Mainframe Gateways

IT is experiencing a revolution. Data is no longer locked up in the glass-walled, air conditioned, computer room. Information is a key part of a company's strategy, and as such, must be easily accessible. The network can be used to bring that corporate data to the desktops of the users. Once the data appears on the user 's PC, he or she can manipulate it by changing variables , trying different views, and plotting graphs. Or the LAN-to-mainframe link may be part of a mission-critical application such as an airline reservation system. Or perhaps users simply need to share electronic mail with mainframe users.

Getting this information requires a physical link between the PC and the host. A mainframe gateway is a combination of hardware and software that allows PCs, PS/2s or Macintoshes on a LAN to communicate with a mainframe. We will limit our discussion to gateways to IBM SNA mainframes, such as the 30xx, 43xx, and 9370.

Gateway Types

The hierarchy in the mainframe world is straightforward. The mainframe is the central repository of data and applications. Access to this information is granted through a system that runs on the mainframe. This system is usually Virtual Telecommunications Access Method (VTAM). VTAM contains information about every device on the SNA network. Terminals are wired to cluster controllers. The terminal family is referred to as 3270.

Introducing PCs into the equation allows more sophisticated applications to be put into place. It also disrupts the hierarchy, since a PC can act as both a terminal and a cluster controller. Most importantly, the PC brings about drastic changes in the politics in IT.

The most basic micro-to-mainframe link is the coax adapter. The two de facto standard coax adapters are from IBM and DCA (now Attachmate). The PC also runs terminal emulation software so it can act as a 3270 terminal. But the coax adapter is not a LAN-to-mainframe connection. Each PC must have an adapter and coax cable. If it is remote, it needs its own synchronous modem. With a network, however, fewer of these components are needed, and their cost can be distributed among a number of users.

A DFT coax gateway is basically a LAN version of the coax card. Unlike the other types of gateways, a DFT gateway allows a PC to emulate a terminal. Distributed Function Terminal (DFT) is a terminal mode that has largely replaced the older Control Unit Terminal (CUT) mode. DFT assumes the device talking to the mainframe is intelligent (as opposed to a terminal). In CUT mode, the cluster controller is responsible for displaying the data, whereas in DFT mode, many of the controller's functions are distributed to the intelligent device.

Like a DFT terminal itself, a DFT coax gateway may have up to five concurrent sessions. This means that at most five people can communicate with the mainframe at the same time. One way to increase the number of concurrent sessions is to install multiple coax cards in the gateway PC. Another method is to buy a coax gateway that uses multiplexing to support up to 40 sessions on a single card.

A DFT gateway is cost efficient if you already have a cluster controller in place. They are also relatively easy to install. However, they have few sessions to offer users.

Because most users are located at a different location than the mainframe, the SDLC gateway is widely used. A Synchronous Data Link Control (SDLC) gateway connects geographically remote users to a mainframe using modems and telephone lines. One PC on the LAN is designated the gateway and it emulates the 3174/3274 controller. This PC contains the SDLC board, gateway software, and a synchronous modem.

Typically each SDLC gateway supports up to 32 workstations with a total of 128 simultaneous sessions. (The 128 limit was traditionally a limitation of NetBIOS.) As for modems, 9600 bps and below is fine for interactive work, but file transfers and remote applications will require 19.2 Kbps and above. An AT with a standard gateway adapter will support speeds up to 19.2 Kbps. For speeds of 56 Kbps, an intelligent gateway adapter will be required.

Keep in mind that the modem bandwidth and the adapter card's CPU must be divided among the users. Although 128 simultaneous sessions is the theoretical limit, the practical limits are set by the type of applications the users run.

The Token Ring Interface Coupler or TIC is the newest method of connection. Made by IBM, the TIC is basically a Token Ring interface adapter for cluster controllers, midranges, front-end processors, and mainframes. It can be a local or a remote connection.

Depending on the TIC card and Token Ring, transmission is at 4Mbps or 16Mbps; at either speed, it is significantly faster than coax or SDLC methods . The TIC is the most logical method if users on an existing Token Ring need access to a local mainframe. A TIC is also essential for cooperative processing applications built on APPC.

Each workstation on the Token Ring can be addressed directly, or one PC may be set up as the gateway. Although direct addressing provides performance benefits, it increases management efforts. Each workstation must be defined in VTAM as a Type 2 Physical Unit, thereby adding overhead at the mainframe level and increasing the amount of work for the people who maintain the system. From the CPU's and systems programmer's point of view, it is more efficient to designate one PC as the gateway. The mainframe polls the gateway PC, and the gateway is responsible for polling each workstation.

A fourth gateway type is a channel-attached gateway. This is the highest performance method, and is most often used when other mainframes, mini-computers, and engineering workstations need to communicate with the mainframe. Because a coax gateway and an SDLC gateway communicate through a cluster controller or front end processor, they are limited by the controller to a raw throughput of 56 Kbps. A channel-attached gateway is connected directly to a mainframe's I/O channel, and while speed varies according to manufacturer, you can get bandwidth of about 20Mbps.

Session Issues

Most gateways are PC-based. The gateway PC can run either DOS, Windows, or OS/2. The workstations communicate with the gateway via a communications protocol, most often NetBIOS. Some gateways tailored for the NetWare environment use IPX. The OS/2 gateways generally use Named Pipes.

