ch04.htm

this describes managing multivendor networks

the path dynamically and automatically. Data that was sent at the time of failure
will be automatically recovered.</P>
<P>ANR is a connectionless protocol used in the intermediate nodes of the HPR subnet.
The ANR technology helps to control congestion between the various RTP endpoints
by monitoring the amount of data flowing between the endpoints, and making adjustments
to ensure against overload. ARB also guards against congestion by sending data only
in measured amounts, as opposed to uncontrolled bursts.
<H3><A NAME="Heading12"></A><FONT COLOR="#000077">Application/User Relationship</FONT></H3>
<P>From the simplest of perspectives, the relationship between IBM users and applications
is highly structured, highly controlled and menu-oriented. This is true of both the
midrange (System/3X and AS/400) and the System/390 environments because both types
of terminals (5250 and 3270) are block-oriented. Typically, users access the system
and run the program they desire. Although accessing the program can be done in a
conversational manner (as with TSO), the actual application program or utility normally
uses some type of full-screen input/output.</P>
<P>Although IBM's environment is multiuser, each user's activities should not be
construed as sessions, such as those with HP and DEC architectures. With sessions,
users perceive that they have the computer to themselves and that their applications
seem to function independently from other activities. In a multiuser environment
like IBM's, each user accesses the system and then is given a controlled set of choices--
the system guides the user through the selection process based on the user's profile.</P>
<P>Another key difference between the session-oriented computers and IBM computers
is the level at which shared access to a program or system service occurs. With DEC
and HP computers, each person runs, in effect, an individual copy of the program
that shares system resources at a level typically below the visibility of the user.
With IBM, this sharing occurs on a higher plane. In most cases, in fact, programs
on IBM systems are written to accommodate concurrent access from multiple users.</P>
<P>The actual appearance of the user interface and the development of programs are
disparate among the IBM platforms. Developing a program on an AS/400, for example,
has a dramatically different approach and appearance than developing the same program
on a 3090. These differences pose a real problem to IBM because it often prohibits
an AS/400 customer, for example, from upgrading to a 9370, because the customer's
AS/400 programs would not be compatible with the 9370 architecture (nor would the
user's programming staff be competent with the new system).</P>
<P>To address this issue and to provide IBM-wide standards for programming and the
program's appearance to the user, IBM introduced its Systems Application Architecture
(SAA). SAA is a set of routines and transport mechanisms that (theoretically) makes
it possible to develop and implement a program on one platform--a PS/2, for example--and
then move that program to another platform--say, a 9370--with minimal effort. SAA
programming routines perform the following functions:

<UL>
	<LI>Provide a common means of accessing files and database information (regardless
	of where they reside).<BR>
	<BR>
	
	<LI>Supply a consistent way to access a terminal (regardless of the terminal's location).<BR>
	<BR>
	
	<LI>Define and implement both a universal character (workstation) and window-based
	(graphics) appearance to the user.
</UL>

