VNC speed and compression

James [Wez] Weatherall jnw "at" orl.co.uk
Fri, 24 Jul 1998 11:41:59 +0000


On Fri, 24 Jul 1998, Diego Sierra wrote:

> James [Wez] Weatherall wrote:
> 
> > I would tend to agree with this.  If WinVNC is sluggish on a P200 
> > machine then this suggests that either WinVNC has been misconfigured
> > or that something else in the system is taking up most of the
> > available CPU time
> 
> Well, not at all. I have WinVNC configured for just poll both the
> foreground and Console windows. 

The main polling options are Poll Full Screen, Poll Window Under Cursor
and Poll Foreground Window.  The Poll Console Windows Only and Poll On
Event Only restrict the polling of console windows and foreground windows
to only console windows, or to only when a remote mouse/keyboard event has
been recieved.

> This is the scenario I have:
> 
> +---------+  33.6 modem link   +----------+    +----+
> | HOME_PC |--------------------| SW 10Mbps|----| PC |
> +---------+                    | Ethernet |    +----+
>                                +----------+
> 
> The HOME_PC is connected to the network via PPP. I run on the PC both a
> laplink sever and the VNC server, the first one for exactly the same
> purpose than VNC:  to has the PC screen at home. What I mean in my first
> email is that laplink seems faster than VNC, mainly when it redraws a
> window, so I'm just asking if there can be some improve in the
> (compression) code. (Obviously I prefer VNC :-) 

VNC doesn't currently allow caching of bitmap data, so when you minimize
a window, you destroy all the bitmap data it contained and it has to be
re-sent when you restore it.  I assume laplink is using some form of
caching to optimize cases like this.

Another possible cause is latency in the above network - when a server
sends an update to a client, it then won't send another until the client
requests one.  This means that on high-latency networks, even if the
bandwidth is huge, the framerate may be quite low.

Cheers,

James "Wez" Weatherall
--
Olivetti & Oracle Research Laboratory, Cambridge, UK.
Tel : Work - 343000


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