Collision vs Broadcast Domains

A collision domain is simply defined as any physical segment where a collision can occur. 

• A collision occurs when two devices send a packet at the same time on the shared network segment. The packets collide and both devices must send the packets again, which reduces network efficiency.

• Hubs can only operate at half-duplex, and thus all ports on a hub belong to the same collision domain.

In the below example we have 6 collision domains. 

In a "Shared Media" there are no separate channels for sending and receiving the data signals, but only one channel to send and receive the data signals.

We call the media as shared media when the devices are connected together using Bus topology, or by using an Ethernet Hub. Both are half-duplex, means that the devices can Send or Receive data signals at same time. Sending and receiving data signals at same time is not supported.

Layer-2 switches can operate at full duplex. Each individual port on a switch belongs to its own collision domain. Thus, Layer-2 switches create more collision domains, which results in fewer collisions. 

Broadcast Domain
 
A broadcast domain is a domain in which a broadcast is forwarded. A broadcast domain contains all devices that can reach each other at the data link layer (OSI layer 2) by using broadcast. 

• All ports on a hub or a switch are by default in the same broadcast domain. 

• All ports on a router are in the different broadcast domains and routers don’t forward broadcasts from one broadcast domain to another.

In the below example we have three broadcast domains, since all ports on a hub or a switch are in the same broadcast domain, and all ports on a router are in a different broadcast domain.

Like hubs though, Layer-2 switches belong to only one broadcast domain. A Layer-2 switch will forward both broadcasts and multicasts out every port but the originating port.

Conclusion :

• Only Layer-3 devices separate broadcast domains. Because of this, Layer-2 switches are poorly suited for large, scalable networks. 

• The Layer-2 header provides no mechanism to differentiate one network from another, only one host from another.

 

Describe the difference between a collision domain and a broadcast domain.  

 

Collision domain is an Ethernet term used to describe a network collection of devices in which one particular device sends a packet on a network segment, forcing every other device on that same segment to pay attention to it. On a broadcast domain, a set of all devices on a network segment hear all broadcasts sent on that segment.