Collision Domain and Broadcast Domain
What is Collision Domain and Broadcast Domain?
Before going to the collision domain and broadcast domain. First, understand what is collision domain? and How does collision work? Go back to days when we use bus topology on the ethernet network may be 10base2 or 10base5 technologies. The devices attached to shared segment how to make sure there is no currently frame on that shared segment. Before they transmitted it we are not allowed to have more than one frame on a shared segment at any one time. Remember we use a technique called CSMA/CD carrier sense multiple access with collision detection.
Carrier sense meaning they can determine they are attached to the network and they can listen to the network to see a frame is present on the shared segment. Multiple access means that we don’t have to take turns we can have multiple devices accessing the same segment. When it clears the transmit then anybody can transmit and the collision detection CSMA/CD means if a collision does occur if we do have two stations and they transmit at the same time and that two transmissions collide we can detect that and we can retransmit.
What is Collision Domain?
Here’s the challenge, that can occur on a shared segment. Let’s say we have a couple of stations may be two bottom stations that are mentioned in the diagram. They are simultaneously listening to the wire to see its fine to transmit and they heard the same period of silence. And they independently concluded that its fine to transmit so they both transmit at the same time. The frames collide and the collision has occurred. Something that is often misunderstood by the collision you might guess that as soon as a collision is detected. The remainder of the frame would not be transmitted and the pc will say “oh…! no, my frame is collided with the other frame let me stop”.
Actually, that device continuously sending the frame the idea is that we have to make sure that everybody on the shared segment knows that collision occurred and if we stop transmitting immediately as soon as we detected a collision. What is collision domain is explained.
However, other devices might not know so we continue the transmitting that frame this process is known as jamming. The jamming is continuing to transmitting a frame for short time so that the other devices on a shared segment can know that collision did occur. After the collision occurs the devices that send the collided frames they obliviously need to retransmit that frame but they don’t want to collision happens again what they will do they independently set random back off timers. Just as an example the bottom left a device in the diagram set a back off time of 10ms and the device in the bottom right set the back off timer of 20ms. And then when they retransmit after these variables amount of time they less likely to collide again. That’s the way CSMA/CD can help support multiple devices on the same shared segment.
Hub Broadcast Domain:
Today, though we don’t use bus topology like we have here on the diagram. In fact, we have largely far away from ethernet hubs. But if you do need to run an ethernet Hub. you must understand that all of the ports on ethernet hub they are in the same collision domain. Meaning that even though, we are not physically connected in the star topology logically it’s still a bus topology. There is only a single frame on this shared segment at any one time. Hub doesn’t make intelligent forwarding decisions. It just takes bits which it received from one port and sends them out on other ports this can limit scalability. If we have lots of devices on same shared segment it would be going to transmit less frequently to increase the scalability. We will replace that hub with ethernet switch.
Ethernet Switch Broadcast domain:
As in the diagram, there are four devices connected to that Ethernet Switch. As a result, we have got four collision domain. Each port on an ethernet switch represents on its own collision domain. Because we only have one device on a switch port that’s mean we don’t even need to run CSMA/CD. If we don’t have to run CSMA/CD because there is no possibility of collision. That means these stations connected to the switch they can run in the full-duplex mode as suppose to half-duplex mode. In full-duplex mode means that we have a station that is simultaneously transmitting and receiving. If we had a Hub and we have to run CSMA/CD so the requirement is that we have to run in half-duplex mode.
This means that ethernet switch not only helps us to scale our network by helping this out by collision issue. But it also provides more effective use of our bandwidth. We can now use full duplex as opposed to half-duplex communication. So from the collision domain and broadcast domain, the collision domain is completed and I’m pretty sure you will definitely learn from it.
What is Broadcast Domain?
We know that ethernet hub has all of its ports in the same collision domain. In other words, if a collision that occurred on one port will effect on all other ports. Let’s contrast with a collision domain with a broadcast domain. A broadcast domain defines how far a broadcast is going to radiate throughout the network. If we look at layer 2 Mac address of a broadcast frame. The destination Mac address is all ‘F’ address.
There is no device on the network that would have network interface card with a Mac address of all ‘F’. This means that the switch it would never learn all ‘F’ Mac address and what a switch do with an unknown destination Mac address. It floods that frame out of all of the other switch ports other than the port that frame received on. Now get back to our ethernet Hub. All of the ports on ethernet Hub belongs to the same collision domain. They also belong to the same broadcast domain. A broadcast seen on one port will be sent to all other ports on an ethernet Hub.
Switch Collision Domain:
So what about switch collision domain? In ethernet switch, every port has its own collision domain. What about broadcast? Remember we just say that a switch will never learn all ‘F’ Mac address. As a result when it receives a frame on one port that destined to all ‘F’ Mac address or broadcast frame. It’s going to flood that out all over the other switch ports other than the port where the frame was received on. This means all of the ethernet switch ports belong to the same broadcast domain. This could start to impact on the scalability of our network if we have a very large network and we have lots of broadcast traffic. So the big question is What will separate the broadcast domain? A Hub doesn’t separate broadcast domain and ethernet switches don’t separate broadcast domain.
Router Separate Broadcast Domain:
A router is layer 3 device is the network layer of OSI model and a router will separate broadcast domains. Broadcast traffic seen of the one router port will not be seen of the other router port. In other words, a router is not going to forward that broadcast traffic. As a result of a layer 3 device like a router or even a multi-layer switch. A multi-layer switch that can do routing a layer 3 device like that it can separate our network into different broadcast domains.
Below the points present to differentiate between what is collision domain and broadcast domain. What is collision domain it is explained below in bullet points.
- Only one frame is allowed to be on a shared segment at any one time.
- CSMA/CD carrier sense multiple access with collision detection.
- Collision interference occurring as a result of two stations transmitting at the same time.
- The continued transmission of a frame that has collided, in an effort to allow all devices on the shared segment to detect the collision.
- All ports on an Ethernet Hub belong to the same collision domain.
- Each port on Ethernet switch is in its own collision domain.
- Ethernet switch ports can be configured to simultaneously transmit and receive when it is configured as Full-Duplex.
- Full-Duplex mode disables CSMA/CD on a port.
- Ethernet Hub ports can only receive or transmit at any one time when it is configured as Half-Duplex
- Half-Duplex mode uses CSMA/CD.
- Ethernet switch provides scalability to our networks.
- Ethernet switch also provides efficient use of bandwidth.
Collision Domain and Broadcast Domain:
- Collision Domain A shared network segment.
- The broadcast domain defines how far a broadcast travels in a network.
- A Broadcast’s frame’s destination Mac address is FFFF:FFFF:FFFF
- Hub all ports belong to same collision and broadcast domain.
- All ethernet switch ports belong to the separate collision domains but to the same broadcast domain.
- Hubs and switches don’t separate the broadest domain.
- A router can separate broadcast domain.
- Router’s each port (also called an interface) can belong to a separate collision domain and a separate broadcast domain.
Now the difference between what is collision domain and broadcast domain is easily understandable.