# Fibre Channel (SAN)

New Contributor
Posts: 4
Registered: ‎03-25-2009

# ISL distance calculation formula

I have Brocade 5300 switches and I'm running an extended fabric between sites. The vendor is quoting end-to-end latency in microseconds, but the Brocade switches report it in kilometers. However, this is not accurate, as any equipment (muxes, repeaters, etc) on the path will add latency.

I need to talk latency in microseconds. What is the formula the 5300 uses to convert its latency measurement to km?  Is there a way to get the raw latency number from the 5300?

Anonymous
Posts: 0

## Re: ISL distance calculation formula

I would suggest that you use first the LD mode from the switch. The switch will measure a "virtual" distance which includes all latency of all components.

You will receive the "virtual distance" let call it VD in km and then use following formula to calculate the Round trip time RTT in us:

RRT = VD*1000000/205479*2.

If your question goes in the direction that your links did not perform as you expect you have to keep in mid your avg frame size. If you have a very small frame size the through put is low but the link is utilize to 100%. In that case you have to increase the number of buffers which the link can use.

Please let me know if you need further information.

I hope this helps. If you are happy with this answer please mark and rate the answer.

Regards,

Andreas

New Contributor
Posts: 4
Registered: ‎03-25-2009

## Re: ISL distance calculation formula

Thank you for  your reply. I'm using LD mode and getting a distance measurement, which I want to convert into latency time.

One question about your formula is where the parentheses go, Is it

RRT = VD*(1000000/205479)*2.

or

RRT = VD*1000000/(205479*2)

Also, I want one-way time.

Dave

Anonymous
Posts: 0

## Re: ISL distance calculation formula

sorry first one is correct. You will get round Trip time.

What distance does the switch estimate in  LD mode?

Andreas

New Contributor
Posts: 4
Registered: ‎03-25-2009

## Re: ISL distance calculation formula

I'm getting  139km. I think that's one-way distance. It says I need 284 buffers.

Thats not a problem, my dispute with the vendor is that I told them I wanted less than 100km, which is what they promised.

Anonymous
Posts: 0

## Re: ISL distance calculation formula

You have at 138km 1,4 seconds round trip time.

This means if you have disk access over this links that the LUNs will perform very bad.

Your alocated buffers looks very low.

Which speed are you running on that link?

Which average frame size do you have?

Maybe you and your provider are both correct.

The provide talks about 100km fiber lenght but you talk about total &quot;virtual lenght&quot; which includes the latency of the devices as well,

I have seen some devices which adds 20km distances to the fiber lenght.

100km is a very long distance for FC. What kind of traffic are you planning to push over the line?

Hopfully no disk access. In that case you have to wait for a verylong time to get the transactions completed.

I hope this helps.

Otherwise let me know.

Andreas

Anonymous
Posts: 0

## Re: ISL distance calculation formula

> You will receive the "virtual distance" let call it VD in km and then  use following formula to calculate the Round trip time RTT in us:

> RRT = VD*1000000/205479*2.

I assume, that RRT is in miliseconds , VD is in kilometers

2 is because it is round trip time (go there and back)

Can you explain what is 1000000/205479 ?

Regards

Anonymous
Posts: 0

## Re: ISL distance calculation formula

It's stupid to answer myself ;-)

I believe there is an error in previous post: it's NOT 1.4 second, it is 1.4 milisecond.

Speed of light in Fiber Optic (glass) is approx 200 000 km/s

Round trip distance is twice 134 km: 2x134=268km

Time = distance/velocity

T= 268km / (200 000 km/s)

T= 0.00134 s = 1.34 ms

I believe that 205474 is more accurate value of speed of light.

Regards

Anonymous
Posts: 0

## Re: ISL distance calculation formula

Sorry for my typo: ms is correct.

205479 km/s is the speed of light in a fiber.

Andreas