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-- by Anne Fischer Lent
Like a dinosaur that somehow eludes extinction, the mainframe continues to withstand the test of time. Despite the rise of client/server networks, corporate intranets and Windows NT Server, IBM says most mission-critical data still resides on mainframes and AS/400s. In fact, tried-and-true mainframes are back in vogue now that IBM has shipped big iron based on economical CMOS (complimentary metal oxide semiconductor) technology.
NT administrators face the challenge of sorting through a maze of connectivity options and linking mainframes to TCP/IP or IPX/SPX networks running NT Server. The choices include direct connections, server gateways, terminal emulation software and emerging technology based on the ever-expanding World Wide Web.
In the early days of PC networking, many companies linked PCs directly to host computers via token ring or Ethernet networks using the 802.2 data link control (DLC) protocol. Many vendors still develop direct-connect software. Hummingbird Communications Ltd., for one, makes HostExplorer, which lets you link hosts with all of your Win16 and Win32 desktops. Similarly, IBM's Client Access for Windows 95 and NT lets you connect Win95 and NT desktops to AS/400 systems, and should be shipping by the time you read this.
Many companies still use this direct-connect approach today, but it has drawbacks. Generally, each PC must be configured with the host's network address. And whenever you reconfigure your host, all connected PC desktops must also be reconfigured-a daunting task for large companies with thousands of distributed PCs. Further complicating matters, the DLC protocol is a TSR program. The program requires a real-mode NDIS or ODI driver, which can cause memory problems on DOS and Windows 3.x PCs.
Rather than invest in complicated direct connections, many companies now opt for mainframe gateway software, which runs on a server and acts as a doorway between LANs and host computers. A mainframe gateway, also called an SNA (Systems Network Architecture) gateway, offers several advantages over various direct-connection methods.
For instance, an SNA gateway can store all of your host addresses centrally for simplified administration. Moreover, an SNA gateway eliminates the need for TSR programs and drivers, saving valuable real-mode memory on DOS or Windows 3.x desktops. Instead of endlessly polling every PC to maintain direct connections, the host simply communicates with the SNA gateway. This can dramatically reduce network traffic and also decrease session time-out problems.
Novell vs. Microsoft
The mainframe gateway market-like the server software market-is dominated by Novell (NetWare for SAA, co-marketed with IBM), with Microsoft (SNA Server) coming on strong. Each product costs about $100 per desktop. Naturally, there are pundits galore for both products. Which gateway is better? That depends on which server OS you've standardized on. Predictably, SNA Server only runs on NT Server, and Novell's gateway only supports NetWare and IntranetWare (a rebranded NetWare 4.11 with bundled intranet software)
Several prominent customers, including TWA, have standardized on SNA Server because of its tight integration with NT Server. A few former NetWare for SAA shops have even converted to SNA Server. Hansen and Associates, a Seattle systems integrator, used both NetWare for SAA and SNA Server 2.11, but changed course when SNA Server 3.0 arrived last fall. "NetWare for SAA had high overhead and SNA [Server] 2.11 didn't support dot-matrix printers for 3270 printing," said owner Pete Hansen, adding that SNA Server 3.0 addressed those concerns.
Customers typically deploy SNA Server using a branch, centralized or distributed model. In the branch model, SNA gateways are placed in remote offices and communicate with your host using native SNA protocols, either directly via SDLC (Synchronous Data Link Control) leased lines, an X.25 network or via the 802.2/LLC (Logical Link Control) networking standard. You can also interlink branch offices using APPC (Advanced Program-to-Program) connections between the remote SNA Server gateways.
Many early SNA Server users adopted the branch model, but Microsoft says new customers are increasingly focusing on centralizing their SNA servers. In this model, SNA gateways are placed in your data center and connected to your host using native SNA protocols. The centralized SNA gateways provide split-stack or TN3270 service for local and remote PCs. The PCs connect to the gateways using popular networking protocols, such as TCP/IP. This centralized model keeps SNA traffic-which many network administrators aren't familiar with-in the data center.
Some customers prefer using SNA Server in branch offices and the central site. This distributed deployment model relies on SNA Server's Distributed Gateway Service (DGS) feature. DGS connects branch office SNA servers to centralized SNA servers using native TCP/IP, IPX/SPX or Banyan Vines protocols. Next, the centralized SNA servers are connected to the host via a direct channel attachment or a local token ring network using native SNA protocols. This distributed deployment model is more expensive than the alternative models (since you're essentially doubling your SNA Server purchases), but it improves your network's redundancy.
SNA Server was the first mainframe gateway for NT networks, but it's no longer the only one. IBM hedged its NetWare for SAA bet by porting its own Communications Server (already available for OS/2 and AIX) to Windows NT earlier this year.
CommServer for NT (http://www.networking.ibm.com/csn/csnprod.html) allows SNA applications to move over TCP/IP networks. On the other hand, TCP/IP sockets applications, such as ftp and telnet, can flow across SNA networks. CommServer ($995 per server; $69 per desktop) can also work hand-in-hand with IBM's other server apps, such as Lotus Notes.
