I thought a bandwidth is a number represent maximum amount of traffic. But it seems not.

Let me show the Network map in my company.

enter image description here

In this image. WAN has a 155Mbps bandwidth and there are a lot of PC and mobile devices. I tested network benchmark from 6 PCs at the same time. The result of min download speed is 91.2 Mbps and min upload speed is 88.7 Mbps. For this reason I wonder how this is possible or it works even though WAN has 155 Mbps.


I used Mbps as mega bit not byte.

It seems I gave you confuse what I want to know. I'm not good at English so It was my best But let me try more to explain specific.

I did't mean why the speed less than 155 Mbps even though the bandwidth of WAN is 155 Mbps.

The thing is how 6 hosts can speed almost 100 Mbps at the same time. WAN is the Internet bandwidth which is ISP(Internet Service Provider) gave our company and that moment may be more than 100 hosts use the Internet when I tested. So How can 100 hosts have speed over than 90 Mbps at the same time even ISP gave us the 155 Mbps for the Internet?

  • 1
    What did you use to view the speeds? Is it possible that it was showing Megabytes rather than Megabits?
    – SleepyMan
    Jan 12, 2017 at 12:02
  • 1
    You should directly connect to your internet circuit and test. Definitely don't test from the PCs connected to a Hub (if that truly is a hub in your diagram). Better yet, throw that hub away and replace it with a switch.
    – John K.
    Jan 12, 2017 at 15:34
  • @SleepyMan it is mega bit
    – the1900
    Jan 13, 2017 at 9:50
  • @JohnK. The question is not about slow things. Sorry about my English.
    – the1900
    Jan 13, 2017 at 9:50
  • So you're receiving 600Mbps from a 155Mbps WAN? That's clearly not possible. Maybe there's some kind of compression or caching going on? Are you sure that you are accessing through the WAN and not locally? Jan 18, 2017 at 19:57

4 Answers 4


There are many possibilities and substantial further testing is needed to narrow down which ones apply in your case.

One is that the speed test you are using only actually tests for a very short period, so although you think you are testing multiple machines at the same time you are not.

Another is that the speed test is downloading the test file from a cache in your network, not the internet.

Another is that the 155Mbps is not in fact a hard limit but a soft limit which is only enforced after it is exceeded for a sustained period.

Another is that your ISP only enforces the speed cap on traffic it considers "out of network" and there is a speed test server for the site you are using within your ISPs network.

Another is that your ISP simply forgot to set up the limit on your line. ISPs are run by humans who screw up from time to time.

Another is that your ISP is playing the "cheat at speedtests" game and this has resulted in speed test traffic bypassing the limiter.

  • Nice answer. A good summary of some of the pitfalls with so-called speed tests.
    – marctxk
    Jan 20, 2017 at 10:42
  • Well I tested on speedtest.net. It seems not cache right? and what are soft, hard limits?
    – the1900
    Jan 25, 2017 at 15:48

You really don't provide enough information to accurately answer the question.

WAN circuits can use physical circuits that are different speeds than the LAN connections used by hosts. It is possible that the hosts connect to workgroup switches at only 100 Mbps, despite the 1 Gbps connection between the firewall and router.

Also, the diagram shows a hub, and those connections are half-duplex. The hosts connected to a hub compete for sole use of the hub for each frame, and collisions will slow the effective throughput.

With 1412 PCs, you certainly have a large bandwidth oversubscription with many hosts competing for the limited upstream bandwidth resource. If you divide 1000 Mbps (1 Gbps) by 1000 hosts, each would average 1 Mbps. You don't give us how many hosts are competing for the upstream bandwidth.

  • thanks to comment I edited my question so Could you read the question again? I'm sorry to bad in English
    – the1900
    Jan 13, 2017 at 9:49

your link on PC's is up with 100Mbps (because of 100base-t standard) but in SDH (WAN) link, it has 155Mbps (because of STM-1 standard).

on 100 Mbps you can have 90Mbps throughput because of tcp syn/ack and re-transmission. (with udp you can have 100 Mbps)

if you want to test your SDH(WAN) link, you should use 4 PC. two pc in your company and two in other place. you sould use two iperf server and client in same time.


Hope that my answer will clear what you are asking: first of all lets talk about the speed test and what does it actually mean. When you do any speed test from any pc at your network (recommended to do the test directly from the ISP router)well,it shows the bit rate for the download direction that can be transferred from the pinged server to your pc and the upload speed will show you the bit rate from your pc to the pinged server. and when you do the speed test ,you are not actually using this data rate ,you are only sending the ping message and getting the response. but the real usage will happen when your pc and the other pcs in your network will try to actually download/upload data from/to specific web server.

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