Evaluation of Shared
Display in WebEx and VNC
Sangmi Lee
(slee@csit.fsu.edu)
1. Introduction
The WebEx is one of the collaboration services provides a range of real-time communications services. WebEx provides Web meeting, sharing documents, sharing applications, giving real presentation, and various environment for collaboration work. The Virtual Network Computing is, in essence, a remote display system which allows users to view a computing 'desktop' environment not only on the machine where it is running, but from anywhere on the Internet and from a wide variety of machine architectures.
These two systems have different goal
of development. The WebEx is developed for providing the environment of
collaboration work for business. And VNC is the system for sharing display and
application between different machines. However, both of systems provide shared
display, export, shared event control in common. Nevertheless, the techniques
used in both systems are different. This evaluation compares and evaluates the
shared display in WebEx and VNC.
2. The Key Features
2.1 WebEx
2.1.1 Meeting center
·
Give any
presentation to anyone, anywhere
·
Demonstrate
software, live
·
Allow anyone in the
meeting to view, annotate, and edit any document electronically
·
Share an application
on your system or share the entire desktop
·
Use remote control
to provide support on the web
2.1.2 OnCall
·
Show and annotate
diagrams and schematics
·
View and diagnose an
application running on a customer's system
·
Upload customer
files for analysis
·
Download patches or
updates
2.1.4 OnStage
·
Provide online
registration, confirmation, notification, and instruction
·
Cover production
issues in advance with training sessions and rehearsals
·
Present graphics and
text -- any graphics and text
·
View any application
in real-time
·
Poll your audience
·
Interact with the
"white-board"
·
Chat live
2.2 VNC
·
No state is stored
at the viewer. This means you can leave your desk, go to another machine,
reconnect to your desktop and finish your job.
·
Small and simple.
The Win32 viewer, for example, is about 150K in size and can be run directly
from a floppy. There is no installation needed.
·
Platform-independent.
A desktop running on a Linux machine may be displayed on a PC, PDA. Or a
Solaris machine. Or any number of other architectures. There is a Java viewer,
which will run in any Java-capable browser.
·
Sharable. One
desktop can be displayed and used by several viewers at once, allowing
CSCW-style applications.
·
Free! It is
downloadable. Both binaries and source code are available from
http://www.uk.research.att.com/vnc , along with a complete copy of documentation.
3. The Shared Display
3.1 WebEx
3.1.1 Vector Graphics
lines, rectangle, text,
etc.) and not as bitmaps. This produces high
quality images and also
drastically reduces the size of the data.
3.1.2 Compression
All large data transfer and
file uploads are compressed. This not only
reduces network traffic it
also adds a level of ‘encryption’ to the data stream.
3.1.3 Incremental Update
During application sharing the contents of the window are dynamic.
WebEx MeetingCenter incrementally
updates only those portions of the screen that have changed. Furthermore the
updates are transmitted as vector graphics commands and not as bit maps.
3.1.4 Video Compression
Several video compression and optimizations have been incorporated
into meeting center. The data stream produced by video transmission is directly
related to the rate of change of the video.
3.1.5 Network communication
The shared display of the WebEx is based on real-time multi-point
data communication. It is following
industry standard, T.120.WebEx has optimal protocol. WebEx MeetingCenter is able to work
through all firewalls using the HTTP protocol. However, it first checks to see
if communication can be established using the lower level TCP protocol. This is
more efficient and reduces network traffic. Using the TCP protocol reduces
network traffic by about 10% when compared to HTTP.
3.2 VNC
3.2.1 Bitmap image
Since the bitmap is easy to handle on the various platforms, all
images from the VNC server are in bitmaps.The
VNC simply works with a server to update the framebuffer displayed on a viewer.
Because it works at the framebuffer level it is potentially applicable to all
operating systems, windowing systems and applications.
3.2.2 Incremental Update
When the
VNC client or server is sharing the contents of the window or events VNC
incrementally updates only those portions of the screen that have changed.VNC has a variety of different encoding schemes for the pixel data, and
it can select the appropriate scheme for each rectangle the server sends, and
make the most of network bandwidth, client drawing speed and server processing
speed.
3.2.3 Adaptive update protocol
The
update protocol is demand-driven by the client. That is, an update is only sent
by the server in response to an explicit request from the client. This gives
the protocol an adaptive quality. The slower the client and the network are,
the lower the rate of updates becomes.
3.2.4 Input protocol
The input side of the protocol is based on a standard workstation model of a keyboard and multi-button pointing device. Input events are sent to the server by the client whenever the user presses a key or pointer button, or whenever the pointing device is moved. These input events can also be synthesised from other non-standard I/O devices.
3.2.5 Network communication
The
VNC operates over any reliable transport such as TCP/IP, and it is basically
point-to-point network. Each client uses differnet number of port and each
platform is accessed via pre-defined port. Thus, the server should open the
connection for each client and send image even if they are all same
images.