Collision Domain And Broadcast Domain in Computer Network

In a telecommunications network, understanding how data travels from one device to another is very important. It helps to build an efficient and scalable communication system.

The arrangement of devices in a network—whether they belong to the same collision domain or broadcast domain—determines how far traffic from a particular host can travel. Proper segmentation using routers, switches, and technologies like VLANs helps minimize unnecessary traffic, reduce congestion, and improve overall network performance.

In this blog post, I will introduce Collision Domain And Broadcast Domain in computer networks. I will also explain how routers, switches, and VLANs can be used to segment networks effectively, reducing congestion and optimizing data flow.

Whether you’re a seasoned IT pro or just starting your journey in networking, understanding these concepts is key to grasping how data moves through networks and why certain network designs work better than others.

What is Collision Domains?

A collision domain is a network segment where only one device can transmit data at a time. If multiple devices attempt to send data simultaneously, a collision occurs, leading to potential data loss.

Imagine you’re at a party where everyone’s trying to talk at once. It’s noisy, chaotic, and hard to have a proper conversation, right? Well, that’s pretty much what a collision domain is in the world of computer networks.

In networking terms, a collision domain is a part of the network where data packets can collide with each other. These collisions happen when two or more devices on the same network segment try to send data at exactly the same time. It’s like two people starting to speak simultaneously at that noisy party – neither message gets through clearly.

When a collision occurs, it’s not just a minor hiccup. It actually slows down the entire network. Why? Because when data packets collide, the devices involved need to wait a random amount of time before they can try sending their data again. It’s as if those two people at the party decided to wait a few seconds before trying to speak again, hoping the other person won’t start talking at the same time.

The bigger the collision domain (i.e., the more devices that are part of it), the higher the chance of these collisions occurring. This is why understanding and managing collision domains is very important for maintaining a smooth, efficient network.

Hubs and Collision Domains

Hubs might seem like simple, innocent network devices, but they can be the cause of collision in a communication network.

Here is how hubs can contribute to collision in a computer network:

1. In a hub-based network, every single port on the hub is part of one large collision domain. This means that if you have a hub with 8 ports, and you connect 8 computers to it, all of these computers are in the same collision domain.
2.  Half-Duplex Communication: Hubs operate in what we call “half-duplex” mode. This means they can only send or receive data in one direction at a time.
3. When a computer connected to a hub sends data, the hub doesn’t discriminate. It takes that data and broadcasts it out to every single one of its ports. This means every computer connected to the hub receives the data, whether it was meant for them or not.

Let’s put this into perspective with an example. Imagine you have four computers connected to a hub. Computer A wants to send data to Computer B. Here’s what happens:

1. Computer A sends the data to the hub.
2. The hub receives the data.
3. The hub then sends that data out to all of its ports, including to Computers C and D, who don’t need this information.
4. If Computer C tries to send data at the same time Computer A is sending, it will result in a collision. Because of this, the more devices you connect to a hub, the more likely collisions become.

Switches and Collision Domains

Switches were created to tackle the collision domain problem, and they do a pretty fantastic job of it. Here’s how switches help to solve the issue of collisions in a network as an alternate to hub:

1. Unlike hubs, where all ports are part of the same collision domain, each port on a switch is its own separate collision domain. This means that if you have an 8-port switch, you effectively have 8 collision domains instead of one big one.

2. Smart Forwarding: Switches are much smarter about how they handle data. Instead of broadcasting data to all ports like a hub does, a switch only forwards data to the specific port where the intended recipient is connected.

3. Full-Duplex Communication: Switches operate in full-duplex mode, which means they can send and receive data simultaneously.

Let’s revisit our example with four computers, but this time connected to a switch instead of a hub:

1. Computer A wants to send data to Computer B.
2. Computer A sends the data to the switch.
3. The switch looks at the destination address of the data and says, “Ah, this is for Computer B.”
4. The switch forwards the data only to the port where Computer B is connected.
5. Meanwhile, Computer C can send data to Computer D at the same time, without any risk of collision.

