it is lapd --link access protocol for d channel, it is the protocol used in gsm for communication between bts and msc

CDMA stands for Code Division Multiple Access. It is a channel access method. It employs spread spectrum technology (Direct Spread Spectrum).
Transmitter Side:
In CDMA, a narrowband message signal is multiplied by a very large bandwidth signal celled the spreading signal. This spreading signal is a Psuedo Noise Code signal. All users in CDMA system use the same carrier frequency and may transmit simultaneously. Each user is identified by it's own psuedorandom codeword. This psuedorandom codeword is orthogonal to one another. In this way, each user is separated from the others.

Receiver Side:
For detecting the message signal, the receiver needs to know the codeword used by the transmitter.

255 because the max 8 bit number is 11111111 which has a decimal value of 255.

stored in flag register part of PSW(Program Status Word).
Voice Signal Band will be 0-4KHZ. Therefore the heighest frequency will be 4KHZ. According to nyquist the sampling rate would be 8 KHZ

8 KHz as the sampling frequency will be double the maximum frequency component for no aliasing effect, as per the nyquist theorm. as the maximum frequency allowed by the filter is just 4khz. so the sampling frequency is 8KHz