the network is busy, the device waits for a random amount of time before trying again.
- Multiple Access (MA): This signifies that multiple devices can connect to and share the same transmission medium (e.g., a single cable segment or hub). All connected devices have the potential to access the medium and transmit data.
- Collision Detection (CD): If two devices happen to start transmitting at roughly the same time (because they both sensed the network was clear), their signals can collide on the shared medium.
- Devices continue listening while they transmit.
- If a device detects a collision (indicated by a distorted signal or increased signal amplitude), it immediately stops transmitting.
- It then sends a special signal called a "jam signal" to ensure all other transmitting devices detect the collision.
- After sending the jam signal, each device involved in the collision waits for a random period of time (using a process called "backoff") before attempting to retransmit the frame. The random delay helps prevent the same devices from colliding again immediately.
Steps in Ethernet Frame Transmission (using CSMA/CD)
Here's a simplified sequence for a device wanting to send an Ethernet frame:
- Listen: Is the network medium idle?
- If Idle: Begin transmitting the Ethernet frame.
- While Transmitting: Continue listening for collisions.
- If Collision Detected: Stop transmitting, send a jam signal, wait a random backoff period, and then go back to step 1.
- If Not Idle: Wait for a random amount of time and then go back to step 1.
- Successful Transmission: If the entire frame is transmitted without detecting a collision, the transmission is successful.
Note: While fundamental to traditional Ethernet, CSMA/CD is less relevant in modern switched Ethernet networks where dedicated connections between devices and switches largely eliminate collisions within individual links. However, understanding CSMA/CD provides valuable insight into Ethernet's historical operation and its design principles for managing shared media.
