Collision Domain Explained

A collision domain is a group of devices that share the same network segment, where data frame collisions can occur when multiple devices transmit simultaneously. When such collisions happen, all devices must wait until the network is clear before attempting to send data again.

When only two devices are directly connected by a cable, collisions do not occur because the communication channel is dedicated and not shared with other devices. To connect more than two devices, a central networking device such as a repeater, hub, bridge, or switch is required. The likelihood and management of collisions depend on the type of device used.

When a hub or repeater is used, all connected devices share a single collision domain. In contrast, with a bridge or switch, only devices connected to the same port are within the same collision domain. Therefore, hubs and repeaters establish a single collision domain for all connected devices, whereas bridges and switches create separate collision domains for each port.

Single collision domain

Hubs and repeaters operate at Layer 1 of the OSI model. A hub functions as a multiport repeater, with its primary distinction being the number of ports it offers. Repeaters amplify signals, while hubs connect multiple devices. Both devices process electrical signals without interpreting data frames. When an electrical signal is received on a hub port, the hub retransmits the signal to all other ports except the source port. Hubs do not interpret Ethernet frames.

If multiple devices transmit signals simultaneously, the signals collide and become corrupted. The hub continues to repeat all incoming electrical signals, regardless of simultaneous transmissions. For example, consider the following scenario. In this network, PCs A and B transmit frames simultaneously. The hub repeats both frames and forwards them to PC-C, resulting in a collision on the segment connected to the hub.

Per port collision domain

Replacing the hub with a switch prevents collisions on the shared segment. At Layer 2, the switch reads the destination address of each frame to determine how to forward it. If multiple frames must be sent from the same port, the switch queues them for sequential transmission.

Key points:-

• A collision domain is a group of devices that share a collision domain.
• In a collision domain, traffic generated by one device adversely affects other devices in the same collision domain. It does not affect devices outside the collision domain.
• A hub or a repeater creates a single collision domain.
• A bridge or a switch creates a per-port collision domain.
• Devices outside a given collision domain are unaffected by collisions within it.

Conclusion

A thorough understanding of collision domains is crucial for designing efficient and reliable networks. The selection of networking hardware significantly influences collision management, with switches and bridges offering substantial benefits over hubs and repeaters in contemporary network environments.

A collision domain refers to a network segment in which data packets may collide when transmitted over a shared medium. If multiple devices within the same collision domain send frames simultaneously, collisions occur, resulting in corrupted data and necessitating retransmission. The type of central networking device, such as a hub, switch, or router, determines the number of collision domains present in the network.

Hubs and repeaters operate at Layer 1 and form a single collision domain, regardless of the number of connected devices. These devices repeat electrical signals across all ports without recognizing frames or addresses, making them incapable of preventing or managing collisions. Consequently, simultaneous transmissions by two devices result in signal collisions that impact all devices within the domain.

Switches and bridges operate at Layer 2 and create separate collision domains for each port. By reading Ethernet frames and utilizing destination addresses for intelligent forwarding, these devices often queue frames to prevent simultaneous transmission. This segmentation significantly reduces the likelihood of collisions, as only devices connected to the same port are affected.