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LAN Gateways and Microsoft's SNA Server

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I can always tell when I walk into a shop that has just connected PCs to an AS/400 through a LAN. The data processing manager or programmer responsible for this usually doesn't have a single hair left on his head. He's pulled every one out, even the eyelashes, and he's reduced to mumbling about multiprotocol stacks and 640K memory limits. It's an ugly sight.

It doesn't have to be that bad. There's another way, and it's called a Systems Network Architecture (SNA) gateway. An SNA gateway provides a single system that handles all communication between the PCs on a LAN and the AS/400. You only have to configure one computer to talk to the AS/400, instead of 20, 300, or however many PCs your network supports.

In the past we didn't have many choices. Gateways were often expensive and difficult to install, configure, and support. We have another option now, Microsoft's recently released SNA Server Version 2.1. While previous versions had some significant limitations, this version creates a new standard for the market. SNA Server has very competitive pricing, is easy to install and configure, and has Microsoft behind it for support. So the good news is, you can keep your hair!

A Very Tight Fit

All AS/400 LAN installations face a basic problem. PCs on a LAN generally don't like to talk SNA, and AS/400s don't like to talk anything but SNA. TCP/IP, IPX/SPX, and NetBEUI are the protocols that perform the best and have the greatest support on PC networks.

Even though IBM has worked on improving the performance of the AS/400's TCP/IP in V3R1, SNA is still going to be the best and fastest link to an AS/400. As far as the other protocols go, they might as well be Greek to an AS/400. It doesn't natively support most other protocols and probably never will.

To solve the problem, people try to run two protocols over the same network adapter in the client PC. This can be done, but there's a problem-DOS and Windows. To run these protocols, you normally have to load device drivers and protocol managers into conventional memory. If you use too much conventional memory, you're going to have problems, such as applications not loading, general protection faults in Windows programs, and intermittent failure of LAN connections.

1 shows an example of a typical LAN in which PCs connect to both an AS/400 and a server. The PC client talks to the server using NetBEUI and to the AS/400 using SNA. This shows the problem configuration.

Figure 1 shows an example of a typical LAN in which PCs connect to both an AS/400 and a server. The PC client talks to the server using NetBEUI and to the AS/400 using SNA. This shows the problem configuration.

Using an SNA LAN gateway solves some of these problems because an SNA LAN gateway uses a PC as a server. The PCs talk to the gateway server in whatever protocol you select for your LAN, and the gateway converts the traffic intended for the AS/400 into SNA. This lets you pick the best protocol for your LAN and talk to the AS/400 in the protocol it prefers.

2 shows the same basic LAN as 1, but using an SNA gateway. The difference is that the server is directly connected to the AS/400 and has two LAN adapter cards in it. The client PC now uses only the NetBEUI protocol, even for data destined for the AS/400. The server is the only system using more than one protocol.

Figure 2 shows the same basic LAN as Figure 1, but using an SNA gateway. The difference is that the server is directly connected to the AS/400 and has two LAN adapter cards in it. The client PC now uses only the NetBEUI protocol, even for data destined for the AS/400. The server is the only system using more than one protocol.

You also have the option to have the AS/400 connected to the LAN instead of the server. This is cheaper, since you need only one LAN adapter card in the server, but less efficient because all data sent to the AS/400 is transmitted over the LAN twice. The data travels from the client PC over the LAN to the server and is then transmitted over the LAN to the AS/400.

Microsoft's SNA Server

Up to this point, everything I've discussed is true of any SNA gateway. As I noted earlier, most gateways are expensive and complex. SNA Server, Microsoft's SNA gateway product, solves these problems. Microsoft released Version 2.1 late last year. I'll cover the new features, but first I'll discuss SNA Server in general.

The foundation for SNA Server is the Windows NT operating system. Since there is some confusion about how Windows NT has evolved, I'll explain it a bit. Originally, there were two types of Windows NT available-Windows NT and Windows NT Advanced Server (NTAS). Both were released at Version 3.1. The main difference between the two is that Microsoft targeted NTAS for larger-scale LANs.

