We sell gigabit services, as both Ethernet and FTTP. We see other ISPs selling 950M or 900M,
why? Will I get a gigabit?
A gigabit means one billion bits of data per second.
There are a couple of gotchas to start with - firstly a gigabit is 1,000,000,000 bits per second, it is not 2³⁰ (i.e. 1,073,741,824) bits per second. A lot of files on computers are measured in gibabytes or mebibytes, not gigabytes or megabytes. The second gotcha is this is bits, not bytes. There are 8 bits in a byte, so a gigabit is 125,000,000 bytes per second.
The answer is that when packets of data are sent to you, i.e. to your equipment in your home or office, those packets of data have a sequence of bits that are coming at 1 billion bits per second for the whole of each packet.
Well, that is how it works, packets of data are sent. You don’t actually get a “file” in one go, it comes as packets of data, and there can be lots of other packets of data for other reasons, and even gaps between the packets.
The main reason for gaps between the packets is that the Internet as a whole, and lots of the bits of infrastructure to you, are a shared service. You are sharing infrastructure with other people.
How much of that is shared and where it is shared depends on the service you have and where you are transferring data from. An un-contended Ethernet service from us means we can send
back to back packets at a gigabit from our data centre to you and get that true full gigabit speed with no gaps - yay! But even then, our equipment and the rest of the Internet is shared with other
people. For services like FTTP, some of the infrastructure between us and you, and even between the exchange and you, is shared.
Being shared does not necessarily mean you don’t get the full gigabit though - that happens when there is contention, and lots of the Internet is built with spare capacity at all times. We aim for A&A never to be the bottleneck, for example, so yes, most of the time, if you have a gigabit service you can expect a gigabit throughput.
Even when there is no contention and packet are coming back to back with no gaps at that full gigabit speed, that does not mean you can transfer a file at a gigabit (i.e. 125MB/s). But why?
The answer is that packets don’t just contain the data in your file. Even a full sized packet carrying, say, 1440 bytes of data from your file, will have extra bits. Typically 20 bytes TCP, 40 bytes IP, 26 bytes Ethernet with VLAN tag, 8 bytes PPPoE, all of which take up space in that packet. That means over 5% is not part of a file transfer even with full sized packets. That is one reason to see services sold as 950M, not because the actual service is not gigabit, it is, but because the way you are using it (Internet access) has overheads.
All of what we have said is true for any speed of Internet connection, contention and overheads mean the speed of a file transfer does not look the same as the underlying speed of the bits on
the wire, which is what you are buying. So what makes gigabit special? Well, a gigabit is fast. Indeed, with even just tens of milliseconds latency, it is way faster than TCP/IP was originally designed for. But let’s look at a few of the reasons a gigabit can be a challenge.
YMMV, as they say (Your mileage may vary). I personally have a gigabit fibre but using a FireBrick FB2900, and in my case, most of the time, the FB2900 is the bottleneck. Even so I have seen sustained downloads of a game s/w update at 850Mb/s. Some people with faster equipment than I have do get much higher speeds. A gigabyte (allowing for those packet overheads) is possible, but it will depends on a lot of factors, as you can see
