Q. What are the supplementary services?

new services are called supplementary services and some of the prominent ones are as below:

Category 1:

• Abbreviateddialling
• Recorded number calls or no dialling calls
• Call back when free.

Category 2:

• Call forwarding
• Operator answer.

Category 3:

• Calling number record
• Call waiting
• Consultation hold
• Conference calls.

Category 4:

• Automatic alarm
• STD barring
• Malicious call tracing

A subscriber issues commands to an exchange to deactivate or activate a service, record or clear data in subscriber line data area or solicit an acknowledgement from exchange. As an instance, a user can disable or enable STD facility on his line by using a command. A command may have or may not have data associated with it. Number of digits in the data, when present, can vary depending upon the command. Consequently, subscriber commands are designed to be of variable length necessitating use of an end-of-command symbol.