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When I check my upload/download speed it’s 4 Mbit/s. This means that I can download/upload at 4 megabits per second. But I do not completely understand what this means.

For example, if I am uploading a 4 Mb file (considering this size for simplicity) through any web application from London and my Internet speed is 4 Mbit/s, will I be able to upload this file in 1 second irrespective of the actual physical location of the server (be it Australia or New York or any other location in the world)? So, when we say that the upload speed is 4 megabits per second, to which location does that apply? Surely it should always be in respect of the destination location, is that not so?

Update:- All i just considered file size as 4 Mb for sake of simplicity. I also understand 1 Mbps is not equal to 1 MBps. 1 MBps = 8Mbps. This question is actually what speed means practically ?

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    File sizes are in MegaBytes (MB), but line speed is in MegaBits (Mb/s). 1MB takes 8 seconds to transfer at 1Mb/s (because there are 8 bits in a standard byte). Your ISP can only advertise the speed they provide, so once your traffic leaves their network, and crosses other networks, there is no guarantee that the speed will remain the same. so no, it is not without regard for location. In practical terms, it will always take additional time to reach half way around the planet than it takes to walk next door. – Frank Thomas Sep 7 '16 at 12:54
  • @FrankThomas Say i want to upload some file on amazon site. Assume myservice provider is PEAK. Do you mean first data will travel to PEAK(and that speed will will be 4 Mbps . So even if it is 4 Mbps between me and my isp then does it mean my data will reach to my ISP irrespective of its location ? I believe NO). Once data reaches to my ISP, it will try to connect Amazon server. Which network ISP is going to connect Amazon server ? I sit common network or PEAK owns network ? – user3198603 Sep 7 '16 at 17:29
  • @user3198603 - You will connect directly to Amazon. Frank's point is, you can only upload at the speeds your ISP provides, but the slowest common denominator could be the desination. It isn't Amazon in this case, who likely will exceed your ISP's capabilties, by several factors. – Ramhound Sep 7 '16 at 20:37
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    "4 Mb file" Do you really mean a 4 megabit file? Or a 4 megabyte file? – Peter Mortensen Sep 7 '16 at 22:08
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    Note that when you upload a file, it doesn't only transfer the file, but also other meta information, such as HTTP headers, TCP packet wrappers, etc. Depending on the protocol, the actual data you transfer can actually magnify to a notable extent. – SOFe Sep 8 '16 at 8:54
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This speed refers to the link between you and your ISP. It does not guarantee than you can get that speed from any place on the Internet.

Lets create the example where you upload a file from your desktop to a server in London:

  1. Data is on your PC.
  2. Data leave via your local LAN to the default gateway. (Most likely at 100Mbit/sec or 1Gbit/sec if you have a wired network).
  3. Data arrives at the modem and is uploaded at 4Mb/s to your ISP. If that is a global ISP then it will be uploaded to their local data center.
  4. Data is then routed in an unspecified way to the server in London.

Step 4 is intentionally vague. The routing can change if lines go down, if inter-ISP connections are changed. If lines are overloaded and routing is intentionally changed etc. It was intentionally build this flexible. If you want more details in why look up ARPA net and the cold war.

... and my internet speed is 4Mbps, will I upload this file in 1 seconds irrespective of server actual physical location (be it Australia or New York or any other location in world).

Assuming the 4Mb/sec is the slowest link in the path to the destination: Yes.

It might help if you think of these two analogies:

