In my first post of this blog, I will discuss the concept of collision in Ethernet transmissions. We all know that the use of switches instead of hubs in modern Ethernet networks has made collisions a thing of the past. But how?

When I started reading about Ethernet fundamentals, I was a little confused about the process of collision. If we think about the early days of Ethernet, it is easy to see how collisions occured. The standards 10BASE5 (Thick Ethernet) and 10BASE2 (Thin Ethernet) both used coaxial cables and were based on a bus topology. In a coaxial cable, we only have two wires: the central conductor and the outer metallic shield, both separated by a dielectric material. Since all the devices are connected to the same cable, if two devices happen to transmit at the same time, the two signals sent are mixed and a collision occur.

But, what happen in a twisted-pair cable? For example, in the old standards 10BASE-T or 100BASE-T, two pairs of cables are used. Since one pair is used for transmission and the other pair for reception, how can we get collisions? In this case, the collisions are a consecuence of the use of hubs.

Hubs are dumb devices. When a hub receives a frame, it sends the frame out all the ports except the port the frame it was received on. For example, the diagram below represents three devices connected via a hub:

If PC1 sends a frame to PC3, the hub will receive the frame on its Fa0 port and it will forward the frame out ports Fa1 and Fa2. So, PC2 will also receive a copy of the frame destined to PC3, that will be dropped. But, if PC2 also sends a frame to PC3 at the same time, the hub will forward the frame out ports Fa0 and Fa2. So, the hub will transmit two electrical signals corresponding to the frames sent by PC1 and PC2 through port Fa2 (using its transmission pairs 3 and 6). It will generate a collision, and PC3 will receive a corrupted sequence of bits.

The introduction of switches solved this problem because switches are intelligent devices that work at the layer 2 in the OSI model, and they know out which ports send the frames and when to send them. If we replace the hub in the previous topology by a switch, when PC1 sends a frame to PC3 we may find two different scenarios:

If PC3 MAC address is in the MAC address table of the switch, the frame will only be forwarded out Fa0/3. If PC3 MAC address is not in the switch MAC address table, the switch will forward the frame out all the ports except the receiving port, like a hub. In both scenarios, if PC1 and PC2 send a frame to PC3 simultaneously, the switch will not start forwarding both frames time out Fa0/3 generating a collision. One of the two frames will be stored in buffer while the other frame is being sent, therefore, collisions will never occur. In a switch, every port represents a different collision domain. Even though a port is configured in half-duplex mode, collisions will never occur as long as there is only one device connected to that port. If more than one device is connected to a switch port via a hub, then only one device can transmit at a time and collisions will need to be handled by the CSMA/CD algorithm.