User`s guide

Figure 6: VidServer source ﬁles

4.3 Videoconferencing Reciprocity Application

The server application, VidServer, waits for incoming calls on a pre-speciﬁed port (2000 by default).

At most, two incoming calls are accepted. This type of application is also known as a daemon — it

waits for connections and then handles them accordingly.

The client application, VidClient, connects to the server application at a IP address speciﬁed by

the user. The host port that the server listens on is speciﬁed by the user in a text ﬁle. The client

application has a similar text ﬁle, which contains the host port number to connect to, as well as the

COM (serial communications) port to communicate on and the baud rate of the COM port.

The data that is received from the COM port is a projector message (the protocol is developed by

Rutledge Engineering), usually sent to a data projector. Only two commands are recognised, “turn

projector on” and “turn projector oﬀ”. The projector messages consist of a string of ﬁve bytes —

in order, they are: start, projector address, command, checksum and stop (Rutledge 1999).

After receiving a message, an acknowledgement message is sent in reply. This consists of six bytes

— in order, they are: start, projector address, command[0], command[1], checksum and stop.

All message bytes are unsigned. The start and stop bytes indicate the beginning and end of a

message. They are FE and FF respectively. The projector address is used to diﬀerentiate between

multiple projectors. Since we are only communicating with one projector, the projector address is

01. The checksum is the sum of all bytes between the start and stop bytes. A number of command

bytes may be sent. For the purposes of the project, only one command needs to be sent at a time,