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10/90 HOW TO HOOK A NETWARE LAN TO AN AS/400?
by
Skip Addison
Director, Consultant Relations
Novell Communications Products Division
INTRODUCTION
The AS/400 supports numerous connectivity options including twinaxial cabling, SDLC (Synchronous Data Link
Control) lines, ethernet and token ring connections. Novell has received numerous requests for information on
how best to tie a NetWare network to an AS/400. This paper outlines the connectivity strategy that will best
integrate the capabilities of the LAN and the AS/400 into one system providing access to LAN applications,
AS/400 (and S/3x) applications and to new client/server applications using IBM's System Application
Architecture (SAA). We assume that while the reader may be familiar with 3270 communications, 5250
communications or NetWare, they may not be familiar with all three. We also assume that the reader has a
passing familiarity with LAN technology.
BACKGROUND
Today, the IBM mid range product line consists of the older System3X (S/34, S/36, and S/38) series and newer
AS/400 series of minicomputers. The S/3X products were very successful when they were introduced in the
mid 1970s. However, sales began to level in the early 1980s with the increasing popularity of Personal
Computers and Local Area Networks. In addition, 5250 terminal sales declined as PC terminal emulation
replaced many terminals. Although by 1988 there were 300,000 IBM S/3Xs installed worldwide, it was rumored
that IBM might abandon the mid range market. This rumor was dispelled when in Jun 1988, IBM announced
the AS/400. There are nine models of the AS/400, providing from 4 to 96MB of memory and up to 38GB of
disk storage.
AS/400 Configurations
C04 C06 C10 C20 C25
Main Storage, MB 8-12 8-16 8-20 8-32 8-40
Max DASD, GB .96 1.28 1.28 2.56 3.84
Comm Lines 5 5 8 14 14
LAN Interfaces 1 1 1 2 2
Local Devices (twinax) 14 54 40 80 80
B35 B45 B50 B60 B70
Main Storage, MB 8-40 8-40 16-48 32-96 32-196
Max DASD, GB 13.7 13.7 27.4 54.8 54.8
Comm Lines 16 32 32 32 48
LAN Interfaces 4 4 4 4 4
Local Devices (twinax) 160 240 400 600 800
At this time there are over 50,000 AS/400s installed, split roughly equally between the US and the rest of the
world. The key features of the AS/400 are a high-level machine interface, an object-oriented design,
single-level storage with relational database capabilities, and highly integrated systems functions. Although
there are office, electronic mail, and query applications available for the AS/400, the most common applications
are industry specific such as accounting and manufacturing software packages.
COMMUNICATIONS
The AS/400 supports several communications options over several different types of media as shown below:
SDLC Twinax Token Ring
5250 (LU 4 & 7) yes yes no
3270 (LU 1, 2, 3) yes no yes
APPC (LU 6.2) yes yes yes
A REVIEW OF THE TERMINAL TYPES
> 3270
For reasons unknown outside IBM, mainframe computers based on the 370 architecture do not share common
communications with the S/3x minicomputers. IBM mainframes were designed to work with the 3270 family of
terminals. 3270 terminals can talk to a mainframe via either an SDLC line or a Token Ring. Additionally, the
IBM 3270 terminal controller can attach directly to a mainframe's I/O channel. SNA's LU type 1 printer, type 2
display, and type 3 printer describe the 3270 terminals.
The 3270 system was designed with an eye towards large installations. Up to 128 terminal sessions can use a
single terminal controller. By emulating a controller, a single LAN-to-host gateway can represent up to 255
users. Moreover, multiple gateways can share a single attachment to the mainframe.
> 5250
The System/3x machines from which the AS/400 is descended were designed to use the 5250 family of displays
and printers. These communicate with the minicomputer via either SDLC or Twinax hardware.
In IBM's System Network Architecture (SNA), each type of terminal or printer has its own Logical Unit (LU)
type. The LU type number defines the communication protocol, or language, that the device "knows how to
speak." The 5250 terminals are LU types 4 and 7.
Only up to 7 terminals are allowed to connect to a single twinax interface. For single user terminals this was not
a limitation. In the world of LAN gateways, however, only 7 connections is a distinct limitation.
A 5250 terminal can not talk to a mainframe; and until the AS/400 came out, a 3270 terminal could not talk to a
minicomputer. The differences in the protocols have caused significant problems.
AS/400 COMMUNICATIONS
Both the 5250 and 3270 protocols are based on a master-slave relationship. The minicomputer or mainframe is
the master and the terminal is the slave. As desktop devices grew more sophisticated, however, IBM needed to
develop a communication mechanism which would let an application on a desktop computer talk to an
application on a mainframe as peers. IBM called this Advanced Peer-to-Peer Communications or APPC. APPC
uses the LU 6.2 communications protocol. APPC is available on the AS/400 via twinax, SDLC and token ring
connections.
APPC is the primary backbone of future SAA-compliant networks. Using APPC/LU 6.2, desktop applications
can share data in a cooperative processing environment with the mainframe, sending updates and queries to host
transaction databases.
Because of APPC's ability to provide highly functional sessions, whether for terminal service or client server
applications, IBM has put most of its long term strategic emphasis on APPC for connections from desktop
computers. However, the 3270 and 5250 communication protocols will still be used for many years, because of
their support for nonprogrammable terminals and a large installed base.
Because of the communication differences between different terminal systems, IBM encourages developers to
develop their applications in such a way that either 5250 or 3270 terminal users can use them. The long term
direction from IBM is to use APPC/LU 6.2 for all SAA compliant applications.
