[THIN] Re: Limits of Latency
- From: "Jan Broucinek" <tinybeetle@xxxxxxxx>
- To: <thin@xxxxxxxxxxxxx>
- Date: Fri, 26 Sep 2003 08:35:40 -0400
OK Geek, I stand corrected. : ) However I was not thinking of LEOS
(Low Earth Orbiting Satellites) when I wrote my reply. LEOS creates a
whole new problem with hand-off since they are not stationary. That
hand-off will most likely be quite unfriendly to Citrix sessions in
the same way that 1XRT cellular networks are when I'm in the car
trying to use my Citrix connection while someone is driving.
And I think your 250ms is a one-way trip, not round trip. Below is an
excerpt from http://www.sisp.net/broadband/satellite.htm
Q: Why is the latency so high?
A: The way a satellite stays in orbit, without using any type of
engine or rocket to correct its orbit, is to gain a balance between
gravity and centrifugal force. The closer to earth, the greater the
gravity pulling the bird toward earth. The faster it moves the greater
the force pulling it away from the earth. It turns out that there is a
spot, about 22,300 miles above the equator, where the speed an object
must travel to gain equilibrium against gravity is exactly the same as
the speed the earth is turning. By placing a satellite in this spot,
its relative position above the earth stays constant. Satellites used
for broadband internet and TV must stay in one place in the sky so
that you can point your dish in one spot and get the signal. The only
other alternative would be to have a constellation of satellites in a
lower orbit, that would rise and fall like the sun and stars. There
would need to be enough of these flying that there was always one
overhead, which requires a lot of satellites, a lot of infrastructure,
and therefore a lot of money.
In the case of a two-way satellite system, when you request something
by clicking on a link, or any other way, that message travels 44,600
miles just to get to the NOC. The stuff coming back to you must travel
the reverse route, so the round trip is 89,200 miles. The speed of
light is 186,000 miles per second in a VACUUM, slower through the
atmosphere. But even if you assumed 186,000 mps then the total time
taken in space travel is about 480ms. Given the atmosphere problem, it
is actually more like 500ms. Add to that the terrestrial internet
latency, which should be about 100ms. Also you can add delays through
transponders, gateways, proxies, etc.
----- Original Message -----
From: "Brian Madden" <brian@xxxxxxxxxxxxxxx>
To: <thin@xxxxxxxxxxxxx>
Sent: Thursday, September 25, 2003 7:28 PM
Subject: [THIN] Re: Limits of Latency
> Hi guys..
>
> Just thought I'd "geek up" this conversation a bit.. Regarding the
claim
> that satellite will never be any better than 600ms due to the
distance
> involved and the speed of light... I don't think that this is true.
>
> Geostationary satellites (most "one-way" communications satellites,
like
> stock, news, tv, weather, etc) are 22,223 miles up, which of course
makes a
> round-trip impossible in anything under 250ms or so. However, the
newer
> "two-way" communications satellites are in low Earth orbit, only
about 200
> miles up. In this case you could bounce a signal off a satellite and
back
> for about the same time that you could make a land-line call. (This
is what
> Gates' company Teledesic plans to use when it launches its
satellite-based
> broadband service.)
>
> Brian
>
> Brian Madden
> +1 202.302.3657
> brian@xxxxxxxxxxxxxxxx
> --------
> Visit www.brianmadden.com for thin client white papers, books,
product
> reviews, courseware, and training videos.
>
>
> -----Original Message-----
> From: thin-bounce@xxxxxxxxxxxxx [mailto:thin-bounce@xxxxxxxxxxxxx]
On Behalf
> Of Jan Broucinek
> Sent: Thursday, September 25, 2003 5:01 PM
> To: thin@xxxxxxxxxxxxx
> Subject: [THIN] Re: Limits of Latency
>
> It depends on the app. In our experience, If the user is typing,
the max
> would be about 150-200ms. If it is a click and select check-boxes
type app,
> then perhaps as high as 400-600ms would be acceptable. Satellite
will NEVER
> be any better than 600ms due to the distance involved and the speed
of
> light.
>
> Disconnects are generally caused by lost packets, you need to see
how many
> retries and packets you lose on the average connection.
>
> ----- Original Message -----
>
> From: Christopher_Wilson@xxxxxxxxxxxxxx
> To: thin@xxxxxxxxxxxxx
> Sent: Thursday, September 25, 2003 4:39 PM
> Subject: [THIN] Limits of Latency
>
>
>
> Hi, list.
>
> Is there any documentation from Citrix or another source that sets a
> bound on how much latency an ICA session will tolerate?
>
> I have a few new locations that have recently been set up our
> WAN/Satellite guy to use a DirectWay satellite connection to the
internet
> then on into Nfuse and my MF 1.8 farm. The problem is that these
locations
> experience frequent disconnects from ICA sessions to the point of
rendering
> Citrix unusable. (No problems surfing mind you.) A ping from them
to us
> returns with a latency of 850-1000ms. The best I remember hearing
from
> this list and elsewhere, latency beyond 300-400ms degrades
performance and
> can cause disconnects. Is this accurate? What is the max latency
tolerance
> by your experience?
>
> Also, I have already tweaked the recommended ICA time-out and TCP
> retransmission settings with no results for this problem. Is there
any
> other tweaking that might help?
>
> Thanks in advance,
>
> Christopher Wilson
> Citrix Administrator
> BJ Services Network Support
> 713.895.5681
>
>
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