Call Transaction VS Session Method

CALL TRANSACTION USING statement
Example
CALL TRANSACTION TCODE USING BDCDATA
MODE CTUMODE
UPDATE CUPDATE
MESSAGES INTO MESSTAB.
CALL TRANSACTION USING, faster than with batch input sessions. In CALL TRANSACTION USING there is no automatic support of interactive correction or logging functions.
The program prepares the data to be transferred once the data is prepared the desired transaction is called and then the data is immediately transferred to SAP database.
CALL TRANSACTION USING supports
Synchronous processing
Transfer of data from an individual transaction each time the statement CALL TRANSACTION USING is called
You can update the database both synchronously and asynchronously
The program specifies the update type
Separate LUW (logical units of work) for the transaction
The system executes a database commit immediately before and after the CALL TRANSACTION USING statement
No batch input processing log
Session Method
In this method sessions are created in a queue and they are managed. There is support for playing back and correcting sessions that have errors also a detailed log is generated automatically.
The program prepares data and stores in in a batch input session. The session thus created can be for more than one transaction. And this session is maintained in the batch input queue. The sessions thus created can be maintained in a queue and processed either in the back ground or in the fore ground.
Your program must open a session in the queue before transferring data to it, and must close it again afterwards. All of these operations are performed by making function module calls from the ABAP program.
The program to create sessions must make calls to SAP function modules for example
BDC_OPEN_GROUP
BDC_CLOSE_GROUP
BDC_INSERT
Session Method Supports the following
Asynchronous processing
Transfers data for multiple transactions
Synchronous database update
During processing, no transaction is started until the previous transaction has been written to the database.
A batch input processing log is generated for each session
Sessions cannot be generated in parallel
The batch input program must not open a session until it has closed the preceding session.