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Micro House PC Hardware Library Volume II: Network Interface Cards And Modems Micro House PC Hardware Library Volume II: Network Interface Cards And Modems
by Micro House International, Inc. and Scott Mueller
Que, Macmillan Computer Publishing
ISBN: 078971664x   Pub Date: 06/17/98
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Descriptions of the seven layers follow:

  Transport. When more than one packet is in process at any time, such as when a large file must be split into multiple packets for transmission, the Transport layer controls the sequencing of the message components and regulates inbound traffic flow. If a duplicate packet arrives, this layer recognizes it as a duplicate and discards it. SPX and TCP are Transport layer protocols.
  Session. The functions in this layer enable applications running at two workstations to coordinate their communications into a single session (which you can think of in terms of a highly structured dialog). The Session layer supports the creation of the session, the management of the packets sent back and forth during the session, and the termination of the session.
  Presentation. When IBM, Apple, DEC, NeXT, and Burroughs computers want to talk to one another, obviously a certain amount of translation and byte reordering needs to be done. The Presentation layer converts data into (or from) a machine’s native internal numeric format.
  Application. This is the layer of the OSI model seen by an application program. A message to be sent across the network enters the OSI model at this point, travels downward toward Layer 1 (the Physical layer), zips across to the other workstation, and then travels back up the layers until the message reaches the application on the other computer through its own Application layer.

One of the factors that makes the NOS of each vendor proprietary (as opposed to having an open architecture) is the vendor’s degree and method of noncompliance with the OSI model. Sufficient protocol standardization has been implemented to allow all Ethernet products to function interchangeably (for example), but these standards do not direct comply with the OSI model document.

Using Low-Level Protocols

The MAC method for most LANs (part of the Data Link layer functionality discussed above) works in one of two basic ways: collision-sensing or token-passing. Ethernet is an example of a collision-sensing network; Token Ring is an example of a token-passing network.

The Institute of Electrical and Electronic Engineers (IEEE) has defined and documented a set of standards for the physical characteristics of both collision-sensing and token-passing networks. These standards are known as IEEE 802.3 (Ethernet) and IEEE 802.5 (Token Ring). Be aware, though, that the colloquial names Ethernet and Token Ring actually refer to earlier versions of these protocols, upon which the IEEE standards were based. There are minor differences between the frame definitions for true Ethernet and true IEEE 802.3. In terms of the standards, IBM’s 16Mbps Token-Ring adapter card is an 802.5 Token-Ring extension. You learn the definitions and layout of Ethernet and Token Ring frames in the sections “Using Ethernet” and “Using Token Ring” later in this chapter.

Some LANs don’t conform to IEEE 802.3 or IEEE 802.5, of course. The most well-known of these is ARCnet, available from such vendors as Datapoint Corporation, Standard Microsystems, and Thomas-Conrad. Other types of LANs include StarLan (from AT&T), VistaLan (from Allen-Bradley), LANtastic (from Artisoft), Omninet (from Corvus), PC Net (from IBM), and ProNet (from Proteon). All of these architectures can be considered archaic, however, and are almost never used in the construction of new LANs anymore.

Fiber Distributed Data Interface (FDDI) is a new physical-layer LAN standard. FDDI uses fiber-optic cable and a token-passing scheme similar to IEEE 802.5 to transmit data frames at a snappy 100Mbps. There are also new standards now on the market designed to upgrade Ethernet networks to 100-Mbps. Some of these, such as 100VG AnyLAN, can no longer be considered as Ethernet and use brand new methods for gaining media access. Some of these new standards are covered in the following section.

Evaluating High-Speed Networking Technologies

If you have fast workstations and a fast file server, you will want a fast network as well. Even the 16Mbps supplied by Token Ring may be too slow if your applications are data-intensive. The explosive growth of multimedia, groupware, and other technologies that require enormous amounts of data has forced network administrators to consider the need for high-speed network connections to individual desktop workstations.

Networking at speeds above 16Mbps has been around for several years, but it has primarily been limited to high-speed backbone connections between servers, due to its additional expense. Several new technologies are available today, however, that are designed to deliver data at high speeds—up to 100Mbps and more—to standard user workstations. Real-time data feeds from financial services, videoconferencing, video editing, and high-color graphics processing a just some of the tasks now being performed on PCs that would benefit greatly from an increase in network transmission speed.


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