The z/OS operating system includes a software component called z/OS Communications Server. z/OS Communications Server implements the SNA and TCP/IP protocols.

SNA applications and transaction servers (like CICS) can use SNA or TCP/IP to send and receive data. Industry standard internet applications can use TCP/IP to send and receive data. For example, a z/OS server may run FTP, telnet, web servers (HTTP), and mail programs (Simple Mail Transfer protocol, or SMTP).

z/OS Communications Server provides a set of communications protocols that support connectivity functions for both local and wide-area networks, including the Internet. z/OS Communications Server also provides performance enhancements that can benefit a variety of well-known TCP/IP applications. These performance enhancements, which may be software-based or hardware-based, are discussed in their appropriate contexts.

Figure 1. z/OS Communications Server

As shown in Figure 1, z/OS Communications Server includes three major components, which are:

• The TCP/IP protocol stack.
• The SNA protocol stack contained in Virtual Telecommunications Access Method (VTAM).
• The Communications Storage Manager (CSM), which provides a shared I/O buffer area for both TCP/IP and VTAM data flow. The CSM function allows authorized host applications to share data without having to physically move the data.

Similar to TCP/IP functions, SNA functions are implemented on a number of platforms besides z/OS, for example, AIX, AS/400, Microsoft Windows, and Linux. As a result, z/OS application programmers can exploit technological advancements in communications (information access, electronic commerce, and collaboration) across distinctly different operating systems.