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So this really baffles me.

Apparently because 1Gbit can transmit data in both directions simultaneously it should be possible to get 2Gbit of data transfer on a single NIC (1Gbit flow seend and 1Gbit receive). People claim that because 1Gbit is full-duplex (almost always) it is exactly 2Gbit in total. My intuition and electrical background tells me that something is not right here 4 twisted pairs 250Mbit capacity each gives 1Gbit. Unless it is really possible to transfer data in both directions simultaneously.

I did a test with iperf. Ubuntu server 12.04 <--> MacBook Pro. Both with decent CPU speed. Tested speed of connection individually and on Mac I can see 112MB/s regardless which direction data is going. On Ubuntu with vnstat and ifstat I got 970Mbit speeds. Now, launching iperf in server mode on both machines at the same time and sending data using 2 iperf clients shows that I'm for example on Ubuntu box sending at 600Mbit, and receiving 350Mbit. which adds up to pretty much 1Gbit link.

So to me there is no magical 2Gbit. Can someone confirm that or tell why I'm wrong?

Another thing that confuses me i the fact that e.g. 24-port switch has for example: Throughput»up»to:»50.6Mpps Switching»capacity:»68Gbps Switch»fabric»speed:»88Gbps

Which would suggest thay can handle 2GBit per port.

UPDATE

I did test again with iperf -s iperf -c 10.0.20.91 -d -t60

which sets window to 212KB. In last test and I got

rx: 961.41 Mbit/s 97603 p/s tx: 953.53 Mbit/s 84725 p/s

on the server NIC, so it's definitely 1GBit each way, simultaneously.

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ivenhov
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2 Answers2

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Duplex is a bit of a misnomer in Gigabit Ethernet as there are not separate send and receive channels like in 10Mb or 100Mb Ethernet. In the lower speeds 2 wires are used to send, and 2 to receive. The other 4 wires aren't used at all (for data anyway).

In Gigabit Ethernet all 4 pairs are used to send and receive. It uses a 2 of 5 trellis coding: For simplicity sake we'll say that each end uses 2 of 5 possible "voltages", one for "1" and the other for "0". It measures the "voltage" on the line, subtracts the "voltage" of what the near side is transmitting and thereby knows what the far end is sending.

The actual details of how this works are more complicated, but the end effect is the same. Regardless Gigabit Ethernet can transfer 2Gbps of aggregate data under ideal circumstances. "Real World" tests will frequently be slower. Also, the Ethernet frames suck down about ~10% overhead, TCP and IP suck down another ~10%; so TCP based tests will usually max out in the mid-80% range.

Your switch must have more than just 24 ports as the switching capacity is more than 48Gb (the 24 ports x 2Gb). The switch fabric is the internal switching capacity and they usually limit the advertised number to the external switching capacity.

Chris S
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There is nothing to baffle- instead of testing, reading would have been enough.

1Gbit is per Definition full Duplex, i.e. send and receive at same time.

TomTom
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