  1. Build a chain with different thickness of links. The chain is as strong as the weakest link. Or a set of pipelines. Your local pipelink is 4cm. Flow though ti will not go faster if it is connected to a bigger pipe. If can slow down if it needs to go though a thinner pipe (e.g. if the server in London is on 33600 bps
  2. Qua routing: You do not set up a full path to the destination. It is more like posting a letter. if it is for a local house, put it in theior mailbox, else put if in the postbox. You do not care how the mail flows internally, just as long as it arrives. Routing for IP is similar.
  • Say i want to upload some file on amazon site. Assume myservice provider is PEAK. Do you mean first data will travel to PEAK(and that speed will will be 4 Mbps . So even if it is 4 Mbps between me and my isp then does it mean my data will reach to my ISP irrespective of its location ? I believe NO). Once data reaches to my ISP, it will try to connect Amazon server. Which network ISP is going to connect Amazon server ? I sit common network or PEAK owns network ? – user3198603 Sep 7 '16 at 17:29
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    Correct. (7 more chars to go with needless stuff to get to the required minimum comment lenght). Argh. I really only needed just one word and a dot. – Hennes Sep 7 '16 at 18:35
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    What is "Qua routing"? – cutrightjm Sep 7 '16 at 19:10
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    So, really, you can not assume that once data arrives at your provider, everything is fast. Like hobbs said, that very fast line is also shared by very very very very very very many people! Oh, and also at the very end of its travel, the data might again arrive at some guy just like you who has only a very slow connection. – Jörg W Mittag Sep 7 '16 at 20:28
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    @user3198603, nobody seems to have mentioned protocol overhead or round-trip time. Most file transfers use TCP, and TCP is limited by factors other than bandwidth, such as round-trip time and window size. Also there is TCP slow start, TCP retransmits where required plus any application layer overheads. This all adds up to extra delay, and this means that for small files at least you will getter better data transfer rates accessing a server that is close, rather than one that is on a different continent. – marctxk Sep 8 '16 at 9:33
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Bits vs Bytes

Bit = A Single 1 or 0

  • = _

Byte = 8 1's or 0's

  • = _ _ _ _ _ _ _ _

  • To get [Bytes per Second] (or megabytes, giga, etc) Simply take ___ Bits and divide by 8

Storage is measured in bytes, why?

  • Bytes are [Data] because a Byte, being 8 1's and 0's add up to make [A Single Letter] Letters are information to a computer. But a single bit means nothing until you get 8 bits.
  • Bytes are 8 bits.
  • 1 MegaByte is 1000 KiloBytes. 1000 MegaBytes make a GigaByte etc... metrics.

==>> Important Update <<==

For those of your trying to correct my Bytes

Please visit Wikipedia /wiki/Mebibyte

Thankyou

End Of UPDATE

Data Transfer is measured in BITS, why?

Because the lowest piece of information you can send is a 1 or a 0 (on or off). So if you turn on a flashlight, thats "On" thats a 1, and turn it off thats "Off" or 0. - This is how computers talk to each other by pulsing each other 1's and 0's.

But how fast do they pulse at each other in a second?

Well that would be how many bits per second?

So we say "Bits per second".

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    Correction: 1 Megabyte = 1024 Kilobytes = 1024*1024 Bytes. It isn't perfect metric, it's just close (it's really 2^10, since binary. It just happens that 2^10 is 1024, which is really close to a thousand so we just use that even though it's wrong by 2%). – Delioth Sep 7 '16 at 20:45
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    In (async) serial communication 1 byte can effectively be 10 bits or more due to the overhead of the start bit and stop and/or parity bites, etc. – Peter Mortensen Sep 7 '16 at 22:11
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    @Delioth Not true. Megabyte = 1000*1000 bytes. Mebibyte = 1024*1024 bytes. See en.wikipedia.org/wiki/Binary_prefix . Please don't continue spreading the confusing overloaded units. – Nayuki Sep 8 '16 at 3:24
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    Megabytes are 1000 bytes => No. Kilobytes are 1000 bytes. – Thomas Ayoub Sep 8 '16 at 7:40
  • @Nayuki Simply using the SI units without clarifying only adds to the confusion of "Why is my 1TB drive showing up as only 931GB?" I don't see the issue with adding that explanation. You're not likely to be dealing with SI units in throughput. If you really hate the confusion, the answer should stick only to kibibytes and mebibytes. – Arda Xi Sep 8 '16 at 9:51
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I'm assuming the file size is 4 mega bits, even though file sizes are usually measured in bytes (8 bits). this means the file is 4,000,000 bits large.

If the connection between you and the receiving party is 4 mb/s (4,000,000 bits per second) exactly, without speed changes during the transfer, the transfer will take exactly 1 second to complete. The total time between you starting the transfer and it actually completing may be larger due to the latency between you and the recipient.

