Security during pass parameter value by ajax client side

Question

Recently i'm implementing a ERP module.i have written a .asmx web service for the total database operation.and all aspx page i want to call those web method written in that web service using ajax client side call.Everyone says that client side call is good than a service side call method.As client side call is more fast than a server side call operation. But i have a Question regarding security of my data.

 

1>Can a hacker change the parameters of the during call the web method ?

 

2>Is there any secure way to send data through ajax call?Any encription/Decription Technique??