Storage Devices and Network Hardware: Question 8
Syllabus 3.4
A small office originally connected its four computers using an old hub. The office manager noticed that whenever one computer sent a large file to a second computer, all four computers' network activity lights flashed at once, and the two computers not involved in the transfer noticeably slowed down while browsing the internet, even though the file was only meant for one of them. The office replaced the hub with a switch that has the same number of ports, and this problem no longer happens.
(a) Explain how the hub handled the incoming file, in a way that caused all four computers to be affected even though the file was intended for only one of them. [2]
(b) Explain how the switch is able to send the same kind of file so that only the intended computer receives it, without affecting the other two computers. [2]
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Worked solution
Part (a): Why the hub affected all four computers
A hub has no intelligence about where data on the network should go. When it receives data on one port, it simply broadcasts (floods) that data out of every other port, sending an identical copy to every connected device regardless of which one the data is actually meant for. In this scenario, that means all four computers received a copy of the file being sent between two of them: each device’s network activity light flashed as it received the data, and processing (or discarding) the unwanted copy created extra network traffic, which is what slowed down browsing on the two computers not involved in the transfer. [2 marks]: [1] for stating that a hub broadcasts/sends data to every connected device, [1] for linking this to why all four computers were affected by data meant for only one of them.
Part (b): How the switch avoids this problem
A switch, unlike a hub, keeps a table of MAC addresses, recording which device is connected to each of its ports. When data arrives at the switch, it reads the destination MAC address carried with that data and looks it up in this table to find out exactly which port the intended device is connected to. The switch then forwards the data only out of that one port, rather than out of every port. Because the file is sent only to the port the intended computer is plugged into, the other two computers never receive a copy of it, so their network connections are not affected. [2 marks]: [1] for stating that a switch uses a table of MAC addresses (or otherwise identifies the intended device), [1] for explaining that it forwards data only to that one specific port/device.
Final answers
- (a) The hub broadcasts incoming data out of every port to every device, so all four computers received the file and the resulting extra traffic slowed down the other two.
- (b) The switch uses a table of MAC addresses to identify which port the intended computer is connected to, and forwards the data only to that one port, leaving the other computers unaffected.