switching:::mutiple devices sending data

[iamtheone12345]

Hi Guys,

Lets say there are 4 pcs connected to a switch.PC1,PC2,PC3,PC4.

Both PC1 and PC2 sends data to PC3 at the same time? what happens?

assuming that the switch knows the mac addresses for all PC's.

[burleyman]

PC3 will get the data form PC1 and PC2. Think of the wire as a highway with two ramps and the cars as packets. The packets will find open space and fit in the data stream and PC3 will assemble the two things. It would be like uploading two files to your PC from two different servers.

Mike

[iamtheone12345]

Is the switch gonna buffer data ?and then send the data from one PC at a time to PC3?Because if the switch has to forward data from both PCs to PC3 at the same time will collisions not occur?

[burleyman · Accepted solution]

Yes the switch will buffer the data as needed, but it does not need to wait till one is completed before the other can send it will do First in First out (FIFO). Again think of the highway with on ramps, as the cars go by the one on the ramp will look for an opening and merge right in if it finds one. if the traffic is so heavy that there is not an opening right away it will wait (buffer) until it sees an open spot.

Mike

[iamtheone12345]

Thanx mate!!!

[Joseph W. Doherty]

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Posting

Many advanced switches can also prioritize data.  I.e. if PC1 and PC2 data arrives same time, advanced switches can send the "more important" traffic 1st.

Collisions happen on shared medium where there's no buffering, e.g. on a hub.

[burleyman]

No problem Arjun....thanks for the rate.

Mike

[Edwin Summers]

I know this was already answered, but don't forget there are multiple layers at play in most cases.  Above these layer 2 frames could be, for example, TCP at layer 4 which is also metering traffic.  If both PCs are sending sustained traffic to PC3 (i.e. - we're not just talking about what happens when two frames arrive at the same time, but when two systems are simultaneously sending data to the same destination), TCP would meter the traffic.  Being acknowledgement based, each sending PC would wait for the acknowledgement from PC3 before putting more data on the wire (...before presenting it to layer 2 for transmission).

Once you reach the point of total saturation (congestion), TCP will react by cutting back the window.  UDP would continue to provide "best effort" service.