<P>The full definition of SAA specifies and defines the underlying formats and transport
mechanisms upon which these three higher-level functions rely. The term <I>SAA-compliant</I>
indicates that a particular feature or function is included in the SAA definition.
For example, the LU 6.2 interface is deemed SAA-compliant but LU 0 is not. Also note
that the actual implementation of SAA is a long-term strategy that involves releasing
pieces of SAA as time progresses.
<H3><A NAME="Heading13"></A><FONT COLOR="#000077">Terminal Attachment Philosophy</FONT></H3>
<P>IBM uses a hierarchical architecture between terminals and computers. This architecture
is similar in the midrange systems and mainframes, although the midrange implementation
cannot accommodate the large numbers of terminals supported by the mainframe architecture.</P>
<P>In the mainframes, the 3270 family of terminals and printers is the backbone of
the user interface. In Figure 4.5, terminals attach to a cluster controller (also
known as a remote control unit) via coaxial cable. Some example workstation controllers
include the 3174, 3274, and the 3276 (which also features a built-in terminal). Each
cluster controller can support multiple terminals and printers (typical numbers include
8, 16, and 32, although cluster controllers can be configured to support larger numbers).</P>
<P><A HREF="javascript:if(confirm('http://docs.rinet.ru:8080/MuNet/ch04/04fig05.gif  \n\nThis file was not retrieved by Teleport Pro, because it was redirected to an invalid location.  You should report this problem to the site\'s webmaster.  \n\nDo you want to open it from the server?'))window.location='http://docs.rinet.ru:8080/MuNet/ch04/04fig05.gif'" tppabs="http://docs.rinet.ru:8080/MuNet/ch04/04fig05.gif"><B>FIG. 4.5</B></A> <I>IBM 3270 Connectivity</I></P>
<P>Each cluster controller interfaces to the central mainframe (or a remote front-end
processor). At the central site, the actual interface can be an Integrated Communications
Adapter (ICA) for smaller networks or a front-end processor in larger networks. With
the exception of small 4300 sites, the interface is normally to a front-end controller
such as a 3705, 3720, 3725, or 3745.</P>
<P>The front-end communications controller is in itself an intelligent device. The
front end runs software called the Network Control Program (NCP), which defines and
controls the network. The NCP interfaces with VTAM (or some communications access
method subsystem) in the host. The attachment between the front-end processor and
the mainframe is a high-speed channel that facilitates a larger volume of high-speed
data.</P>
<P>For the 5250 family of terminals and printers used with midrange systems, the
architecture is similar but not identical. Like the 3270s, the 5250 terminals attach
to a controller, but they use a twinaxial cable connection that daisy-chains the
terminals on the cable to the controller (see Figure 4.6). In smaller networks, the
controller is integrated directly into the midrange computer. In larger networks,
remote controllers (5294 or 5394) serve to funnel the terminals back to the midrange
host.</P>
<P><A HREF="javascript:if(confirm('http://docs.rinet.ru:8080/MuNet/ch04/04fig06.gif  \n\nThis file was not retrieved by Teleport Pro, because it was redirected to an invalid location.  You should report this problem to the site\'s webmaster.  \n\nDo you want to open it from the server?'))window.location='http://docs.rinet.ru:8080/MuNet/ch04/04fig06.gif'" tppabs="http://docs.rinet.ru:8080/MuNet/ch04/04fig06.gif"><B>FIG. 4.6</B></A> <I>IBM 5250 Connectivity</I></P>
<P>The critical difference between the midrange and mainframe terminal architectures,
however, is the midrange's lack of a front-end communications processor. This device
enables mainframes to handle large terminal networks.


<BLOCKQUOTE>
	<P>
<HR>
<B><font color=#000077>NOTE:</font> </B>Both product families use SDLC as their communications methodology.
	Messages from the 3270 family are carried in the LU 2 and LU 3 formats, while messages
	from the 5250 family are carried in LU 7 format.&#160;n 
<HR>


</BLOCKQUOTE>

<H3><A NAME="Heading14"></A><FONT COLOR="#000077">Peer-to-Peer Relationships</FONT></H3>
<P>Given the hierarchical nature of SNA and its structured PU layout, it is sometimes
difficult to determine what is a peer to what else, let alone what relationship those
two peers can have. Previously in this chapter, the fundamental concepts of PU 2.1,
APPN, LU 6.2, and APPC were presented. However, it is often unclear how (or if) these
topics are related.</P>
<P>Recall that a Physical Unit Type 2.1 device is an intelligent device capable of
conversing with other PU 2.1 devices and is also capable of routing or rerouting
messages through other PU 2.1 devices. Application Peer-to-Peer Networking (APPN)
describes the interaction between PU 2.1 devices. A Logical Unit Type 6.2 is a data
format that enables two programs to communicate across SNA. A program implementing
the LU 6.2 interface must reside on a system that is operating at either the PU 2.1
or PU 5 level. Application Program-to-Program Communications (APPC) describes LU
2 conversations.</P>
<P>In addition to these concepts and implementations, SNA also supports the following
interactions, which can be considered peer-to-peer:

<UL>
	<LI><I>SNA Distribution Services (SNADS).</I> As the name implies, SNADS is a distribution
	service used to move information, such as documents, electronic mail, and files,
	throughout the SNA physical network. Please note, however, that the information sent
	through SNADS must be selected for distribution and that the distribution does not
	occur on demand (like the U.S. mail, things get there when they get there). SNADS
	uses both APPC and APPN as part of the delivery mechanism.<BR>
	<BR>
	
	<LI><I>Network Job Entry (NJE).</I> NJE exists above the Job Entry System (JES) and
	is used to distribute the processing aspects of batch jobs among systems. In other
	words, NJE links JES processing systems to form a loosely coupled batch system. With
	NJE, a job can be submitted on System A, processed on System B, and printed at System
	C. NJE uses bisynchronous links, SDLC links or direct channel-to-channel attachment
	to facilitate the communications between systems.<BR>
	<BR>
	