When using a site like speedtest.net, you test the transfer speed between your computer and one of their test servers (They show a little map indicating the position of the server). The result of this test depends heavily on intermediate networks, since your final speed will be that of the slowest link in the chain.

  • cascer1, I'm not a networks guy, and the last time I did any kind of calculations was when networks consisted of paper cups and string, so this may seem like a stupid question. My recollection is that we used to assume 10 bits per byte transmitted to account for overhead (check digits, etc.). So if you had a throughput of 4 Mb/s, a 4Mb file would require 1.25 seconds rather than 1 second. Is this no longer the case, or are you just keeping the explanation simple? – fixer1234 Sep 7 '16 at 18:41
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    I'm not a network guy either I'm applying my knowledge of a single semester of network infrastructure together with what I think I know about networking. I've never heard about 10 bits to the byte, but it sounds like a baker's byte – cascer1 Sep 7 '16 at 18:58
  • @fixer1234 Also (I can't edit my other comment for some reason), perhaps that 10 bits per byte was a pretty accurate approximation back in the day but I don't think it's very accurate anymore. Again, I base this basically on nothing except for intuition. – cascer1 Sep 7 '16 at 19:12
  • Looks like things have gotten more complicated since the dawn of time. This triggered my interest and I did a little Googling. tamos.net/~rhay/wp/overhead/overhead.htm and cisco.com/c/en/us/about/security-center/… get into this. Without allowing for losses, it looks like a modern network can be a lot more efficient, with overhead as low as about 2.5% (worst case without losses ~83% for smallest packet size). I'm not sure what the old 25% was based on. – fixer1234 Sep 7 '16 at 19:39
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    10 bits accounts for one parity bit and one control bit. But, as you say, things are actually a bit more complicated. There can even be compression involved, making it on average less than 8bits/byte. – ths Sep 7 '16 at 19:52
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To make it simple, let’s say that yes, 4mbps is your stable/max connection speed: no drops, no increase, etc.

You will upload and download at that speed constantly on your end, but it all depends on the server you’re uploading to or downloading from: if they only have 1Mbps then your max speed will be 1Mbps, but if the max speed of the server is 1GB then you will max out at 4Mbps.

You can only go as fast as the weakest speed.

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In addition to other answers

If you upload your file to a fast server, so the link to your ISP is the slowest link in a chain, then the transfer will be limited to 4 Mbps by your ISP – it's true in general for large files.

But if the hardware can do better (i.e. the 4Mbps is an arbitrary limit set by your ISP because you pay for this option only, not some faster one) and your file is small enough then you may experience "burst upload speed".

It means your ISP may allow you to send the first 10 MB (an example, the actual value is set by your ISP) of a file a lot faster than the declared 4 Mbps if your link has been idle (or almost idle) for a while. After these 10 MB the link gets "saturated" and the 4 Mbps limit applies for remaining data. Make your link almost idle for another while and you will be able to burst again.

In other words: if you keep your upload low for a few seconds you gain a credit of some bytes you can upload a lot faster before your limit kicks in.

This mechanism makes your web browsing more snappy, improves uploads of files.

From ISP's point of view many of their clients generate such upward burst only. They (as a group) hardly ever saturate the ISP's link to the Internet (its upload bandwidth) because the bursts occur randomly at different moments, so there's no point in limiting every single connection. Thanks to the bursts these clients are happy with more responsive Facebook when they post their food photos. On the other hand few clients using P2P or sending large files at the same moment may saturate the link to the outside, so they are limited when they exceed some threshold.

A file of 4 Mb in your example is rather small, it is 0.5 MB. It may be transferred a whole within a single burst if your ISP supports it. You may hit some other limit (or temporary network slowdown) somewhere beyond your ISP though.

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    I think you are being down voted because your explanation of saturation and burst. Usually ISP's advertised speed is usually the burst speed, and rarely is line saturation an issue. Considering that cable can get up to 300Mbps+ now and VDSL can do more, depending on the DSLAM, etc, your answer is a bit off. – Ian M Sep 8 '16 at 5:39

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