Ethernet traffic speed

Posted on 2010-11-13
Medium Priority
Last Modified: 2012-08-13
Hi experts,

I have a question that I think a lot of people don't think about and even some high level cisco experts won't know the answer. It's not something one would always think about.

My question is in regards to Ethernet speed. If I have site A and Site B are connected by a Meto Ethernet Circit that has a 1GB CIR, and a computer with a gigabit NIC at Site A sends traffic to a server with a gigabit NIC at Site B, would traffic actually go at a gigabit speed?
Also switches of the PC at SiteA and the Server at SiteB are directly connected to the ISP's NTE switch.
I find it odd that i have this setup at one of the offices yet if I monitor the switchport of SiteA that connects to the NTE using solarwinds, I only see it peak at avg 8-14mb. There's never a bandwidth issue, but I would think if the PC connects to the Server, it would go at wire rate or near it.
Question by:trojan81
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LVL 20

Expert Comment

ID: 34128220
It depends on the protocol.

You are measure throughput of some specific protocol on the wire.  Many protocols use windowing to determine the rate of communication.  The throughput is not a limitation of the network but of the endpoints.
LVL 10

Expert Comment

ID: 34128302
Rather, it is limitation of distance and attenuation, and how many hops between. For every stop along the way, you lose something and gain something (repeaters). Even over fiber, you have distance limitations (2miles?). Wired, you have need of repeaters because you are going to lose signal dependent on location of wire (power lines, lights, etc) and how many others are using the line at any point in time. Broadband is especially known for this issue.  

Author Comment

ID: 34128474
I was doing some resarch and I think I figured out the answer.

My basic question was, if siteA and siteb are directly connected with a gigabit circuit and two devices with gigabit nics talk to each other, shoudlnt they communite near wirespeed? If I monitor the uplinks on each switch on both side that connect to the ISP's NTE, why am I only seeing it at 10-15mb?

I think it's because the traffic is getting monitored at 15 min intervals. So it takes the average amount of traffic over 15 min...does that sound correct?
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LVL 20

Accepted Solution

RPPreacher earned 375 total points
ID: 34128600
Both latency and the windowing on the protocol will make sustained 1 Gbps impossible.

Solarwinds doesn't report the average.  It polls every 15 minutes and reports the current level.
LVL 10

Assisted Solution

koudry earned 375 total points
ID: 34128851

If you connect 1GB port to another 1GB port back to back, you should get near if not 100% of 1Gb traffic output. You also need to know the capacity of each node in the network, i.e. the maximum speed that the card at the node can carry otherwise you have a situation of a bottleneck.

This may not help much but I discovered that the NIC is not the only factor that determines the speed you get. You may have a fast NIC but the back plane on the actual device may create a bottleneck, i.e. does not give you 100% of the speed of the NIC. I discovered this with Cisco 2651XM and 1701 routers on their DSL cards where you cannot get more than 2.5M downstream no matter how fast the DSL circuit or NIC.

My suspicion is that you have a bottleneck somewhere.

Good luck
LVL 84

Assisted Solution

by:Dave Baldwin
Dave Baldwin earned 375 total points
ID: 34129744
According to this page http://en.wikipedia.org/wiki/Gigabit_Ethernet , the spec distance for a wire connection is 100 meters.  Anything beyond that requires fiber to get gigabit speeds.  Did you get a fiber drop from your ISP to both computers?

In addition, you need to make sure the units of measurement are the same.  1000Base-TX is 1 giga-bits-per-second while throughput measurements are often in mega-Bytes-per-second.  10 mega-Bytes-per-second is roughly equal to 100 mega-bits-per-second.  I wouldn't mind having an 10 mega-Byte-per-second connection to my ISP.
LVL 32

Assisted Solution

aleghart earned 375 total points
ID: 34133776
Most computers use the OS for TCP wrap and unwrap, so even if they're sitting next to each other, they'd never approach 1Gbps.  On top of that, hard drives could not handle that fast I/O.   Throw routers into the mix, and you've got another hop or two or three between switches.  1Gbps is a theory and a spec, not a reality.

Even on the same LAN, you'd need two machines with NICs with TCP offloaded from the CPU, and fast RAID arrays with a good amount of fast cache.  1Gbps = 125MB/sec.  Maybe jumbo frames supported through all the devices can reduce the number of packets and keep the speeds up.

Author Closing Comment

ID: 34227711
appreciate the responses although everyone was focused on the fact that you can't pull 1gbps speeds rather than my real question of an explanation of not even getting near a fraction of a 1gbps speed.
LVL 32

Expert Comment

ID: 34233023
Having a circuit labeled "gigabit" has very little to do with the throughput of the traffic.  It's like an automobile speedometer.  It might say 140mph...which may be theoretically possbile...but the label has no effect on your actual measured speed at any point in time.

If your sending node cannot spool its drives and cache stored data faster than 20MB/sec...then you'll never get any faster than a 160Mbps burst, even ignoring overhead and TCP packet wrapping.

You never clearly stated whether you were measuring 8-14MB/sec of data stored, or 8-14Mbps of data transferred.  With the wording of the question, I didn't see it as the big mystery you claimed "even some high level cisco experts won't know".  I saw it as perhaps not understanding what happens during a file transfer, or why actual speeds are different than an arbitrary or nominal label.  Nominal 1Gbps is just that...it's theoretically available, but real-world use would vary by any number of variables.

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