ISDN supports  three  techniques  to control and invoke  supplementary services namely keypad protocol,  feature key management  protocol and functional protocol.

The first two  techniques  are known as stimulus  protocols as they do not  require  the  user's  terminal  to  have  any  knowledge  of  the procedures that are used to control and invoke supplementary services. Whereas, the functional  protocols are protocols that include specific messages  that are  generated  by the user's  terminal  to invoke  and control specific supplementary services.

The Keypad Protocol

It evokes memories of pushing and a pound sign and a couple of numbers to tell the LE to  forward  all  calls.  The  user's  terminal  puts a sequence  of  keypad  strokes  (0-9,  #,*)  into the  keypad  facility information  element of SETUP or  INFORMATION  message and sends it to the network.  The network  interprets  the sequence of keystrokes  and checks  if it is  complete.  If not, it sends an  INFORMATION  message which would cause the user's  terminal  to display a message / signal. After  it had  received  the  complete  information,  it  invokes  the appropriate action.

The Feature Key Management Protocol

It is the other  stimulus  protocol that is used to invoke and control supplementary  services.  A key stroke on the dialing pad  generates a feature  identier number and is transmitted to the LE.  The LE inturns interprets this  identifier and takes  necessary  action.  To use this protocol, the user's  terminal  puts the feature  identifier  into the feature activation element in a SETUP or INFORMATION message and sends it to the  LE.  The LE  sends  back  an  INFORMATION  message  if  the information is  incomplete.  The LE then takes  appropriate  action to provide  the  requested  service  as  soon  as it  gets  the  complete information.

The Functional Protocol

It is similar to basic call control  protocols  with respect to user's terminal and LE.  Terminals that use functional protocol must know the meaning  of  the  messages  they  send  and  receive.  The  functional protocol is divided into two  categories of  procedures:  the seperate message approach and the common information element procedure.

The seperate message approach uses a distinct set of messages for each desired  function.  For  example,  there are six distinct  messages for call hold  feature.  To place a call on hold, the user's  terminal  or the  network  transmits  a HOLD  message  and gets a HOLD  ACKNOWLEDGE message in  response.  To retrieve a call, a RETRIEVE  message is sent and a RETRIEVE  ACKNOWLEDGE  message is received in return.  The other two  messages  , HOLD  REJECT  and  RETRIEVE  REJECT  are  used if the requested action is not performed.

The common information element procedure uses a single, common element (the  facility  information   element)  to  signal  for  supplementary services.  It  applies   only  when  the  service   does  not  require synchronization  between user and a message The  facility  information element looks like more like message than an  information  element; it comprises  variable number of components that are similar in format of information   elements.  The  optional   components  of  the  Facility information element are

* Invoke: Indicates the supplementary service being requested.

* Return result: Response to the invoke component.

* Return error: Indicates the previus reuest for a supplementary service cannot be provided.

* Reject: Indicates the previous facility information element was not understood.