Embracing SNA Server
Microsoft is countering IBM's CommServer offensive by enticing third-party software developers to support SNA Server. Among Microsoft's closest partners is Proginet Corp. (http://www.proginet.com). Microsoft recently acquired a minority stake in Proginet; Proginet returned the favor by releasing HostOffice, a suite of apps that integrate Microsoft BackOffice servers with mainframes or AS/400 environments. The HostOffice suite includes Fusion File Transfer Management Systems (FTMS), SecurPass and HostFolder, among others.
FTMS provides management and control when transferring bulk data between IBM systems and Windows networks. It lets you transfer files in the background (unattended or on a recurring basis) between mainframes, midrange systems and BackOffice apps. SecurPass ties together the security systems of NT and your IBM host. And HostFolder can publish mainframe and AS/400 data on the Web using Microsoft Exchange public folders. It also allows hosts to send print queue output directly into e-mail via Microsoft Exchange distribution lists, mailboxes and fax servers.
Once you have an SNA gateway in place, your corporate desktops will need terminal emulators to view mainframe data. You can choose from dozens of products. A safe place to start is by evaluating terminal emulators from market leaders Attachmate and Wall Data.
Attachmate's Extra For SNA Server ($195 per desktop) runs on Windows 3.x, Win95 or NT 4.0 desktops. It supports load balancing, hot backup, SNA print services, LU (Logical Unit, a user's port into an SNA network) pooling and TN3270E (an alternative to TCP/IP for mainframe connectivity). Wall Data's rival offering, Rumba Office 95/NT for SNA Server ($300 per user), supports mainframe, AS/400, Digital VAX and UNIX connectivity. Its graphical interface offers a "notebook" metaphor (resembling Lotus Organizer), allowing you to organize and integrate host sessions with desktop apps. Rumba also contains a family of ActiveX controls, allowing developers to generate custom client/server apps containing host data.
Equipping PCs with terminal emulators can require extensive configuration-and a visit to each and every desktop within your corporate network. For those seeking less complicated host connectivity, Web technology may be the solution.
Virtually every terminal emulation vendor is developing software that allows familiar, easy-to-use Web browsers-running on PCs, network computers or forthcoming NetPCs-to access host data. International Data Corp. of Framingham, Mass., predicts more than 150,000 Web-to-host gateway units will be sold by the year 2000. "Long term, the browser may become the terminal emulation front-end," says SNA Server user Jim Burks, an engineering manager at Harrah's Entertainment in Memphis, Tenn.
The most basic Web-to-host access is simple 3270 or 5250 conversion, which translates 3270 or 5250 data streams into HTML. Conversion software for SNA Server was flooding the market as we went to press. On the upside, conversion software can give you easy access to host data from browsers, and administrators can apply their company's established intranet security policies to Web/host apps.
On the downside, this simple conversion process often requires a separate browser instance for each host session, and some large apps may not run well as a result.
Web browser plug-ins are an alternative to conversion software. White Pine Software's WebTerm Toolbox ($69 per emulator; $169 for the administrator piece), for one, lets you embed terminal emulators within your Web browser. Future releases will support Java and ActiveX, which means you won't be caught in the object crossfire between Sun Microsystems and Microsoft. NetSoft and NetManage are also said to be developing ActiveX-based emulators that expand on their current products (see table above)
Other Web-to-host tools, like Attachmate's Extra Host Publishing System ($25,000), allow software developers to link 3270 apps and relational databases to the Web. The result can be appealing GUI-based apps for Web browsers that look far better than a 3270 screen. However, this approach suffers from some of HTML's limitations. For example, performance can slow to a crawl when real-time host updates refresh a Web browser. Further complicating matters, PF keys must still be supported, and you must still operate in a terminal-like mode-even though you're looking at your browser.
Some companies are already testing Web-to-host connectivity from within their corporate intranets (firewalls protect the systems from "curious" external Internet users). For example, at TWA none of the airline's Web-to-host transactions are publicly accessible, according to David Lash, a senior technical consultant at the airline in Kansas City, Mo. Lash expects that when TWA rolls out the production Web pages, there won't be direct access to mainframe data. For security reasons, the transaction "will take a few hops" before it connects to the host.
Linking to the future
NT administrators are seeking to streamline host access methods with SNA gateways and, longer term, Web-to-host connections. But before you toss aside aging 3270 terminals, examine the entire marketplace. Attachmate, IBM, Microsoft, Novell and Wall Data-to name just a few-want your host-access business. Only you can decide which method suits your company best.
Anne Fischer Lent is a freelance writer who specializes in Windows NT issues and trends. She is the author of The Windows for Workgroups Bible (Addison-Wesley, 1993) and The Ultimate Desktop Publishing Starter Kit (Addison-Wesley, 1995). She has also written extensively for WINDOWS Magazine's sister publication, InformationWeek, among others. Contact Anne care of the editor at the e-mail addresses here.