Hi Nitin
PCM is pulse code modulation.It converts anlog signal into digital signal. The sampling frequency is 8000hz, that is to say u have 8000 samples per second(this process is called as sampling). each sample is represented by 8bits(Quantising and coding).PCM30 or E1, has 32time slots and each time slot is of 64kbits/sec so the transmission rate for one PCM-30 is 2048kbits/sec.