In general, the Ethernet Switch use RJ-45 link to connect the Hosts and the Switch uses Store and Forward mode.

So, why is there a saying:

Switch can segregate the collision domain

I thought the Switch Store and Forward mode and RF-45's full duplex mode eliminates collisions.


A collision domain is a network segment connected by a shared medium or through repeaters/switches where data packets may collide with one another while being sent. The collision domain applies particularly in wireless networks, but also affected early versions of Ethernet. A network collision occurs when more than one device attempts to send a packet on a network segment at the same time. Members of a collision domain may be involved in collisions with one another. Devices outside the collision domain do not have collisions with those inside.

Since every port in a switch is its own collision domain, a host will never collide unless it's medium is running half duplex.

That is why a switch segregates collision domains, because every port is an individual collision domain.

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The main point with switches is that they buffer network frames. This enables a switch to receive a frame and then forward it later when the egress link is idle. This decoupling of receive and transmit operations enables a network that works with flows that are largely independent from each other and only compete for link bandwidth.

This is in stark contrast to repeater hubs that repeat incoming bits as they are received. A hub can't buffer anything, so a collision on an egress interface needs to disrupt reception on the ingress interface - the hub needs to propagate an upstream collision back to the source. This way, all nodes connected to a hub (or potentially chained hubs) form a single, common collision domain. Only one node at a time can transmit.

A switch connected to a hub (or another non full-duplex capable device) can still use half-duplex mode on any of its ports but due to the buffering between the ports, any collision will not propagate across the switch - the switch will simply retransmit the buffered frame later on. Therefore, a switch segregates collision domains or removes them completely on full-duplex links.

Note that half-duplex Ethernet and hubs are very much things of the past and only interesting for historical reasons or under very specific circumstances. Practically all Gigabit and (by standard) faster Ethernet links have dropped support for half-duplex communication and you need to use switched or point-to-point connections exclusively.

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I thought the Switch Store and Forward mode and RF-45's full duplex mode eliminates collisions.

That's what "segregate collision domains" means. The switch eliminates collisions because the collision domain for each port is limited (segregated) to just that port. Combine this with full duplex mode and only one device on a port, and there's nothing left to collide with.

However, you can still have collisions if there is an old-school hub connected to the switch port and multiple devices connected to the hub. Thankfully, we try not to use hubs anymore, but this is why it's more correct to say "segregates the collision domain" over "eliminates collisions"; it's not that hard to contrive situations where collisions can still happen.

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