The most basic function of a mainframe gateway is distributing the mainframe's sessions to the PC users. Gateways allocate these sessions or Logical Units (LUs) statically or dynamically. A PC or terminal explicitly defined in VTAM is said to have static or dedicated LUs. So a particular user, say John in Accounting, always gets a particular session, say LU #2. In dynamic pooling, sessions are grouped together. So one pool could be printer sessions, another, terminal sessions. Pooled sessions are distributed on a first-come, first- served basis.

Users with high priority or who deal with sensitive information should have dedicated sessions. Dedicated LUs simplify management, since the mainframe personnel always know who has access to which session. The disadvantage is that VTAM tables become very large. And every time a user is added, the tables must be updated. Dynamic allocation insures flexibility and eliminates waste. A mainframe gateway should support both methods.

On the whole, mainframe gateways offer very little in the way of management capabilities. Functionality can range from the very simple-reporting which user has which LU-to the very complex-reporting information back to IBM's NetView. This is one area where improvement is needed.

Workstation Software

The gateway PC takes care of the basic communication and session handling with the mainframe. But each workstation must run software as well. The first key item is terminal emulation. The PC running DOS, OS/2, Windows or the Macintosh must be able to act as a 3270 terminal. Of the 3270 family, the 3278 and 3279 color displays are the most widely emulated. The 3178 and 3278 monochrome displays are also popular. If you need to display the full screen formats of the larger displays, each PC may need to be equipped with a VGA or better graphics adapter.

In addition to terminal emulation, a gateway may also need to provide printer emulation. This will permit PC users to print mainframe data to their local and LAN printers. Most gateways emulate the 3287 printer; some emulate the 3286.

File transfer is as essential as terminal emulation. Most support IBM's host-based file transfer program called Send/Receive or IND$FILE. Most have a proprietary file transfer that is generally a great deal speedier than IBM's. Some increase the speed of Send/Receive by using larger buffers. On the other hand, that requires more memory on the PC side. The gateway should also allow users to transfer files in background mode. With background file transfer, a user can set up a file transfer and switch to an interactive terminal session while the transfer takes place.

Most gateways allow users to have multiple, simultaneous sessions on the mainframe. Some gateways limit this to a specific number, say two terminal sessions and one printer session per user. Others allow one user to hog every session the gateway has available. Most people would find it difficult to keep track of more than three simultaneous sessions (one for calculation, one for interactive work, and one for printing). It is very useful if the software supports windowing, so the various sessions can be overlaid on a single screen.

The PC keyboard must be remapped to emulate the 3270 terminal keyboard. The difference between the <ENTER> and the <RETURN> keys on the 3270 keyboard must be reconciled. The PF keys must be added to the PC keyboard. These problems are compounded by the differences in the XT and AT keyboards. Often, the workstation software allows the user to pop up a map of the keyboard. Some allow the user to remap the keys on the fly. Another handy feature is to allow users to build macros to automate common functions, such as logging in or retrieving mail.

If the users need to access a mainframe application that makes use of SG2 or APA graphics, special software is required. SG3, the older graphics method, has been replaced by vector graphics (which is also called All Points Addressable). In APA graphics, the mainframe sends only the vector points, and the terminal draws the image on the screen. The 3179-G and 3279-SG3 displays support graphics.

By the time an operating system, a network operating system, and gateway software is loaded into a PC's memory, there's precious little RAM left. Generally, the amount of available RAM and the functionality is a tradeoff . The high end functions, such as graphics or APPC, tend to require a great deal of RAM. If you need these high-end features, consider a gateway that supports expanded or extended memory or one that is able to page to disk. Or consider one that supports OS/2 workstations.

This brings us to a hotkey to DOS. While many gateways offer the user the ability to hotkey to DOS, in reality, there is often little RAM left to do more than a directory listing. Also, when you switch to DOS, often times the file transfer or other process will stop because DOS is not a multitasking operating system. Some vendors have developed a workaround. But if you want true multitasking, you will need OS/2, 32-bit Windows, or UNIX.

Programming Interfaces

Terminal emulation, file transfer, and print services are basic, but essential functions. As the PC becomes a partner with the mainframe, rather than a souped-up terminal, more sophisticated applications are possible. Application programming interfaces (APIs) are the enabling mechanism. APIs allow programmers to move part of an application down from the mainframe and onto the PC. So for example, a loan application can be entered and verified on the PC, then sent to the mainframe. Mostly, APIs are used for far simpler tasks. Users can automate simple tasks , such as downloading files.

Nearly all gateways have some type of programming interface, whether proprietary to that manufacturer or compatible with IBM's. HLLAPI (High Level Language Applications Programming Interface) and 3270-PC API are two of the best-known IBM APIs. They allow you to present information in a way different from the traditional mainframe. For example, mouse support is enabled through HLLAPI. EEHLLAPI or Entry Emulator HLLAPI is a subset of HLLAPI. These APIs are popular because they do not require any changes to be made on the host side.

Enhanced Connectivity Facilities/Server Requester Programming Interface (ECF/SRPI) is a newer API for cooperative applications. It is easier to implement than Advanced Program-to-Program Communications (or APPC). APPC is used in applications where the PC or midrange is considered the peer of the mainframe. This is a radical departure from the traditional terminal-midrange-mainframe hierarchy. APPC is primarily used in high-end transaction processing applications, such as done in airline reservations or banks.

This tutorial, number 22, was originally published in the May 1990 issue of LAN Magazine/Network Magazine.