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New Member

Fastethernet throughput

Hi all,

I was told by a cisco expert that 100mbps full duplex fastethernet means 200 mbps of total throughput. Is it practical or marketing term ?


Re: Fastethernet throughput


I wouldn't call it a marketing term.

You can send and receive 100 mbps respectively at the same time.

Many applications typically send/receive asymmetric traffic volumes but voice is a good example for sending and receiving pretty symmetric volumes.



Super Bronze

Re: Fastethernet throughput


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BTW, FastE full duplex is optional.  Although most devices, especially today, that support FastE will support full duplex.  Also BTW, FastE hubs (which you're unlikely to see much of) have additional restrictions beyond what Ethernet hubs had.

If full duplex is provided, you often get some benefit even when the bulk of your traffic is flowing in just one direction.  This because, often there is some back flow (e.g. ACK packets).  With half duplex, you can collide against those.  Full duplex doesn't have this problem.  And, as Rolf noted, you do have the bandwidth bidirectional.  However, although the bandwidth is practical, moving from half duplex to full duplex won't double your transmission rate transferring a file from one host to another.  So, marketing likes to mention it.  (Note: They don't push this nearly as much as when full duplex was "new".)

Lastly, and on the subject of transferring something like a file between hosts, remember your payload rate won't see the wire rate due to L2 and L3 overhead.  Reason I mention this, people often wonder why their transmission rate is often lower, possibly much lower, then the nominal wire bandwidth.  For example, a minimum sized Ethernet packet, 46 bytes, will use 84 (or more) bytes of the wire's bandwidth.  Then there's also higher protocol overheads too.

Hall of Fame Super Silver

Fastethernet throughput

I'd say it's a bit of both.

Rolf's answer is correct with respect to most applications' traffic characteristics. While voice is pretty symmetric, even the most bandwidth-intensive voice codecs use less than 0.1 Mbps. (Reference) Even an HD 1080 30 fps video conference uses only about 6 Mbps.

At the application and file system layer you also have to take into account any overhead for Layer 2+ encapsulation (Ethernet header, IP addressing, TCP checksum etc.) There are also other factors at play such as the Ethernet inter-frame gap that prevent even the wire speed maximum throughput from reaching 100 Mbps.

So the bottom line is that while you can theoretically send 100 Mbps while also receiving it on a full duplex 100 Mbps port, you very very seldom will.

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