Hi,
Protocol is your choice. You can expand it depending on the method. Eg. If you make both transmitter and receiver:
start bit, field bits(0-63 or 0-127), control bit, address bit, command bit. You could make use of the command and control bits to expand your circuit to way beyond 127. It all depends on how you put things while coding.
Sony's SIRC is also very popular, esp. in commercial applications. All my IR devices use SIRC as the SIRC based transmitters, ie remote controls are widely available and mostly people use ready-made transmitter of Sony. So all I have to do is make the receiver according to what I want and it's very easy.
Hope this helps.
Tahmid.
---------- Post added at 07:36 ---------- Previous post was at 07:34 ----------
As for the number of channels in SIRC, there are 2, there's a 13-bit version and an 18-bit version. The 18-bit version can be used for many many channels.