it is the combination of both top-down and bottom-up 
approaches . we can also say hybrid appoarch

big bang approach is nothing but system approach.after
completion of all the modules and integrate it, testing is
done.

it is a type of integration testing in which s/w elements, 
h/w elements, or both are combined all at once, rather
than in stages

In big bang Integration testing, individual modules of the 
programs are not integrated until every thing is ready. 
This approach is seen mostly in inexperienced programmers 
who rely on 'Run it and see' approach.

all the modules are integrated first at once and then 
testing prformed . Disadvantage of this is we cannot point 
out  the origin of the bug , i mean to say if there is any 
bug , then it is difficult to developer to find out which 
part of application is not working or whether their is any 
mistake while doing the integration of modules