Host Access Solutions: Terminal Emulation Software and Host Integration

Migrating Your Terminal Emulator from SNA to a Web Platform

Evolution of the TN3270 and TN3270E standard

Protocols used to access the IBM® mainframe have changed dramatically over the past two decades. Desktops too, have been through a considerable evolutionary process. Microsoft® Windows XP now being positioned as the operating system of choice for desktops and servers in large mainframe sites. The Internet and the industry's strong endorsement of IP have moved companies from SNA to a web platform. Now, a mature TN3270E standard stands as the preferred path for WEB TO HOST access. This article focuses on the evolution of the 3270 standard, the migration of host access from SNA to a web server platform, and many of the challenges you could face in making the transition.

A look into the past reveals the incredible changes in the area of host access and the stability IP promises to bring. Since the mid 80's when companies began to replace 3270 terminals with PCs, there have been several generations of protocols used to gain host access. It began with the IBM® 3274 cluster controller and the DCA IRMA coax adapter using simple CUT mode coax communication. Later it migrated to the IBM® 3174 controller and the expansion toward a more advanced function, or DFT, coax market.

However, coaxial communication began to lose its appeal once LANs were created. IBM® took the lead in LAN connectivity with various 802.2 Token Ring DLC connectivity options (front end processors, 3174 TIC attachments and PC-based LAN gateways). Yet the success of the 802.2 protocol would be in a sense short-lived.

Once NetWare began to dominate the market as a LAN operating system, most Novell customers selected Novell's NetWare for SNA, and later NetWare for SAA, as their standard solution for host access. DOS memory limitations greatly assisted Novell in the sale of this solution since the use of both IBM® and Novell drivers on the desktop proved to be very memory intensive. Other LAN gateways also entered the market, such as those based on NETBIOS, but their success was limited by the inefficiencies of this protocol.

The Arrival of TCP/IP

The arrival of TCP/IP as a standard LAN protocol (especially once promoted by Cisco®, IBM®, Microsoft®, SAP and others), brought about a universal change unprecedented in the history of host connectivity. Soon most 3270 terminal emulation providers would support the TN3270 and TN3270E standards for IP communication.

TN3270 Matures into TN3270E

TN3270 is a standard protocol that utilizes TCP/IP for communication with the IBM® mainframe. It also addresses access to the host via the Internet. Details of the TN3270 standard are referenced in the IETF RFC 1576 specification. However, TN3270 primarily defined the standard display of 3270 data on the desktop and left open a number of important SNA issues. This void in the standard opened the door for the more mature TN3270E specification.

TN3270E incorporates a set of enhancements to the 3270 specification and is a superset of the original RFC. TN3270E is referenced in the IETF RFC 2355 TN3270 Enhancements Specification (which replaced RFC 1647). TN3270E addresses critical issues such as:

• IBM® 3287 LU 1 and LU 3 host printing
• The use of specific LUs, associated LUs and LU pools
• The addition of "SNA-like" features in TN3270E (including the SNA Bind and Unbind Command, responses and sequence numbers)
• A standardization of the 3270 Attention and System Request keys
• Access to the SSCP-LU session
• Host Printing

One of the main issues addressed by TN3270E is the ability to emulate IBM® 3287 host printers. There are two types of 3287 host printers emulated: LU Type 1 SCS (SNA Character String) printers and the older LU Type 3 DSC (Data Stream Control) host printers. In order to accommodate 3270 host printer data streams, "SNA-like" features have been added to the TN3270E protocol. For printing, response requests are used to pace the sending of print data from the host to the PC. In such a case, the TN3270E client acts as a throttle between the host, PC and printer. Sequence numbering is also implemented to ensure that the data isn't received out of order.

LU Flexibility

Another major feature of TN3270E is the ability to connect to a host session using a specific LU name, the use of LU pools or the association of multiple LUs (i. e. one for display session, one for a host printer session). Using TN3270E, a device name (also referred to as an LU name) can be configured for each session. This ensures that a session of a particular type will always be available. Using basic TN3270, sessions are connected to an LU obtained from a generic pool of LUs and it is possible to run out of LUs. Also, each time a connection is made, you more than likely will not be connected to the same LU. However, TN3270E does support generic pools, as well as named pools of LUs.

When configuring printer sessions, TN3270E offers the ability to "associate" or "pair" a printer session to a specific display session. When a printer session is associated to a display session, the printer session uses the same device name as the display session. This ensures that the specific display - printer session combination is always used together. This feature is helpful when administering the assignment of display and printer sessions.

Standard Attention Key

Another item addressed by TN3270E is a standardized method for the implementation of the 3270 Attention Key function. This is important to remember, as the original TN3270 specification did not address the use of the Attention Key. As a result, 2 different methods emerged; the "IBM®" and "Telnet" solutions. In older IP host stacks, use of the Attention Key was unavailable. This caused various combinations of code mismatches, which resulted in the 3270 Attention Key not functioning with the session managers and host applications that use this function.

