The INVITE client transaction is created by the transaction user(TU) at the client side. When the transaction user wants to initiate the session, it creates the INVITE client transaction. Then the transaction user send this INVITE request to the transaction layer. As shown in Fig.

3(a), there are five different states that INVITE client transaction can be entered:\1.~Calling~~~~~~~~~~~~~~~~2.~Proceeding~~~~~~~~~~~3.~Accepted\4.~Completed~~~~~~~~~~~5.~Terminated\When a INVITE client transaction is initiated we entered the extit{initial} extbf{ extit{Calling}} state. The transaction layer passes this INVITE request created by the transaction user(TU) to the transport layer.

Transport layer then passes this INVITE request to the server side and start extit{timer B}. If an unreliable transport is used start extit{timer A}. In the extit{initial} extbf{ extit{Calling}} state six events take place for the INVITE client transaction.

When extit{timer A} fires it reset the timer by pushing the transaction and retransmit the INVITE request to the server. The client transaction change the state from extit{initial} to extit{terminated} when extit{timer B} fires. The extit{transport error} occur when the transport layer is unsuccessful to forward the INVITE request across the network, then the transaction entered the extit{terminated} state and update the transaction user(TU). If the extit{provisional response}(1XX) is received, the client transaction deliver this request to the transaction user(TU) and enter into the extbf{ extit{Proceeding}} state. In case of receiving the final extit{successful response} (2XX), i.

e.server accept the INVITE request successfully, the transaction notify its transaction user(TU) regrading the request and entered into the extbf{ extit{Accepted}} state. When the transaction receives the extit{non-success response} (3XX-6XX), i.

e. a server receive the INVITE request but does not accept it, the transaction passes the response to transaction user(TU) and generate an acknowledgment(ACK), forward this ACK to the transport layer, and enter into the extbf{ extit{Completed}} state.\If the transaction is in the extbf{ extit{Proceeding}} state, the transaction can receive one or more extit{provisional responses} (1XX), notify its transaction user(TU) and continue to exits in the extbf{ extit{Proceeding}} state. It can also receive a final extit{successful response}(2XX),passes this response to the transaction user(TU) and enter into the extbf{ extit{Accepted }} state, or it can also receive a extit{non-success response}(3XX-6XX) forward the response to the transaction user(TU) and generate an ACK, passes the ACK to the transport layer and enter into the extbf{ extit{Completed}} state.\