What is Broadcast Domains?

While collision domains deal with data transmission at a lower level, broadcast domains operate at a higher level of network communication.

A broadcast domain is a part of the network where devices can receive broadcasts from other devices on the same network.

But what exactly is a broadcast? It’s a type of network communication where a device sends out a message that’s meant to be received by all other devices in the same broadcast domain.

Imagine you’re in a large office, and someone stands up and shouts, “Who wants to go for lunch?” That’s essentially what a broadcast is in network terms. It’s a message sent out to everyone, regardless of whether they’re interested or not.

Why is Broadcast Message Important?

Here’s why broadcasts are important:

1. Device Discovery: When a device wants to find another specific device on the network, it often starts by sending out a broadcast message asking, “Hey, is Device X out there?”

2. Network Announcements: Broadcasts are used to announce important network information, like the presence of a DHCP server that can assign IP addresses.

3. ARP Requests: When a device knows the IP address of another device but needs to find its MAC address, it sends out a broadcast ARP (Address Resolution Protocol) request.

The key thing to remember is that both hubs and switches will forward broadcasts.

Example of a Single Broadcast Domain

Let’s look at a practical example of a single broadcast domain;

Imagine you have a small office network with the following setup:

– 4 computers connected to a hub
– 4 computers connected to a switch
– The hub and switch are connected to each other

In this scenario, you have one large broadcast domain that encompasses all eight computers.

Here’s what happens when a broadcast is sent:

1. Computer A (connected to the hub) sends out a broadcast message.
2. The hub receives the broadcast and forwards it to all its ports, including the other three computers and the switch.
3. The switch receives the broadcast from the hub and forwards it to all of its ports, reaching the four computers connected to it.
4. As a result, all eight computers receive the broadcast message, regardless of whether they’re connected to the hub or the switch.

This setup works fine for small networks. All devices can easily communicate with each other, and the amount of broadcast traffic is manageable. However,  this can become problematic as the network grows larger.

 

Separating Broadcast Domains with Routers

So, what if a network administrator wants to split this single broadcast domain into two separate ones? This is where routers come into play.

Routers are like the traffic cops of the networking world—they control the flow of data between different network segments.

Here’s how routers help manage broadcast domains:

1. Broadcast Barrier: Unlike hubs and switches, routers do not forward broadcasts messages by default. They act as a barrier, retaining broadcasts within their original network segment.

2. Network Segmentation: Routers allow us to create logical separations within our network. Each interface on a router typically represents a different network segment and, consequently, a different broadcast domain.

3. Intelligent Forwarding: Routers make decisions about whether to forward packets based on the destination IP address. This allows for more controlled communication between different parts of the network.

Let’s modify our previous example by adding a router:

1. We place a router between the hub and the switch.
2. Now, we have two separate broadcast domains:
– Broadcast Domain 1: The four computers connected to the hub
– Broadcast Domain 2: The four computers connected to the switch

In this new setup:

– If Computer A (connected to the hub) sends a broadcast, it will reach the other three computers on the hub, but it won’t cross the router to reach the computers on the switch.
– Similarly, if Computer E (connected to the switch) sends a broadcast, it will reach the other three computers on the switch, but not the computers connected to the hub.

This ability to block broadcasts is what makes routers so important in network design. They allow us to create logical separations within our network.

Purpose of Multiple Broadcast Domains

At this point, you might be wondering, “Why go through all this trouble to create multiple broadcast domains? Isn’t one big network easier to manage?”

While it’s true that a single broadcast domain can be simpler in very small networks, there are several reasons to create multiple broadcast domains as networks grow: viz;

1. Reduced Network Traffic: Each broadcast sent on a network has to be processed by every device in that broadcast domain. In a small network, this isn’t a big deal. But as networks grow, the amount of broadcast traffic can become significant. By creating multiple smaller broadcast domains, you reduce the number of devices that need to process each broadcast, leading to less overall network traffic.