Also, the AS/400 provides no documented method of supporting non-programmable terminals and APPC from
the same gateway. Therefore, whether using 3270 or 5250 terminal emulation, a separate gateway will be
required for SAA compliant client/server applications.
> 3270 SNA for AS/400
When IBM introduced the AS/400, they reduced its dependency on 5250 terminals by allowing 3270 terminals to
run applications that had heretofore required 5250 terminals. This is made possible by transparently translating
between the 3270 and 5250 data streams "on the fly". That is, the application running "sees" a 5250 terminal
although the user may actually be using a 3270 terminal.
The translation is not perfect, however. The principal difference is that the 3270 keyboard is missing three keys
that exist on the 5250 keyboard ("Field +", "Field -" and "Field Exit").
"PC SUPPORT"
IBM's AS/400 PC Support program allows PCs to connect to an AS/400 via token ring, ethernet or twinax. In
this case, the PC is emulating a 5250 terminal, but the 5250 information is "buried" in LU 6.2 packets for
transmission across the token ring. (Note that the chart above indicates that 5250 communications can not be sent
directly over the token ring.) Unfortunately, IBM's AS/400 PC Support program degrades performance of the
AS/400 and consumes a sizeable portion of the PC's RAM. The determination of whether these detractions are
prohibitive or not must be made by the customer.
There is no support from IBM for third-party equivalents to AS/400 PC Support. The method used by AS/400
PC Support to encapsulate the 5250 data stream in APPC/LU 6.2 is not documented publicly.
NOVELL AS/400 COMPATIBLE COMMUNICATION PRODUCTS
LAN to mainframe and LAN to minicomputer products are a major part of Novell's communication product
line. We have products for all three communication protocols (3270, 5250 and APPC). These are described
below.
3270 and APPC Communications.
There are two products which provide 3270 and APPC communication protocols, the NetWare 3270 SNA
Gateway and NetWare 386 Communication Services for SAA (which will begin shiping later this year). In the
SNA Gateway and Communication Services for SAA, a PC on the LAN emulates an IBM cluster controller. This
gateway PC can attach to the AS/400 using either SDLC or token ring. Other PCs on the LAN run the NetWare
3270 LAN Workstation software to emulate 3270 terminals and use APPC.
Novell's 3270 LAN Workstation has always included a keyboard definition utility permitting users to permit
users to redefine the keyboard for 5250 emulation. On October 18th we will begin shipping our SNA product
with 5250 keyboard emulation as an option (but the 3 "missing keys" will still be missing.
The gateway plays a central role in the interaction between users and the AS/400. It performs the low level
interaction with the AS/400, freeing up memory and processing power in the workstation. The high level
interaction is handled at the workstation. In SNA terms, the gateway emulates a PU, the workstation a LU.
Due to limitations in the AS/400, one gateway can not support both 3270 and APPC simultaneously. This
limitation does not exist in the mainframe world.
5250 Communications.
The NetWare 5250 Gateway, Version 2 emulates a 5250 terminal controller (IBM 5251 Model 12, 5294, or 5394)
so that up to 7 or 9 workstations (depending on configuration) on the LAN can function as 5250 terminals on a
System/34, System/36, System/38 or AS/400. The gateway connects to the AS/400 via either twinax or SDLC
lines. In concept, it is identical to the NetWare SNA Gateway except that it facilitates emulation of 5250
terminals rather than 3270.
STRATEGIC DIRECTION
As we have seen above, in connecting NetWare to an IBM mainframe there is little question about how to
proceed. NetWare SNA Gateways provide highly reliable, fully functional 3270 terminal emulation, the same
type of communications mainframe applications expect in normal day-to-day operation, with or without a LAN.
There has been some confusion, however, in the area of connectivity to the AS/400. This confusion has arisen,
in part, because of the dependence of many System/3x applications on 5250 communications on the one hand,
and the historically poor industry support (including support from IBM) for 5250 communications.
One can see, however, that IBM's trategic direction is to provide the broadest level of support to 3270 and APPC
communications. While the AS/400 can support 3270, there is no support for 5250 terminals on mainframes.
While the token ring can support 3270 controllers, there is no direct 5250 terminal support on token ring, except
for AS/400 PC Support. It is clear that IBM's primary strategic direction is focused on APPC and cooperative
processing. Novell is supporting this direction and increasing NetWare's connectivity to the AS/400.
RECOMMENDATION
1. All new applications being developed for use by non-programmable terminals should support 3270 terminals
as an option.
2. All cooperative processing applications for the IBM environment should use APPC/LU 6.2.
3. Users should look to 5250 gateways and terminal emulation (including AS/400 PC Support) as a migration
strategy for existing applications.
By following these recommendations users can expect the following benefits:
>Support for existing applications
>Positioning the network for future enhancements
>Better support for advanced communications
Existing applications have largely been written to use the 5250 data stream. For many users, 5250 gateways are
the preferable choice to ease the transition to LAN based systems without retraining.
While providing support for these existing applications, MIS staff and application developers can best position
themselves and the corporate network to take advantage of future communication enhancements by developing
all new applications in such a way that they can be used by either 5250 or 3270 users. Applications to be used
by PC users (ie. no nonprogrammable terminals) should be written to use APPC/LU 6.2.
Micro to AS/400 applications which are implemented using APPC in compliance with SAA provide the highest
level of functionality today and are in the best position to support all the features of the network of the future.
CONCLUSION
The changing world of data communications offers ever better means of communicating between the desktop
and the minicomputers and mainframes. Today's changes, however, must preserve yesterday's investments. So
when it comes time to determine the best strategy for tying the AS/400 to a LAN, the recommendation is: APPC
for the long term strategy; 3270 and 5250 as a migration tool and for support of existing applications.