Access to VTAM Characteristics

TN3270E provides a TN3270E client access to the VTAM terminal characteristics. The TN3270E server encapsulates and sends the actual SNA Bind command to the TN3270E client. The SNA Bind information includes items such as the 3270 screen size, extended data stream attributes and the LU type.

SSCP-LU Session

TN3270E also provides an interface to the SSCP-LU session, which is sometimes referred to as the VTAM Logon Screen. This is a session that's established with VTAM before an actual LU-LU application. The VTAM Logon Screen is typically an unformatted green screen with a simple menu and is usually different for each mainframe installation. It provides the ability to log onto various host operating systems within the IBM® mainframe network, such as a CICS, MVS/TSO or VM/CMS host system.

Standard System Request Key

TN3270E also implements a standard System Request key, which switches between the LU-LU and SSCP-LU sessions. A user may terminate a host application by pressing the System Request key, then keying in "LOGOFF" followed by the ENTER key. TN3270E uses "SNA-like" encapsulated versions of the SNA BIND and UNBIND commands to switch the 3270 terminal emulator between SSCP-LU and LU-LU sessions.

TN3270E - A Client/Server Standard

It's important to understand how the TN3270E standard works and what it offers in terms of features, because all web to host solutions are based on this standard.

To access the mainframe from a Windows desktop using TN3270E, there are 2 required components; a TN3270E client and a TN3270E server. The TN3270E server could be integrated IP code running on the mainframe, a LAN-based PC gateway, or a Cisco® or IBM® router that has the ability to translate SNA data to IP. As time goes by, most companies directly access the OS/390 acting as a TN3270E server.

When early IP adopters migrated from SNA to IP, many consolidated their SNA servers to a handful or even a single TN3270E server. Yet they simply reconfigured their existing 3270 emulation for the TN3270 or TN3270E protocol. In many cases, problems began to appear -- session lockups, 3270 display problems, cursor ghosting and a long list of others oddities. Today however, the TN3270E client market is undergoing it's own consolidation from deployment on the desktop to deployment from a web server platform.

WEB TO HOST Solutions

From an architectural perspective, there are two platforms for a web to host terminal emulator - Java™ and ActiveX. IBM®, for example, have based their products on Java™ for portability reasons. Zephyr has taken the Microsoft® recommended route for application development with ActiveX. One of the biggest differences between the two is the size of the interface - 8-bit for Java™ and 32-bit for Microsoft®'s ActiveX.

Challenges

One of the biggest challenges in moving your 3270 terminal emulator to a web server is the dependency of many users on scripts and APIs that must also be ported to the web server. Older HLLAPI applications, some even developed for Windows 3.1, simply need to be redeveloped. The good news is that tools exist today that not only easily automate 3270 functions, but also alter the screen to have a GUI look and feel. This can be a first step toward enterprise application integration (EAI) - an emerging market for host access providers.

Another area that can differentiate web to host solutions is network administration. Since the terminal emulation is centrally administered and deployed from a web server, the options available for the network administrator in terms of the storage of macros, APIs, session profiles, end user customization of sessions, etc. can be crucial to evaluate. For companies with tens of thousands of host users, this aspect of the web to host solution can make or break an opinion of a particular product.

TN3270 and the Web Solves Problems

There are substantial benefits for consideration of TN3270 as a standard protocol for web to host access, including:

• Uses TCP/IP as universal, non-proprietary communications protocol
• Supported by entire 3270 communications industry
• Simplifies remote access to the IBM® mainframe
• Offers fast, reliable communication
• Ideal for large LAN/WAN environments, more stable than older LAN-to-host protocols that weren't designed for the complexity/size of today's networks

One reason for the stability of TN3270 is due to the clear separation in the responsibilities of the SNA and 3270 processing layers. In the past, SNA processing has been shared between the gateway and the 3270 terminal emulator. With TN3270, all SNA processing is performed by the TN3270 server. The TN3270 client handles only the TN3270 data stream processing. This separation of responsibilities sets boundaries that are clearly defined.

In Closing

Companies are quickly migrating host access to a web server platform using the TN3270E client/server standard. Now, most use the mainframe as their TN3270E server. On the client side, traditional, desktop-based 3270 terminal emulation is moving to a web server platform for centralized administration. New tools allow host data to be viewed much like a web site, integrating the mainframe with the Internet.

Quicktrial or Download Options

There are two ways to evaluate the PASSPORT WEB TO HOST® terminal emulator: either download a copy of the software or do a QuickTrial of the PASSPORT WEB TO HOST® application. With QuickTrial, there is no server installation, you simply download the client and run the application from our web server. This is a fast and easy way to look at the PASSPORT WEB TO HOST® software. For those that want to install the PASSPORT WEB TO HOST® software on their own web server, you can download the fully functional 3270, 5250, SCO ANSI, VT100/VT220/VT420 and Wyse 60 emulator trial and complete a full evaluation.