The release of SNA Server Version 2.1 was concurrent with the release of Windows NT Version 3.5, also known as Daytona. There are still two flavors of NT, but their names have changed to Windows NT Workstation and Windows NT Server. NT Workstation is a full 32-bit, preemptive, multitasking desktop operating system targeted primarily at developers and power users. It's what I run on my PC both at work and at home. Windows NT Server is Microsoft's network operating system (NOS) designed to compete with Novell NetWare.

Based on Microsoft's performance testing, SNA Server requires NT Server. It will also run on NT Workstation but Microsoft doesn't support this. However, there are some hard-coded connection limits built into NT Workstation, which isn't optimized for use as a server. Because of the limitations associated with running SNA Server on NT Workstation, most people will only do so when they want to learn how to install and configure it.

SNA Server's integration with Windows NT has a number of benefits.

o The monitoring tools and utilities provided with Windows NT work with SNA Server.

o When you use the Windows NT performance monitor, you have options to monitor the performance of SNA Server.

o Windows NT puts errors related to SNA Server in the event log, which is similar to the QHST log on the AS/400.

o SNA Server supports any of the LAN protocols that Windows NT supports, including TCP/IP, IPX/SPX, NetBEUI, and AppleTalk.

Windows NT is a reason to feel confident in the reliability of SNA Server. Because Windows NT runs applications in different memory spaces, applications that crash (even 16-bit Windows applications) are less likely to cause problems with SNA Server. Most problems that you have won't require you to disrupt connections to the AS/400.

Running under Windows NT isn't the only reason for the effectiveness of SNA Server. One of SNA Server's strengths is its modular design. Microsoft intentionally designed SNA Server so that third-party vendors can extend it by following the standards. This allows vendors to support new communications tech-nology when it becomes available or to support features Microsoft doesn't necessarily think are cost effective.

For example, Microsoft supports a number of interfaces to the AS/400, including token-ring, Ethernet, SDLC, and X.25. It does not support twinax cabling. Microsoft apparently didn't feel there would be enough demand for twinax support, possibly because of its relatively low throughput. Another company, however, disagreed. It felt there was a market for twinax connectivity for SNA Server. By working with Microsoft and writing a driver conforming to the SNA Device Interface Specifications (SNADIS) standards, that company is able to provide twinax support which Microsoft will bundle with SNA Server.

To complement SNA Server's strengths, here's what's new in Version 2.1. The number of clients and sessions supported has increased to 2,000 and 10,000 respectively; Version 2.1 runs on all of the processors that NT Server supports and supports the new protocols available, including native TCP/IP, AppleTalk, and Banyan Vines.

Problems With LAN Gateways

One problem with any gateway is that all connections to the AS/400 are lost if the server goes down since you are routing all SNA traffic through a server. However, Microsoft designed SNA Server so it supports hot backup if you have two or more servers. This allows you to define more than one route to the AS/400 and have users switch to an alternate route on their own when a failure occurs. Because this requires more than one server, it may not apply in a small shop.

Performance is another issue. Since a single resource supplies services for a number of clients, there is naturally going to be a bottleneck. In the case of SNA Server, the potential resource bottleneck is the server PC, but, if you have multiple servers, SNA Server supports load balancing. This allows dynamic routing through a server based on the load on that server.

Another solution from Microsoft is to simply refute the contention (pardon the pun). Microsoft studied the delays involved in transmitting data through an SNA Server configuration. According to the information provided by Microsoft, the delay imposed as a result of the server was negligible. Almost all the time was spent in the physical transmission of the data and in the processing time the host needed to receive the data.

From my perspective, a simpler answer is that the performance of the server is something you can control. Earlier, I explained that Microsoft integrated SNA Server with Windows NT, including the performance monitor. If you suspect that the server is a bottleneck, you can use the performance monitor to find out if that is true and why. That way you can find out if you need to add more memory or a faster processor, or if the problem is not with the server in the first place.

