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For a duplex 1 GigBit NIC, [134,217,728 Mbytes], does this mean it can receive AND send both at 134MNBytes per second, which is 268 MByte total capacity; Or Send and Receive share the 134 Mbytes, so each can have about 67 MByes per second?

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up vote 8 down vote accepted

Full-duplex means that your transmit and receive are independent: you can transmit at 1Gbps and receive at 1Gbps at the same time.

The question of whether you'll achieve full line rate or not is another matter - there's plenty of factors which will prevent you getting full line rate. That's beyond the scope of this question though.

Also, minor point, but network bandwidth is measured using SI units, so a gigabit is 10^9 bits per second, not 2^30 bits per second. So your theoretical maximum is (1000*1000*1000)/8, not (1024*1024*1024)/8, or in other words, it's 125MB/sec in each direction.

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It depends on it being configured for full-duplex or half-duplex.

Usually, it should be full-duplex (full bandwidth in each direction).

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The ethernet spec actually mandates that gb ethernet must be full duplex. – chris Aug 20 '09 at 20:55
Correct. 'duplex' on its own doesn't mean anything, it's either 'half' or 'full' – Mark Henderson Aug 20 '09 at 21:03

Sounds like you're using Cisco Marketing bandwidth specs.

Mostly people pay attention to capacity in a specific direction without protocol overhead when they talk about a specific media type's bandwidth. 100mb ethernet, for instance, is not called 200mb ethernet, even if you would need that much capacity if you were to monitor a link running at full bandwidth in both directions.

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