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

Text and graphics are sent in a vector format (i.e. drawing primitives

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.