	<LI><I>Distributed Data Management (DDM).</I> DDM is used primarily in the midrange
	systems. DDM is a shared-file product that enables midrange systems to share access
	to data files on a file or record basis. DDM can be run over a Token-Ring network
	or over SDLC lines. Support for DDM is also available for versions of CICS, thus
	enabling mainframes and midrange system to share file-based information using DDM.
</UL>

<P>Again, given the size and complexity of SNA, other peer-to-peer facilities are
available. The capabilities presented herein, however, are the most commonly used.
<H3><A NAME="Heading15"></A><FONT COLOR="#000077">PC Integration Strategy</FONT></H3>
<P>IBM has adopted the IEEE 802.5 Token-Ring specification as the basis for its PC
and PS/2 LAN implementation. This ring topology features a central ring from which
devices (such as PCs and PS/2s) are attached (see Figure 4.7). In this LAN architecture,
a token is passed along the ring. When a unit stops transmitting or has nothing to
transmit, it relinquishes the token to the next unit on the ring.</P>
<P><A HREF="javascript:if(confirm('http://docs.rinet.ru:8080/MuNet/ch04/04fig07.gif  \n\nThis file was not retrieved by Teleport Pro, because it was redirected to an invalid location.  You should report this problem to the site\'s webmaster.  \n\nDo you want to open it from the server?'))window.location='http://docs.rinet.ru:8080/MuNet/ch04/04fig07.gif'" tppabs="http://docs.rinet.ru:8080/MuNet/ch04/04fig07.gif"><B>FIG. 4.7</B></A> <I>Token-Ring Topology</I></P>
<P>The original IBM Token-Ring implementation operated at a rate of 4 Mbps. A higher
data rate, 16 Mbps, was made available in the late 1980s. To make the physical attachments
to the ring easier, additional concentrators and attachment units were released.
Two common units are the eight-port medium access unit (MAU) and the two-port hub;
these units extend the ring without corrupting its fundamental topology. Gateways
and bridges can be used to tie two or more Token-Ring networks together.</P>
<P>With the physical Token-Ring LAN, stand-alone PC networks can be implemented using
IBM's network software or other network software such as Novell's NetWare. To incorporate
a Token-Ring LAN into a global SNA network, however, IBM's networking software must
be implemented with attachments to the SNA network. The nature of this attachment
depends on the processor or terminal types closest to the LAN.</P>
<P>For example, in a small midrange environment, a Token-Ring LAN can be connected
to the midrange via a midrange interface controller. In this case, the midrange system
actually participates in the Token-Ring LAN; a PC product named PC Support is available
to enable the PCs or PS/2s to access the midrange applications and files. (Moreover,
when multiple midrange processors are present on a Token-Ring LAN, special peer-to-peer
capabilities are available to share files and log on to the various systems.)</P>
<P>In larger SNA networks, however, the attachment of the LAN to the network depends
on the location of the Token-Ring LAN. If the Token-Ring LAN is remote to the central
mainframe, it can be attached to the remote cluster controller, which acts as a gateway
to the SNA network. If, however, the LAN is near the central processor, it might
also be connected to the front-end communications processor, which serves this gateway
function.</P>
<P>For a more detailed comparison of the Token-Ring topology to other topologies
(such as CSMA/CD), refer to Chapter 7, &quot;LANs and WANs&quot; (p. 139).
<H3><A NAME="Heading16"></A><FONT COLOR="#000077">Office Automation</FONT></H3>
<P>IBM offers several products to address the area of office automation. Its two
basic products that offer office functions (a calendar, electronic mail, information
management, and so on) are the following:

<UL>
	<LI><I>PROFS (PRofessional OFfice System).</I> PROFS is a software package available
	for the VM operating system. Because PROFs depends on VM and because VM is not usually
	implemented on larger mainframes, the 9370 is often the host of choice. PROFS exchanges
	messages with other mail systems through SNA Distribution Services (SNADS) or DISOSS.<BR>
	<BR>
	
	<LI><I>AS/400 Office.</I> AS/400 Office is a software package for the AS/400. AS/400
	Office exchanges messages with PROFS or through SNADS or DISOSS (Distributed Office
	Support System).
</UL>

<P>To provide an office automation solution that integrates PC and PS/2 functions
with the PROFS and AS/400 Office products