Deployment And Development

SNA Server supports just about every client PC you can think of, including MS- DOS, Windows, Windows NT, as well as Macintosh operating systems and versions of OS/2 that support LANs. The software you need for client PCs is included on the SNA Server CD-ROM. Microsoft has a method to create install diskettes for the client software; you can also install the client software from the server.

Installing and configuring SNA Server is a simple process. A future article on SNA Server implementation will provide more details. One of the reasons it is so simple to configure is because you use processes like drag-and-drop to configure the connections as you do with other PC software.

Another reason the software is easy to use is the support you get from Microsoft. The CD-ROM contains all of the documentation, and Microsoft has included a 90-minute videotape that explains the basics of SNA. The tape also goes into detail on how to configure SNA Server both for the mainframe and for the AS/400.

You can obtain support through a forum (MSSNA) on CompuServe and an Internet FTP (FTP.MICROSOFT.COM) site. Microsoft also has pay-per-incident telephone support and two additional levels of support: priority and premier. Additional support is available from Microsoft's Solution Providers and Consulting Services.

If you are developing applications for or need support for certain APIs, SNA Server supports all of the SNA APIs, including APPC, CPI-C, and EHLLAPI. The CD-ROM contains a number of example programs demonstrating APPC and CPI-C. The irony is that many of the sample programs come from IBM.

Microsoft includes a 5250 and 3270 emulation applet (a scaled-down emulator) with SNA Server. It's only for "demonstration, evaluation, training, [and] support" purposes, and the license only allows you to run it on one PC at a time. A number of other vendors supply 5250 emulation programs for both Windows and Windows NT. Many of these vendors have included coupons with the SNA Server package, allowing you to obtain a copy of their software free or at very little cost.

SNA Server also works with Microsoft's Remote Access Service (RAS). RAS is a protocol that allows a remote client to dial-in to the network. Everything appears as though the user is locally attached to the network. RAS also allows you to access an AS/400 remotely through SNA Server. RAS services include dial- back capability.

Is an SNA Gateway for You?

The greatest problem with SNA gateways has been the price. With SNA Server, you pay a per-seat license charge, and you pay for a copy to run on each server. With Version 2.1, Microsoft has changed the pricing structure and, more importantly, dropped the overall price as a result.

There are a number of quantity purchase programs available, ranging from a fairly small quantity of 50, up to 10,000 client licenses. Microsoft sells the license packs in units for single clients and 20 clients through a number of retail outlets. This makes it easier to get the exact number of client licenses you need, which can save you money.

There are a number of questions you can ask yourself to help determine if an SNA gateway will help you. Answering "Yes" to any of the following questions means you should be looking at an SNA gateway.

o Are you running more than one protocol on the client PCs?

o If so, are you having problems with conventional memory?

o Is the fact that you need to connect to an AS/400 determining which protocol you use?

o Do you want more control over how resources are allocated for LAN-connected PCs?

o Are the PCs you have attached using too much memory and too many CPU cycles?

o Is setting up LAN clients that connect to the AS/400 too difficult?

If you decide you need an SNA gateway, the question is, which one? In the past, if you were running a Novell NetWare network, you had only one choice. With SNA Server's better integration with NetWare, you now have a second choice.

The strengths of SNA Server and Windows NT-namely, support for symmetrical multiprocessing (SMP) and non-Intel processors, ease of configuration, performance, and price-mean that it should be at the top of your list of SNA gateways to evaluate.

Product Information

The minimum hardware requirements for SNA Server above what you need for NT Server are 4 megabytes (MB) of RAM and 11MB of disk space. The memory requirements are variable based on the number of clients you will attach, but the speed of the processor has a significant effect as well. The disk requirements are 2.2MB for a DOS client, 4MB for a Windows for Workgroups client, and 7MB for a Windows NT client.

SNA Server Price: $1,059 (10 clients) Microsoft Corporation One Microsoft Way Redmond, WA 98052 800-426-9400; fax: 206-936-7329

Jim Hoopes is a senior technical editor for Midrange Computing.

LAN Gateways and Microsoft's SNA Server

Figure 1 LAN Without an SNA Gateway

LAN Gateways and Microsoft's SNA Server

Figure 2 LAN With an SNA Gateway



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