SNA is the all-encompassing framework for communicating within the IBM environment. It is an evolving communications architecture that is constantly adapting to the changing communications needs of business, research, and government.
While the SNA communications environment is complex and has many functions, the information discussed in this manual is the end-user view of the SNA network.
End users need two types of communications service:
3270 interactive terminals to communicate with mainframe applications, and
Remote Job Entry for printing reports.
IBM 3270 terminals are used to view mainframe data and enter transactions. These 3270 terminals are central to order entry, airline reservations, programmer workstations, and a number of other similar applications.
The IBM 3274 Control Unit connects many 3270 terminals to a single link with the mainframe (host). Many times the 3274 Control Unit is referred to as the 3270 Control Unit. The 3274 Control Unit can be locally connected with the host or reside at a remote site via a synchronous modem connection.
A coaxial (coax) cable connects the 3270 terminals to a 3274 Control Unit. Coax is the same type of cable used for cable TV.
While most data entry and control of the mainframe is performed on 3270 terminals, printing and file transfer are most efficiently handled through a Remote Job Entry workstation.
Remote Job Entry (RJE) workstations print reports at remote sites, send large files directly into the job entry system, and control status of mainframe jobs.
An RJE workstation can be an IBM 3777-3, System/36 running the Multiple Session RJE program, or better yet—a PC running BARR/SNA RJE or BARR/RJE+3270.
Like the Remote 3274 Control Unit, SNA RJE also uses a synchronous modem connection to the mainframe.
The SNA protocol connects both 3270 and SNA RJE to the host. SNA is a layered communications protocol with the lowest level known as Synchronous Data Link Control (SDLC). Data Link Control (DLC) assures that frames (blocks) of data are sent correctly through an error-recovery procedure. If a frame is received incorrectly, SDLC asks that it be re-sent. A frame-numbering scheme assures that frames are received in order with no missing or duplicate frames.
SNA defines the entities that exist within the communications network. The main communications entity visible to the end user is the Physical Unit (PU), which is further subdivided into Logical Units (LU).
The independent units in SNA architecture are referred to as Physical Units (PUs). The most common physical units are the 3274 Control Unit and the RJE workstation.
Each physical unit on an SDLC line has a one-byte address with the address being the first byte of the frame. This means that up to 254 physical units can share a single SDLC line. When more than a single physical unit is attached to an SDLC line, that line is called a multipoint or multidrop line.
Within the 3274 Physical Unit, separate Logical Units (LUs) represent each of the 3270 terminals. Within the BARR/3270 Physical Unit, separate logical units are allocated for printers and terminals. Each logical unit is an independent entity within its parent physical unit.
The host software that implements the 3270 sessions function consists of three components: Application Program, VTAM, and NCP.
The Application Program interacts with the user’s terminal via the VTAM communications control program. The Application Program writes screens to the 3270 terminal and requests responses. The program allows the user to perform queries, data base updates, order entry, and statistical analysis. At Logon the 3270 user enters the APPLID (Application’s ID) of the Application Program.
Virtual Telecommunications Access Method (VTAM) from IBM is the single most important product in the SNA environment and is common to all host SNA systems.
VTAM is an operating systems program that controls communications between the mainframe applications software and the remote terminals.
The mainframe is attached to one or more IBM communications controllers namely 3704, 3705, 3725, and 3745. Each communications controller has many lines, connected to one or more remote terminals. NCP, Network Control Program, resides in each one of the controllers. In short, the
Application Program talks to VTAM.
VTAM talks to NCP.
NCP talks to a host modem.
Host modem talks to a telephone line.
Telephone line talks to a remote modem.
Remote modem talks to BARR/3270.
Before using BARR/3270, you must first coordinate specific information with the VTAM communications programmer who supports the host environment. We recommend sitting down together at a PC to configure the 3270 Description and Communications Link parameters.
The host programmer must describe your remote to each of the three host software components: Application Program, VTAM, and NCP. The host programmer is the only person who will know the values of the parameters to be entered in the BARR software Communication Link screen (see Section 5.3.0 through Section 5.3.8, Communication Link).