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• Token Ring: With this network transmission format, everyone transmits signals when it is "their turn ." Token Ring networks are popular in IBM environments and offer speeds of 4 Mbps or 16 Mbps.
• Asynchronous Transfer Mode (ATM): This is a relative newcomer on the market. It is not common in local area networks (LANs) yet, but many manufacturers are starting to design and sell adapters using this technology. ATM relies on relatively high-speed burst transmissions.

The fourth layer (the transmission protocol) is similar to the envelope in which you send a letter by mail. This layer assembles the data you're transmitting into packets that can be routed through the network. Some of the more common transmission formats follow:

• Transmission Control Protocol/Internet Protocol (TCP/IP): This is the basis of the Internet you probably have read about. It started with a U.S. government research project and turned out to be a standard that people could rally around. TCP/IP is the most common format I have come across for client/server computing and connecting to UNIX-based computers.
• Internetwork Packet Exchange/Sequenced Packet Exchange (IPX/SPX): These are the transmission protocols used in Novell networking environments. Their use typically is limited to communications between PCs and Novell servers.
• NetBEUI: This is a protocol that some IBM and Microsoft networking products use for basic communications between PCs and LAN servers (such as Microsoft Windows NT). You probably will encounter this protocol only if you are running the Workgroups version of Oracle.
  I've had trouble using NetBEUI for client/server communications. I recommend that you consider using TCP/IP for client/server applications.
• System Network Architecture (SNA): This is actually more an architecture than a protocol. It is the main environment for IBM mainframe shops .

The fifth layer is where the database-unique processing begins; I call it lower middleware. Middleware is a term you encounter often in database networking; it refers to any supporting software you need to connect your application or database management system (DBMS) to the networking utilities on your computer. I made up the term lower middleware to refer to products (such as Oracle's SQL*Net) that transmit database requests in a predefined format (such as TCP/IP) to the networking software on your host computer.

The next layer is what I call upper middleware. These products are designed to enable a variety of applications to interface with lower middleware products to interface with a database on a remote computer. I qualify Microsoft's Open Database Connection (ODBC) standard as upper middleware; it takes queries and transactions from products such as the Visual C compiler or the Microsoft Query product and formats them to interface with a specific lower middleware

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product such as SQL*Net. Some products, such as Oracle's SQL*Plus, already have a direct interface to the lower layer of middleware (SQL*Net) and therefore do not need a separate upper middleware software package.

Finally, at the top of my stacking model are the applications that most users really care about. Of course, on the server end is the DBMS. On the client end is the financial accounting or sales forecasting system that users interact with on a daily basis. As I discuss later in the chapter, you have the option of using the PC and network to emulate a dumb terminal. In this host/terminal scenario, the PC application you run is a terminal emulator that connects directly to the network transmission utilities.

I included this discussion of a seven-layer database model because one of the greatest challenges I have faced when integrating computer systems is getting all the drivers, network interface cards, operating systems, middleware, and application packages to work with one another. Don't be too alarmed; they work very reliably after you purchase the right components . The trick is figuring out the correct equipment and software and configuring them properly.

Figuring it out is where the database seven-layer model comes in handy. Each of the layers corresponds to a product you have to purchase. Your trick is ensuring that whenever two layers touch one another, the products on either end are compatible. This compatibility has to be specific to your host computer environment (for example, a Hewlett-Packard H50 UNIX server or Packard Bell Force 101CD running Windows 95), and the exact versions of the products on either side must be compatible with one another (for example, SQL*Net TCP/IP 2.3 for Windows 95 and the Microsoft Windows 95 TCP/IP stack). Sales people tell you that the products work with a particular environment, but they might not know the gotchaswhether you can use only a particular network interface card, for example. When you're choosing products, it is a good idea to draw out your database seven-layer stack and look at the product specifications to ensure that all components are compatible with one another.

An Overview of Oracle Networking

At this point, you might be wondering how Oracle fits into the networking picture. The good news is that Oracle provides a large number of networking options ( unfortunately , for the systems folks and DBAs, that is also the bad news). Oracle started out as a host-based application. However, the company quickly recognized the advantages of splitting processing between the host computers and the increasingly intelligent workstations and PCs that were appearing on users' desks. This led to client/server products such as SQL*Net and PC development tools such as Oracle Forms. The client/server environment evolved to include interfaces to non-Oracle development tools and databases through the neutral ODBC and OLE interfaces, which I discuss later in this chapter.

Databases have grown in number and size . With a single, huge database to process information, many organizations have elected to build a series of smaller databases connected to one another. This is where the Oracle SQL*Net, Distributed options, and Gateway products come into play. I discuss each of these concepts in more detail in the next few sections.

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Of course, no discussion of modern networking is complete without the words World Wide Web prominently displayed. Although many companies are building products for the Web, few have the enthusiasm with which Oracle approaches this medium. It is very rare that Larry Ellison or other senior Oracle officials speak without mentioning the Oracle Web products, the Network Computer, or Web commerce. Almost all Oracle products are being Web- enabled (including the Oracle Applications suite), and Oracle has joined a number of joint ventures supporting Web-based environments. Therefore, I have included a section about Web/Internet networking for Oracle products. Figure 55.3 summarizes the Oracle networking environment.

Figure 55.3.
An Oracle networking
overview.

Before I conclude this discussion, I want to point out something that I always considered a little odd. If you are working with the earlier versions of the Oracle Workgroups Server products on a Microsoft Windows NT Server (for example, Version 7.1), you actually use networking software to communicate from your regular Windows applications (such as SQL*Plus) that are running on NT Server to your Oracle database, which is designed to run in native NT mode. This happens because regular Windows applications run under an NT subsystem known as Windows on Win32 (WOW). Oracle linked the 16-bit and 32-bit sides of the Windows NT system via SQL*Net. If you use Named Pipes as your communications protocol, you do not need to have a network interface card installed, but if you use TCP/IP as your protocol, you must have a network interface card. It's just something to remember if you are working in this environment.

Host/Terminal Connections

A good place to start is where Oracle startedwith databases located on host computers that are accessed by using terminals (or PCs acting as terminals). Figure 55.4 shows the basic configuration of the architecture; it has the advantage of being very simple. The host computer system provides facilities to connect terminals and supports one or more terminal types (such as DEC VT-100) to which it can send output and from which it can receive input. This type of connection usually has the following characteristics:

• The interface usually is capable of displaying only text and not graphics.
• You typically control the interface through a series of menus or command-line inputs.

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