[roughbeast]

Originally Posted by watercooled

You are correct about it being overheads. When we talk about gigabit Ethernet it literally means 1,000,000,000 bits per second transferred on the wire, but not all of this is usable for actual data because of the way it is arranged into frames/packets. Each frame/packet needs a header to say where it's going e.g. a MAC header on layer two, then an IP header on layer 3, then more for TCP/UDP above that - you get the point though. More information available online if you search for those terms.

In an overly simplified way and ignoring multiple layers, lets say a packet can hold 95 bits of data with a 5 bit header. All 100 bits still need to be transferred over the Ethernet link but only 95 bits of actual data (payload) are being transmitted per packet. Translating that into speed, instead of 100 bits per second say, you now only have 95 bits per second. It is still literally 100 bits being sent mind, that's not in question, but like you say some is overhead because of how networks work.

If you have two computers on a LAN, set up file sharing and share a file between them, you'll see a similar limitation which tends to be around 930-950Mb/s throughput depending on things like packet size, protocol used, etc.

This is also true with DSL - if you're close enough to the cabinet for VDSL you will sync at 80Mb/s on the highest Openreach package, but do a speed test and this shows more like 76ish Mb/s for exactly the same reason. This is also why it's now advertised as such because that's the number that shows up in speed tests.

The reason this is different with Virgin is because what they sell as 200Mb is not 200Mb - go check your modem stats. Virgin have changed these profiles from time to time but at the time of making this post my profile speed is 230,000,061 bps. So pretty much 230Mb/s.

Hope that explains it well enough!

That's one of the clearest explanations I have seen. So, until cards,switches, ports etc all become significantly >1000Mbps no one is going to be achieving anything in tests over 930-950Mb. However, a connection over one gig still benefits a household or business with multiple devices clamouring for more and more bandwidth.Most of us just can't measure up to that higher speed yet - hence the Samknows realspeed facility.