The encoding law used is the µ-law as specified in Recommendation G.711. The sampling rate, load capacity and the code are also specified in that Recommendation. The number of quantized values is 255. Two character signals are reserved for zero value (11111111 and 01111111). In some networks the all-0 character signal (00000000) is eliminated to avoid loss of timing information to the digital line, resulting in 254 quantized values.
ITU-T G.733 FRENCH-1993 Referenced Document
ITU-T Q.275 6.5 Data Channel Failure Detection - Specifications of Signalling System No. 6 (Study Group XI) 1 pp
ITU-T G.733 FRENCH-1993 history
1993ITU-T G.733 FRENCH-1993 CHARACTERISTICS OF PRIMARY PCM MULTIPLEX EQUIPMENT OPERATING AT 1544 kbit/s