Should be around 70ms RTT for London to NYC on an E1, so maybe a little more or less for a T1. Regards, Neil. -- Neil J. McRae - Alive and Kicking neil@DOMINO.ORG

On Mon, 25 Mar 2002, Pistone, Mike wrote:

I was curious if anybody would share what they consider to be average or acceptable transatlantic ping response times over a T1. I know there are tons of variables here, but I am looking for ballpark figures. Assume that utilization on the circuit is extremely low, and you are measuring point to point across the line. You can also assume no other bottlenecks effecting the response times (router performance, or what not). Should you see a ~150ms trip? 250ms? 450ms???

Something like 70 - 100 ms with small packets.

Is there any equation to estimate response times? For example, if your circuit from A to Z has a 500ms avg response, than that equates to a circuit distance of aprox. 5000 miles or something?

The three main components in the delay are: - serialization delay: it takes a certain amount of time to get a packet out of the interface. This is the size of the packet divided by the bandwidth of link. For instance: 1500 bytes = 12000 bits / 1536000 bps ~= 8 ms. (Double for RTT.) - speed of light: this depends on the medium. For fiber, it's about 200,000 km/s = 125,000 mi/s. So 5000 miles worth of fiber (which could be the atlantic, but your milage may vary) is 40 ms. (Double for RTT.) - queuing delays: this depends on how busy the circuit is and on the number of hops. You can remove the queuing factor by leaving your ping running for a fairly long time and then only look at the shortest RTT. If the shortest and the average RTTs are far apart, the circuit is very busy.

On Mon, Mar 25, 2002 at 09:13:20AM -0600, Pistone, Mike wrote:

Is there any equation to estimate response times? For example, if your circuit from A to Z has a 500ms avg response, than that equates to a circuit distance of aprox. 5000 miles or something?

As I'm sure you remember from your physics class, light travels through a vacuum at 299,792 km/sec (lets call it 300,000). When it travels through other mediums, it moves slower based on that medium's refractive index. For example, water has a refractive index of 1.33, which means light travels through water at about 0.75c, or about 225,000 km/sec. Fiber works on a principal called "total internal refraction", which means that the light is continually reflected into the core with no (or little) loss in the cladding. To accomplish this, different material with different refractive indexes is used. Since the cladding has a lower refractive index than the core, as long as the angle of incidence exceeds a critical angle, the light will be reflected back into the core instead of shooting out the sides. The values of refractive indexes used in fiber are usually something along the lines of 1.46 in the cladding and 1.48 in the core. So if you do a little math, you'll see that light propagates through fiber at around 0.67c, or 200,000 km/sec (or approx 125,000 miles/sec). Putting that in ms terms so even ping monkeys understand, you get approximately 1ms of speed-of-light induced delay per 200 km (or 125 miles) of fiber path. As an example (and to answer your original question), 500ms RTT / 2 is approximately a 50,000 km or 31,250 mile fiber path. Adjust a little for all the microseconds of switching and buffering which happens to your packet along the way, and you can get a fairly good idea how drunk the people were when they laid your fiber. -- Richard A Steenbergen <ras@e-gerbil.net> http://www.e-gerbil.net/ras PGP Key ID: 0x138EA177 (67 29 D7 BC E8 18 3E DA B2 46 B3 D8 14 36 FE B6)