The 70% number came from shared Ethernet, where everybody
effectively shared a single cable, and only one machine could talk at a time
- statistically you could not run data on it more than 70% of the time, and
it is 10Mb/s total across all the connected devices.
Switched Ethernet is different, in that it's only you and the switch on that
single cable.
What about 100 Mbps and 1Gbps ethernet ?
Both of these are only run in switched mode, not in shared mode.
Over that, you need to take care of the usual protocol overhead costs
You can look at tables or standards documents to identify
protocol overheads. In general these are "pretty small", around 10%
or so.
In terms of measuring if you have enough bandwidth available - you can't do
that deterministically on any frame/packet based network. You cannot predict
when another device will send a burst of large packets, which fill up buffers,
or a very large number of small packets, which chew up the CPU. This can vary
on timescales from milliseconds to mega-minutes. All you can do is look at the
path and identify where you might see congestion. Applications running over
IP have to be able to deal with congestion, leading to latency and jitter, or
leading to packet loss, or routing changes that also impact latency and jitter.
A sad fact, but that's also the power of the Internet.
On your local network, the main bandwidth constraint is the switch in the middle,
and then any other applications that are talking to your two endpoints. If you
have two PCs on their own on a dedicated switch and no other applications, you
should be able to get close to 100% throughput. I've seen over 95Mb/s transport
rate on a 100Mb/s Ethernet network.
If you have a need to guarantee bandwidth, there are some ways you can do that
- Ethernet and IP both allow traffic to be tagged with 'priorities', giving
those frames/packets better (or worse) treatment than other traffic. That requires
support from the hardware along the way though, and is not a 100% guarantee
in all cases. To get guarantees like that requires a more deterministic network
like ATM and its virtual circuits.
A PDU is 8 bits/byte times 144 bytes or 1152 bits.
NB. : Mbps = 1000 x 1000 bps, kbps = 1000 bps, Gbps = 1000 Mbps