CSMA/CD, Half duplex, Full duplex, Collision domain, and Broadcast Domain | CCNA Video 04

This video is the fourth video in our ongoing tutorial series on CCNA. In it, we will learn about CSMA/CD, half duplex, full duplex, collision domain, and broadcast domain.

Video description
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A cable creates an electrical circuit between the source and destination devices. The source device puts signals on this circuit. The destination device reads signals from this circuit.
If the destination device wants to reply, it must wait until the circuit becomes free. Once the source stops sending signals, the destination device can respond.
The same rule applies to the source. If it wants to reply, it must wait until the destination device's transmission ends.
This type of communication is called half-duplex. In half-duplex, a device can only send or receive data at a given time. It cannot perform both actions.
However, if the cable provides two circuits, end devices can perform both actions simultaneously. They can use one circuit to transmit and another to receive the data. This type of communication is called full-duplex. In full-duplex, a device can send and receive data simultaneously.
CSMA/CD
The CSMA/CD mechanism allows layer one network devices to forward signals of multiple devices in a queue under certain conditions.

Since layer one devices in half-duplex mode create only one electric circuit for transmission, this mechanism allows only one device to send signals. If another device wants to send the signals, it must wait until the circuit becomes free.
Devices check the connected cables to know whether the media is free for transmission. If they see signals in it, they do not start the transmission. In CSMA/CD, this process is called carrier sense.
This mechanism ensures that only one device sends signals at a time.
As long as it is true, all devices can send signals if multiple devices sense the media simultaneously. Since the media is free for transmission, they both put their signals on their connected cables.
When it happens, signals collide at the hub. This mechanism defines this situation as collision detection.
If a device sees a collision for its transmitted signals, it retransmits them with a JAM signal.
This JAM signal informs all recipients about the collision.
After sending the JAM signal, it waits for a small random period measured in microseconds and senses the wire again.
If collision still exists, it repeats the same steps. If it sees no collision, it retransmits the original signals.
Since devices wait for a random period, the device whose waiting period is over starts the retransmission first, and the remaining device waits til it ends.
This way, CSMA/CD allows layer one devices running on half-duplex to send and receive data.

Collision domain
A collision domain is a group of devices affected by a collision. For example, when a collision occurs in this network, all three devices are affected by it. Since they are all affected by it, they belong to the same collision domain. However, this situation changes if we replace this hub with an ethernet switch.
An ethernet switch creates a separate circuit between the source and destination. When it receives incoming signals, it converts them into frames and forwards them only from the port connected to the destination. It uses the source and destination address of the frame to make forwarding decisions. We will learn more about this in upcoming videos. By now, remember an ethernet switch creates a logical circuit between the source and destination devices. It forwards frames only from the port connected to the destination device. It works in the full-duplex mode. These features create a separate collision domain for each port of the switch. For example, an eight-port switch creates eight collision domains.
However, connecting a port to another switch or a higher-level device such as a router or multi-layer switch does not create a separate collision domain. Connecting two switches creates only a signal collision domain between them.

Broadcast domain
A broadcast domain is similar to a collision domain but works on layer three. It is a group of devices that share the broadcast messages. A broadcast is a layer three message. It reaches all devices within the broadcast domain.
Since it is a layer three message, only a layer three device can understand it. Routers are layer three devices. They define a boundary for broadcast messages. Like a switch port, each router interface creates a separate broadcast domain.