Throughput vs. Sync Rate

So what would be the throughout one should expect to see from a speedtest or downloading if their VDSL line synced at 40000/10000kbps ?

and would the math also be the same for speeds from 2000/2000-> 80000/20000kbps?

probably around 37mbit/sec if all is working ok.

Is their a specific math behind it all... a percentage maybe... do you know any sites where I can get this information?

I calculate around 7.5% loss mark then for throughput over the sync rate

Maximum TCP/IP throughput is 96.8% of the sync rate.

The other differences are down to the various overheads on your home network etc
Even 100 Mbps Ethernet does not deliver 100 Mbps but a couple of Mbps below this.

OK...

So I'm syncing at 66000kbps ... I can only achieve a max throughput of 60700kbps...

I feel this is around 2000kbps shy of its capability...

Nothing shocking I know but curiosity

Yes. The overhead is essentially down to overheads on the framing, IP and TCP overheads.

The usual maximum packet size is 1500 bytes (there are so-called "jumbo-packets", but they are only used for specialist purposes, like local storage over IP and require special support). However, within that 1500 byte packet there is a 20 byte IP packet header and a 32 byte TCP header. That gives you a payload of 1448 bytes. However, there is also an Ethernet framing overhead comprising another 38 bytes. So, for every 1448 bytes of data there are 1538 bytes sent over the link. That's 94.1% payload efficiency, which means for a 40mbps sync there is a maximum of 37.6mbps data throughput available.

The proportion is the same no matter what the sync speed. So an 80mbps will have a maximum TCP payload throughput of 75.2mbps.

*** edited as there was a slight mistake where I'd used 1440 as the payload size ***

Note that this is for IPV4. IPV6 has larger overheads.

Also, there is an overhead for "ack" packets in the opposite direction (About 4%, so downloading at 40mbps requires a minimum upload bandwidth of 1.6mbps).

UDP protocols have different overheads, but that's not normally used for speed tests.

Edited by deleted (Wed 01-Apr-15 14:07:37)

Have you checked the current IP Profile rather than the sync?

In reply to a post by b4dger:

Have you checked the current IP Profile rather than the sync?

64mbps (BTw IP Profile)

Edited by mlmclaren (Wed 01-Apr-15 13:34:29)

That calculation is surely only covering the TCP & IP headers. Surely it's necessary to include the Ethernet frame overheads too (unless the stats reported by the modem include an allowance for that).