2. Improved Performance: With less broadcast traffic cluttering the network, devices can communicate more efficiently. This can lead to faster response times and better overall network performance, especially in larger networks.

3. Enhanced Security: Separating broadcast domains can help isolate different parts of your network. This can be useful for separating departments in a company or for keeping sensitive systems separate from the general network. It’s like having different rooms in a house – you can control who has access to each room.

4. Better Scalability: As your network grows, having multiple broadcast domains makes it easier to add new devices or segments without impacting the entire network. You can expand one segment without affecting others.

5. Easier Troubleshooting: When network issues arise, having smaller, separated broadcast domains can make it easier to isolate and troubleshoot problems. It’s much easier to find a needle in a small haystack than in a giant one!

6. Bandwidth Conservation: In networks that span wide geographical areas (like WANs), separating broadcast domains can help conserve expensive WAN bandwidth by keeping local traffic local.

7. Improved Network Design: Creating multiple broadcast domains encourages better network design practices, leading to more organized and efficient network architectures.

Large Network Example

Now that we understand the benefits of multiple broadcast domains, let’s look at a larger network example to really drive home the importance of proper broadcast domain management.

Imagine a medium-sized company with 500 employees spread across several departments: Marketing, Sales, Finance, HR, and IT. If this entire network were set up as a single broadcast domain, here’s what might happen:

1. Broadcast Storm: With 500 devices all capable of sending broadcasts, the network could easily become overwhelmed with broadcast traffic. Every time a device looks for a network resource or sends out an ARP request, all 500 devices would receive and need to process that broadcast.
2. Reduced Performance: As broadcast traffic increases, the overall network performance would suffer. Devices would spend more time processing irrelevant broadcasts and less time on actual productive tasks.
3. Security Concerns: Sensitive information from one department (like Finance) could potentially be visible to all other departments due to the flat network structure.
4. Troubleshooting Nightmare: If a network issue arises, the IT team would have to sift through traffic from all 500 devices to identify the problem.
5. Limited Scalability: Adding new devices or departments would become increasingly problematic as the network grows.

Creating Smaller Broadcast Domains

The solution to our large network problem is to create smaller, more manageable broadcast domains. Here’s how we might approach this:

1.  Analyze Traffic Patterns: First, we’d look at how data typically flows through the network. Which groups of devices need to communicate with each other most frequently?
2. Identify Logical Divisions: In our example of 500 devices in a network above, the obvious divisions are the different departments: marketing, sales, finance, HR, and IT.
3. Implement VLANs: We can use Virtual LANs (VLANs) to create separate broadcast domains within our switches. Each department would be assigned its own VLAN.
4. Deploy Routers: We’d use routers (or Layer 3 switches) to connect these VLANs and control traffic between them.
5. Configure Inter-VLAN Routing: This allows necessary communication between departments while still maintaining separate broadcast domains.

Here’s how our improved network might look:

– VLAN 10: Marketing (100 devices)
– VLAN 20: Sales (150 devices)
– VLAN 30: Finance (75 devices)
– VLAN 40: HR (50 devices)
– VLAN 50: IT (125 devices)

Conclusion

Here is a recap of the key points we’ve covered in the article so far:

1. Collision domain is about managing data collisions at the lower levels of the network.
2. Hubs create large collision domains, which can lead to network inefficiencies.
3. Switches help by creating smaller collision domains, significantly reducing the chance of collisions.
4. Broadcast domains is about managing the spread of broadcast traffic across the network.
5. Both hubs and switches forward broadcasts, creating large broadcast domains in flat networks.
6. Routers are your go-to tools for creating and managing separate broadcast domains.
7. Implementing multiple broadcast domains through VLANs and routers can greatly improve network performance, security, and manageability.