I am using a break signal from the common channel to indicate tot he 89C51 that what is coming next (on the common channel) is the channel number to which switching is to be done.

89C51 sofwtare looks for one bit period of low on the incoming RX line, understands that next received byte will be channel number(say 1), switches the analog switch so that common channel is connected to that channel (1 in this case).