SNA is the all-encompassing framework for communicating in the IBM mainframe environment. SNA is an evolving communications architecture that constantly adapts to the changing needs of business, research, and government. This chapter describes the SNA network from the end-user’s viewpoint.
As shown in Figure 3-1, the SNA communications network connects user workstations in the network with mainframe applications.
Figure 3-1. IBM mainframes communicate through SNA networks.
The physical link between the PC workstation and the mainframe depends on which links your mainframe supports. Chapter 2 in the Host Definition Guide describes the links BARR/RJE supports.
Figure 3-2 shows the communication components between the PC workstation and the mainframe computer with an SDLC connection as an example.
Figure 3-2. SDLC communication components between the mainframe and BARR/RJE workstation.
As shown in Figure 3-2:
An RJE JES2 or 3270 application program communicates with VTAM.
VTAM communicates with NCP.
NCP communicates with a host modem.
The host modem communicates with the telephone line.
The telephone line communicates with a remote modem.
The remote modem communicates with BARR/RJE.
The SNA protocol, a layered communications protocol, connects both 3270 and SNA RJE to the host. The lowest SNA level is the Synchronous Data Link Control (SDLC). Data Link Control (DLC) ensures that frames (blocks) of data get sent correctly through an error-recovery procedure. If SDLC receives a frame incorrectly, it asks for the frame to be re-sent. A frame-numbering scheme ensures that SDLC receives frames in order with no missing or duplicate frames.
End users need two types of communications service:
Remote job entry for submitting jobs and RJE printers to print reports
3270 interactive terminals to communicate with mainframe applications and 3270 printers
The end-user workstations communicate with mainframe application software to perform tasks. For example, the BARR/RJE workstation communicates with the RJE system for remote printing. The Barr RJE+3270 program communicates with interactive 3270 programs for data entry.
RJE is comprised of two parts: workstations and the mainframe system.
Workstations – RJE workstations print reports at remote sites, send large files directly into the job entry system (JES), and control the status of mainframe jobs.
An RJE workstation can be an IBM 3777-3, System/36 running the Multiple Session RJE program, or a PC running BARR/RJE.
Mainframe System – A mainframe RJE system manages job input and report and file output for remote sites.
BARR/RJE is compatible with host computers that support the SNA Multiple Session RJE communications protocol as the following IBM systems do:
MVS/JES2
MVS/JES3
VSE/POWER
VM/RSCS
VS1/RES
The RJE system communicates with VTAM, the universal SNA control program.
The IBM mainframe environment includes 3270 workstations and applications.
Workstations – IBM 3270 workstations allow you to view mainframe data and enter transactions. These 3270 terminals are central to order entry, airline reservations, programmer workstations, and other applications.
IBM 3270 printers print reports, receipts, or other transactions. You can use them with 3270 workstations. For example, you can make a reservation with the terminal while the printer produces a bill or receipt.
While you perform most data entry and control the mainframe on 3270 terminals, you can handle printing and file transfer most efficiently through an RJE workstation.
Applications – The 3270 application programs on the mainframe interact with your workstation through the communications network. The application programs write screens to the 3270 workstations and request responses. These applications allow you to perform queries, database updates, order entry, and statistical analysis. At logon, the 3270 user enters the APPLID (Application ID) of the application program. Mainframe 3270 applications include CICS, TSO, and IMS.
IBM’s SNA architecture, whose software components are Virtual Telecommunications Access Method (VTAM) and Network Control Program (NCP), defines mainframe communications networks.
VTAM is the most important product in the SNA environment and is common to all SNA host systems.
VTAM is an operating system program that resides on the mainframe. It controls communication between the mainframe application software and remote workstations.
The mainframe connects to one or more IBM front-end processors (FEPs); that is, the 3704, 3705, 3725, or 3745. These communications processors manage communications links and connect to one or more remote terminals. NCP resides in each processor and controls its operation.
SNA defines the entities in the communications network. The main communications entity visible to the end user is the physical unit (PU), which is subdivided into logical units (LUs). The PU corresponds to the physical connection to the host. LUs correspond to application sessions available through each PU connection.
The most common PUs are the IBM 3174 and 3274 control units and the RJE workstation. Each physical connection to the host must have a separate PU definition in VTAM.
Each LU is an independent entity within the parent PU. Separate LUs represent each of the 3270 sessions (terminals or printers) and RJE sessions (printers, punches, readers, console, and commands).
To distinguish the LUs in each PU, each LU has a unique number known as the LU’s local address (LOCADDR). Local addresses can range from 1 to 255. When data flows between an LU and the host, it includes the LU’s local address.
Before you use BARR/RJE, you must configure the software’s RJE Description screen and Communication Link screen parameters to match the mainframe’s parameters. The host VTAM communications programmer must describe your BARR/RJE remote to the RJE system, VTAM, and NCP. The Barr Host Definition Guide documents the parameters the host programmer and the BARR/RJE installer must set. Give one copy of the guide to your host programmer.