Help in writing a synchrnous ppp driver

Hi All,

In linux kernel, what is the difference between the ppp_synctty.c
(PPP synchronous TTY device driver) and syncppp.c (synchronous ppp)
functions..?

I have a High speed serial driver that is capable of doing HDLC framing
using a Network Processor. I want to run PPP on that serial driver. 
The device driver has just provided functions to read/write on the device.

I intend to add a proprietary header to the packets coming out of PPP.
Do i need to use the PPP synchronous TTY device driver or the one similar
to syncppp.c. I am not clear of when to use the syncppp functions sppp_input etc.?

To use the functions in ppp_synctty.c,  then does the driver also needs to be a TTY
driver.?

Whether all the linux driver written for serial devices need to be TTY drivers..?

Thanks,
Vanitha