Auxiliary channels, also referred to as sub-channels, are transmission channels via which control, acknowledgement and monitoring information is transmitted.
The auxiliary channel has a narrow bandwidth and can be operated in half duplex or duplex
justlike the transmission channel. However, it has a much lower transmission speed
than the home channel and usually has separate interface lines.Auxiliary channels with half duplex operation use direction switching
If direction switching is not required during transmission, the additional auxiliary channel interface lines can be omitted; this is then referred to as asymmetrical duplex operation.International Telecommunication Union (ITU) as an acknowledgement channel. Thus in V. 23.