T1 is a protocol for digital transmission over telephone networks.
These bits convey on or off hook, and busy signal status on telephone lines.
The distortion effect on voice and data signals is negligible when a modem is used for modulation.
To remove the errors, data can be transmitted 7 bits at a time instead of 8.
[2] Since robbed-bit signaling could impact voice quality and reduce data bandwidth, it was largely replaced by the Primary Rate Interface standard, which reserves one of the 24 voice channels for signaling information instead of using bits in every channel.