Network Switches and Data Transfer Speeds

Network Switches and Data Transfer Speeds

I recently upgraded my home Wireless Routers from 100 Mb/s to 1 Gb/s by replacing the switches. The main house switch is an unmanaged 1U rack-mounted switch, with a second desktop switch. Out of pure interest, I took the opportunity to do a little bit of speed testing to see how much of a difference upgrading the switches made in terms of actual data transfer speeds.

A few basics to avoid confusion – b/s is bits per second and B/s is bytes per second. All of the reported figures will be in MB, so converting b/s to B/s:
Fast Ethernet = 100 Mb/s = 12.5 MB/s
Gigabit Ethernet = 1 Gb/s = 125 MB/s

100 Mb/s and 1 Gb/s refer to the speed of the underlying technology but data transfers at these rates are never achieved because of protocol overheads and such. As a baseline, if I write a large file (8 GB) to my PC’s local disk, I get a data transfer of between 50-55 MB/s.

On my network, I have two Buffalo Linkstation NAS devices, one with a Fast Ethernet interface and one with a Gigabit Ethernet interface. 2 GB’s worth of data would be written to each of these devices with different Ethernet switches in place to see what actual data transfer speeds would be achieved. The following Linux command was used five times in each situation and the result averaged.
I also carried out two further tests:

With Gigabit Ethernet only, I wrote to both NAS devices at the same time. The data transfer speeds were unaffected.
I connected the two Gigabit Ethernet switches in series and wrote to the NAS. Transfer speeds were reduced by 1 MB/s on the Gigabit NAS to 20 MB/s. The change on the Fast Ethernet NAS was minimal.
There are several things that can be deduced from the information shown in the table above and the other tests.

Actual data transfer rates are considerably less than the theoretical maximums.
There’s no performance difference between rack-mounted and desktop switches.
The write speed of the NAS can be a limiting factor.
Gigabit Ethernet switches give large improvements with Gigabit Ethernet devices.
Gigabit Ethernet switches give small improvements even with Fast Ethernet devices.
Keep the number of switches in the network path to a minimum.
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