Editing 56 kbit/s line (section)

==Speed derivation==
The figure of 56&nbsp;kbit/s is derived from its implementation using the same digital infrastructure used since the 1960s for [[digital telephony]] in the [[public switched telephone network]], which uses a [[sampling rate]] of 8,000&nbsp;Hz for [[Pulse-code modulation|PCM]] audio with 8-bit [[audio bit depth]] to encode [[analogue signal]]s into a digital stream of 64,000&nbsp;bit/s. However, in the [[T-carrier]] systems used in the US and Canada, a technique called [[bit-robbing]] uses, in every sixth [[frame (telecommunications)|frame]], the [[least significant bit]] for [[Channel Associated Signaling]] (CAS). This effectively renders the lowest bit of the 8 speech bits unusable for data transmission, and so a 56&nbsp;kbit/s line uses only 7 of the 8 data bits in each sample period to send data, thus giving a data rate of {{nowrap|8 kHz ×}} {{nowrap|7 bits {{=}}}} {{nowrap|56 kbit/s}}.