Let us see these principal blocks. The first block that we have shown was of A/D or D/A converters. To build a converter, the sample is at fast rate, let's say that the higher is at 70 MHz, it was sampled at 140 MHz per second, so you need a converter that samples at fast rate, operates at high frequences, and it uses a very wide dynamic range.
This means, the issues in ADC and DAC are sampling rate, bandwidth, signal to noise ratio, a spurious speed dynamic range, resolution, and power dissipation.