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I transfer the file(140 MB size) from my network in india to client network in US through VPN. It takes around 40 mins. Here are ISP's speed

My ISP speed is 4 Mbps and clients is also 4 Mbps.

Here is my calculation of max time it should take:-

1 MB = 8 Mb. So with speed of 4 Mbps it will take 2 seconds to transfer 1 MB of data. For 140 MB it will be 280 seconds i.e 5 mins around. Similarly 5 mins for data from client ISP to his network

Total time will be 10 mins.

As all ISP's are connected phonically(i believe fiber optics) through Internet exchange. Time to transfer the data b/w ISP's should verly less.

In any case total time should not be more than 15 mins but it takes 40 to 45 mins. Any thoughts who can be culprit here or something wrong in my calculation ?

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First of all your calculation of time is wrong - in theory it should be 5 mins instead of 10. There is no buffer at Your ISP that would stop and cache the data before continuing to another network. Data transfer goes as fast as the slowest link in chain allows it - thus if both networks have 4 Mbps then the speed should be minimum of (4 Mbps, 4 Mbps) = 4 Mbps and thus 140 MB / 4 Mbps = 280 seconds.

This, however, doesn't apply to long distance transfers. Your ISP and the ISP of your client offer 4 Mbps speed, but only to their parent network. They generally can't guarantee you the same speed to another country or continent because they have no control over it.

These transfers depend on other factors like parent network load or bandwidth limit allowed to your ISP, etc. Many consumer-grade ISPs don't offer special plans specific for long distance transfers.

  • First of all your calculation of time is wrong - in theory it should be 5 mins instead of 10 its 10 mins because of 5 + 5. 5 mins from my computer to my ISP and 5 mins for client ISP to client computer. Hope you agree. – user3198603 Sep 10 '16 at 11:45
  • Regarding This, however, doesn't apply to long distance transfers. Your ISP and the ISP of your client offer 4 Mbps speed, but only to their parent network. Per mine understanding all ISP's are connected physically (thru internet exchange either fiber optics or some other medium), So what do you mean by parent network here ? – user3198603 Sep 10 '16 at 11:48
  • @user3198603 1. That would be true, if the file was first saved at your ISP and THEN traveled to the client's location - but you're transfering it directly. - Here is a paralel example: You want to transfer 14 m^3 of water. You feed the water into pipe with speed 4 liters per second and client extracts it at 4 liters per second - the pipe allows transfer speed 4 liters per second. You can't feed faster, because the pipe cannot inflate. The client can't remove water faster because the pipe would empty out. Thus it will take 14*1000/4 = about 4 hours. – Marek Rost Sep 10 '16 at 12:07
  • @user3198603 2. ISPs are generally connected to a country-wide network, which then connects to international network. At each connection point there are huge network switches and even those switches have limit to how much data they can transfer at once. It's not problem of transfer speed - light and electrons travel almost instantly, but there is a limit on how much data can you transfer at same time. Hope this explained it a bit. – Marek Rost Sep 10 '16 at 12:14
  • @Mark Rost Regarding parallel i was under impression when i say 4Mbps its speed b/w my moden and my ISP.. I mean with in 1 sec , 4Mbdata will be travelled to my ISP but cover further distance like to client ISP. It will take more time. What i get from answer distance is not a constraint here. With in 1 second , 4Mb data will be transferred across any distance . Right ? – user3198603 Sep 10 '16 at 12:21

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