Ciphering
Ciphering is used to protect signaling and user data from intruder. First of all, a ciphering key (Kc) is computed using the algorithm A8 stored on the SIM card, the subscriber key (Ki, also stored in the SIM card) and a random number (RAND) delivered by the network (this random number is the same as the one used for the authentication procedure). Secondly, a 114 bit sequence is produced using the ciphering key, an algorithm called A5 and the TDMA frame number (Fn) number provided by the network. This bit sequence is then XORed (TRUE if only one of the inputs is TRUE) with the two 57 bit blocks of data included in a normal burst.
Modulation
The burst formatted bits are then modulated using GMSK which is nothing but 8PSK modulation technique and transmitted over a carrier. In GMSK the bit stream is first filtered using a Gaussian filter so as to remove the higher harmonics and round off the corners of the bit pulses. Then they are used for modulating the frequency of the carrier using MSK modulation.
That concludes one side of transmission; the decoding part happens in the other side. This is all together how speech transmission occurs in GSM
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Satya Sravan
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