cceps (complex cepstrum) function in Matlab My doubt is in regards to the usage of the function cceps in Matlab. 

The function is giving me the cepstrum for the entire speech signal of 3 or 4sec. This works well when I play back the signal after taking the inverse (using theicceps function). However, in order to analyze the given speech signal, we are supposed to only consider 20ms frame. 

Is taking the complex cepstrum throughout the signal valid? Why is it working out here?
You should split the signal up into overlapping chunks of 20 to 30 ms. It is certainly valid to take the complex cepstrum on the 4 second signal. But, since the frequency characteristics of speech are constantly varying the complex cepstrum on the 4 second signal will average all of these together and you won't know what happened when. If you take it in each 20 ms window then you can see how the properties of the signal change over time.