Consider an Access Point with a packet queue that can hold up to 50 packets and a network whose bottleneck is a link with a datarate of 50 Mbit/s.
There are quite a few nodes connecting wirelessly to the Access Point, one of them uploads a file (1.5 GB) via FTP to a remote server. When observing the average throughput of the FTP upload, I have noticed it is around 13.7 Mbit/s. And now I wonder how this is possible? The bottleneck is 50 Mbit/s, right?
The only reason that I can think of is this: The bottleneck must be somewhere else. And since all physical connections of the network support a higher datarate, it must be with the Access Point.
The TCP MSS = 1460 bytes, its advertised receive window is scaled to 1,460,000 bytes. Is it possible to determine the network’s bottleneck with this data? If so, how?
Answer
Do not assume that components are performing ideally. Test.
Do a pure network throughput test over wireless to the FTP server, such as iperf. This removes the variable about how fast the storage is.
Temporarily plug a client, like a laptop, directly into the wired Ethernet that the AP is using. Do the FTP test and the iperf test.
- If wired FTP is much better, the bottleneck was wireless.
- If iperf is much better but FTP isn’t, there is still a bottleneck in the FTP service.
- If neither improves much, there is a bottleneck somewhere else on the network. Look at each hop, especially slower links like WAN/Internet.
802.11 is shared spectrum, horribly noisy, and prone to interference. You will not get close to the theoretical maximum.
Attribution
Source : Link , Question Author : user503842 , Answer Author : John Mahowald
