> -----Original Message-----
> From: Michael Milette [mailto:[EMAIL PROTECTED]]
>
> Yes, it is true. The ICA protocol is actually optimized for
> low bandwidth.
>
This pretty much sums up ICA's advantage over RDP(Microsofts Protocol) and
RFB(VNC). ICA is fast at any speed. VNC and RDP are good at moderate to
high speed. VNC with the tight encoding is showing very good improvements at
the low bandwidth end of things, so VNC could be considered to be good at
all speeds. Try running a web browser or using Photoshop/Gimp over a VNC
and an ICA connection, and you will like VNC better.
Here is the technical difference.
Both ICA and RFB use the DRI(Direct Rendering Interface) or X11(Unix X
server protocol) calls coming from the windowing environment while VNC is a
"screen scrapper" and uses the actual graphic output to the screen. So if
the windowing env. outputs something like "show the letter 'A'", ICA and RDP
send a command to the remote client telling it to show the letter 'A' using
font Times. VNC actually lets the letter 'A' get written to your a video
buffer once the letter is drawn, VNC scans the letter and transmits the
results.
The only difference between RDP and ICA is that ICA has been optimized in
many ways for low bandwidth connections. If you were running a program over
RDP on a 9600 baud modem and the program was showing a video clip, RDP would
diligently send each and every frame over one frame at a time. ICA, as well
as VNC, would show a frame every so often depending on how big the video
was. This is true because ICA and VNC adjust for slow connections. RDP
might take 5 hours to play a one minute video clip while ICA and VNC will
only take on minute, but may only show 2 frames.
This is a worst case example, but the feedback from the client that ICA and
VNC use makes a big difference in the speed of the product.
The big speed difference between VNC and ICA is latency. Latency is the
amount of time it takes for the client to send a command to the server and
receive a response. ICA uses many techniques to improve latency over high
latency connections. When you click the letter 'A' on the keyboard when
running an ICA client, here is what happens:
1) ICA client draws and 'A' on you screen.
2) ICA client sends the letter 'A' to the server
3) If more letters are pressed before the response from the server returns,
the client keeps drawing letters locally and queuing the key strokes in the
send buffer.
4) When the sever responds with the result of the first key press, the
client draws the result. The server sends all addition key presses to the
server and starts back over at #3.
So if you are typing and email that is 2000 characters long, the client may
only talk to the server 500 times instead of 2000 like VNC would.
When you move your mouse around the ICA clients window, on a slow
connection, you mouse coordinates are sent to the server only every so
often. With VNC, if you move your mouse from the left of the screen to the
right the coordinates are sent as many times as is possible given the
current bandwidth. Tight VNC has actually modified this behavior in their
beta version.
Anyway, sorry for the long post, but someone should really write this up
nice(unlike mine) and put it in the FAQ. Very interesting stuff.
Greg
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