A channel erect and the B channel inverted. Subchannels Each of the two or four channels has the option to be split into four subchannels so that more streams of data can be sent, but at lower rates. These subchannels are numbered 1 to 4. The subchannel numbered 1 is identified by being the character that has the different polarity from the others in the stream. So the subchannels will be arranged like this in the multiplex 4CRC stream where italic means inverted: A1 B4 A2 B1 A3 B2 A4 B3 If a data stream that is half rate is required, then subchannels 1 and 3, or 2 and 4 are combined. A three quarter rate subchannel can be made by combining subchannels 2, 3 and 4. If one half-rate and two quarter rates are required the half-rate gets subchannels 2 and 4. The facility to split the channel is called a channel divider. It would provide a stop signal during the time when a sub-rate user is not receiving a character. Costs for such a circuit were lower than for a full rate. The marking system used enables the receiver to determine the subchannel order. In practice use of subchannels has not been observed although international partial rate fixed services were once available. System setup In order to have a functioning system, a circuit has to be set up consisting of two transmissions in both directions. One station will be the master, and the other will be the slave station. The slave station will determine its timing from the master station. The first stage is automatic phasing, also called phase hunting, where each end will determine the correct phase of the character stream. During this stage a receiver will slip a bit until system phase is achieved and the system phase matches the marking pattern. The slave station is supposed to send seven bits 0000000 or 1111111 in place of the signal repetition. Once synchronisation is achieved data can be transferred. The mark signal will be sent to the connected equipment so signal that the circuit is available. As characters are received at one end they will be inverted according to the marking pattern and checked for the present of three 1 (mark) symbols. If it is an error, a RQ character is sent on the return path. No characters are sent on from the receiver for a period