What are the collision domain reducing devices? if collision reduce how it advantage to network?
A collision is the moment when two or more frames from two or more different devices are in the network at the same time. Collisions are a feature of networks, as ethernet, that were designed to share the same media (collision domain) with a contention-based access method.
When a collision happens, the frames become unintelligible, so the devices that sent the frames have to retry later and other devices in the network have to wait until the collision dissapears to try to use the network.
Having a few collisions is not a problem. However, if 50% of the frames in a network suffer collisions, your network will be slow, and a slow network can produce higher layer problems as application disconnections, errors, etc.
As more devices are connected to a network the chance of having collisions increases, so the best way to reduce collisions is to segment the common media. This segmentation is achieved with switches. Each switch port is a "collision domain", it means that collisions happening in a port are not reproduced in the other ports of the switch.
When your devices are connected to an ethernet hub and device
A sends a frame to device
B, all the other devices see the frame and are unable to use the network until that frame ends.
When your devices are connected to an ethernet switch and device
A sends a frame to device
B, the frames go from port
A to port
B and the other ports don't see the frame so they see the network free and can use it. It means that multiple conversations can be achieved and that collisions happening in one port are isolated to that port.
In networks with a shared medium, a collision domain encompasses the nodes connected to it. In each collision domain, only a single node may be transmitting at any given time.
If two nodes start transmitting at the same time (due to a signal's propagation delay, this isn't noticeable immediately) the data is garbled, so they need to cease transmission and follow the protocol before trying again. Usually, there will be some random delay before that, so the network bandwidth isn't utilized in this time. Bandwidth is wasted for the time before the collision detection (the unsuccessful, partial transmission) plus the jamming time (to make sure each node has detected the collision) plus the necessary delay (to make a repeated collision less likely).
The effect can be reduced by separating larger collision domains into smaller ones or by getting rid of collisions completely. A network device capable of buffering a packet/frame enables separation of the source's collision domain from that of the destination - this can be a switch, a bridge, or a router. This reduction in collision domain size decreases the likelihood of collisions and therefore can potentially increase network throughput.
When both sides of a link are capable of buffering, an appropriate link can even be used for transmitting in both directions simultaneously. This is called full duplex and it removes the collision domain completely. This is the common mode of fully switched Ethernet operation today. Wi-fi on the other hand uses a inherently half-duplex medium and cannot use full duplex (at least not currently).