An Ethernet hub's speed for this network....thanks

[cmcm88992]

sorry for my poor english...i am from china :(

in this case,four computers connected to one hub,and the hub's speed is 100Mbps

normally when four computers send a frame to the hub at the same time,hub's speed should be 25Mbps <---this i know

and when if one computer send to another computer when other pc idle(e.g PC0 send a frame to PC1) the hub's speed should be 100Mbps

but may i ask ....why???

hub's function is when it receive a data frame,it will send to every port included the source

that mean if PC0 send a frame 100Mb to PC1,when the frame go to hub,hub will send the frame to PC0,PC1,PC2,PC3(but only PC1 will get it,other PC will delete the frame)

the speed should be 100Mbps/4 = 25Mbps

why the answer is 100Mbps??

sorry for my poor english again ...thanks everyone  :'(

[Joseph W. Doherty]

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Posting

normally when four computers send a frame to the hub at the same time,hub's speed should be 25Mbps <---this i know

Sorry, your "know" is incorrect.

If all four computers send a frame at exactly the same time, the frames will collide.  All four computer NICs should detect this, and try sending their frame again, but the retries should be at different times, see http://en.wikipedia.org/wiki/Carrier_sense_multiple_access_with_collision_detection.

All four computers, between carrier sense and collision detection, will share the 100 Mbps.  Ideally, each will get its fair share (your theoretical 25 Mbps), if all want to transmit, but because of collisions and back offs, they will realize less (possibly much less).  When there's only one station transmitting to the other, it won't have the collisions and back offs, so it can utilize all 100 Mbps.  However, if the receiver is sending ACKs, that too will cause some collisions and back offs and so the 100 Mbps won't be obtained.

Something to think about, for four PCs, what's better, a 4 port 10 Mbps switch, or a 4 port 100 Mbps hub?

[cmcm88992]

that mean hub's reason of make the bandwidth slowly cause of CSMA/CD to avoid the collision?

if it don't have collision,four computer same a frame at the same time,the speed is 100Mbps also??

and when PC0 send a frame to PC1 , even the frame will also go to PC0,PC2,PC3

totally four destination at the same time,the speed is

PC0 send to HUB 

HUB to PC0 with 100Mbps (PC will delete it)

HUB to PC1 with 100Mbps (PC get it)

HUB to PC2 with 100Mbps (PC will delete it)

HUB to PC3 with 100Mbps (PC will delete it)

those 4 action at the same time,am i right? thanks :)

[Martin Carr]

As said, you have one collision domain, therefore the bandwidth is shared, and a percentage will be lost, due to collisions.

With switching, one advantage (of which there are many) is that each port is in it's own collision domain, therefore to answer Joseph's question, the 10MB switch would be better, in addition CSMA/CD is not required.

Martin

[Joseph W. Doherty]

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Posting

You're sure the 10 Mbps switch is always better than a 100 Mbps hub?  ;)

PS:

BTW, 15 years ago, or so, this was a real question.

[Joseph W. Doherty · Accepted solution]

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Posting

FastEthernet always conveys the bits at 100 Mbps.

When PC0 sends to PC1, the hub will replicate the frame to all other ports but PCO's.  PC0 doesn't send to the hub, it puts the frame on the wire, and if intended just for PC1, the frame will have PC1's MAC as its destination MAC.

All other PC NICs will see the PC0 sent frame, but depending on its addressing, will either ignore it or accept it.  I.e. if destination MAC is the NIC's MAC or a special multicast or broadcast MAC, it's accepted.

It might help if you remember a hub simulates original Ethernet using a single shared wire.  NICs place frames on the wire, all other NICs will then see the frame.  Since a hub has multiple wires, it replicates an ingress frame to all the other ports.  Hubs are also known as repeaters.

Or, think of shared Ethernet, whether shared wire or hub, as similar to multiple people talking within the same room.  When one person speaks, all can hear.  If more than one person speaks, at the same time, often you can not understand either speaker.  When speaking in a room, but information is directed to just another person, often you'll preface person's names at the beginning of the speaking (e.g. like a teaching calling on one student).