IBM Virtual Telecommunications Access Method (VTAM) is a mainframe software subsystem used to provide network connectivity for IBM mainframe operating systems, including z/OS, z/VM, and z/VSE. VTAM facilitates communication between applications running on the mainframe and other systems in a networked environment.

VTAM serves as the primary communication software for IBM mainframes, enabling them to connect to various networks and devices. It provides the necessary protocols and services for communication between mainframes and other systems, such as terminals, servers, and networked devices.

1. **Network Connectivity**: VTAM enables IBM mainframes to connect to a wide range of networks, including local area networks (LANs), wide area networks (WANs), and the internet. 2. **Protocol Support**: It supports various communication protocols, including SNA (Systems Network Architecture), TCP/IP (Transmission Control Protocol/Internet Protocol), and APPN (Advanced Peer-to-Peer Networking). 3. **Session Management**: VTAM manages communication sessions between mainframe applications and external devices, ensuring reliable and efficient data transfer. 4. **Security**: It provides security features such as authentication and encryption to protect data transmitted over the network. 5. **Resource Sharing**: VTAM facilitates resource sharing among multiple mainframe applications, allowing them to access common resources and services.

Official documentation for VTAM can be found on the IBM website: IBM VTAM Documentation

For more information about VTAM, visit the official IBM website: IBM VTAM Official Website

Learn more about VTAM on Wikipedia: Wikipedia - VTAM

Explore VTAM-related projects on GitHub: GitHub - IBM VTAM

1. Establishing a VTAM session in COBOL:

```cobol ENVIRONMENT DIVISION. DATA DIVISION. WORKING-STORAGE SECTION. 01 SESSION-ID PIC X(8) VALUE 'MYSESSION'. PROCEDURE DIVISION.

   EXEC CICS START 
       TRANSID('MYTRAN')
       CHANNEL('MYCHAN')
       SESSION(SESSION-ID)
   END-EXEC.

```

2. Sending a message via VTAM in Assembler:

```asm

        USING MSGTAB,R15
        LA    R1,0(,R15)     /* Length of message */
        LA    R2,MESSAGE     /* Address of message */
        LA    R3,MSGTAB      /* Address of message table */
        LA    R4,2            /* Record number */
        SVC   202             /* Issue the SEND macro */
        RETURN (14,12),T,RC=(15)

MESSAGE DC XL8'Hello' ```

3. Retrieving session information in PL/I:

```pli DCL SESSION_INFO CHAR(100); CALL 'CPSISRIN' USING SESSION_INFO; PUT SKIP LIST('Session Information: ', SESSION_INFO); ```

4. Handling VTAM exceptions in Rexx:

```rexx /* Retrieve VTAM error information */ ADDRESS LINKMVS 'GETMSG' ERRORMSG = MSGTEXT

/* Display error message */ SAY 'VTAM Error: ' ERRORMSG ```

5. Configuring VTAM resources in JCL:

```jcl //DEFINE EXEC PGM=IKJEFT01 //SYSTSPRT DD SYSOUT=* //SYSTSIN DD *

 D NET,DEFINE,LINE=(NAME=MYLINE,ACB=NET,TYPE=POINT)

/* ```

Some popular third-party libraries used with VTAM include:

1. **CICS** - IBM's Customer Information Control System for transaction processing. 2. **IMS** - IBM's Information Management System for database and transaction management. 3. **MQSeries** - IBM's messaging middleware for asynchronous communication. 4. **CA-Top Secret** - A security product providing access control and authentication services. 5. **IBM Communications Server** - Includes various components for TCP/IP networking on mainframes.

Competitors and alternatives to VTAM include: - **HPR/IP** - Hewlett-Packard's network communications software for their HP NonStop systems. - **OpenConnect** - Offers mainframe connectivity solutions similar to VTAM. - **BMC AMI** - Provides mainframe integration and management solutions, including network connectivity. - **Attachmate Reflection** - A terminal emulation software that enables connectivity to mainframe systems. - **Rocket BlueZone** - Another terminal emulation solution for connecting to mainframes and other legacy systems.