COMPUTER DESIGN QUESTION ABOUT LAN. PLEASE HELP AND EXPLAIN.?

Question

Suppose there are five computers connected together to form a local area network (LAN). The maximum data transport (bandwidth) that the network cable can provide is 10 Mbps. If we use a low-end device (Hub) to connect them, all the computers in the network share the 10 Mbps bandwidth. If we use a high-end device (Switch), then any two of the computers can communicate with each other without disturbing the other computers. If you want to download a 10 MB file from a remote server, which is located outside your local network, how long will it take if using a Hub? How long will it take if using a Switch? Assume the other four computers only communicate with each other, and each has a constant data rate of 2 Mbps.

Answer 1

A hub is a 1/2 duplex device plus you are going to have collisions with the hub that you do not see with the switch

With a switch

So if the 4 computers have a constant data rate of 2 Mbps that is using up 8 of your theoretical 10 Mbps bandwidth. That leaves you  2 Mbps for file transfer so it should 5 seconds to download the file

With a hub

Since the hub is a 1/2 duplex device - you cannot get the numbers you say you have

The system would be limited to 5 Mbps

Each computer would have 1 Mbps bandwidth

You might download the 10 MB file in 10 seconds

But you cannot compute collisions and they could easily double your download time and cut your performance by 1/2 again

Answer 2

First off all we cannot assume  a constant data rate because packet switching is not 100 percent efficient. If you require this, then employ an ATM protocol over OSx.

Switches are fast because the use MCUs to multiplex data. Assuming QoS disabled:

10MB = 100Mbits

Hub

100Mbits / 2Mbps = 50 sec

Switch

100Mbits / 10Mbps = 10 sec

Answer 3

Im trying to remember this information from cisco classes a log time ago but i should be pretty close.  Using the switch assuming your getting optimal bandwidth from the remote server at 10Mbps you could receive the file in 8 seconds.  other wise it depends on the servers load from other users and other bandwidth traffic along the way to your network.    Using the hub the time would be undeterminable due to an unpredictable amount of collisions in the hub which would result in packets having to be resent.  It might also cause corruption of the file as well.  Thats about as well as i can explain that hope it helps.

Answer 4

You did not address your Internet speed.  Since you are downloading from a remote server, presumably over the internet or over a private network (WAN), you need to remember that the WAN speed is slower than 10 Mbps in most cases.  Therefore the hub or switch will not be the slow step.