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Wireless Networking Handbook
(Publisher: Macmillan Computer Publishing)
Author(s): Jim Geier
ISBN: 156205631x
Publication Date: 09/01/96

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ANSI Fiber Distributed Data Interface

The Fiber Distributed Data Interface (FDDI) standard was produced by the ANSI X3T9.5 standards committee in the mid-1980s, and it specifies a 100 Mbps dual token-ring LAN. FDDI specifies the use of optical fiber medium and will support simultaneous transmission of both synchronous and prioritized asynchronous traffic. The CDDI (Copper Data Distributed Interface) version of FDDI operates over Category 5 twisted-pair wiring. FDDI is an effective solution as a reliable high speed interface within a LAN or corporate network.

FDDI is an expensive, but effective solution for supporting high speed deterministic access to network resources. Some organizations find it necessary to use FDDI to connect a group of servers in a server pool. It is also beneficial to use FDDI as the backbone for a campus or enterprise network. The synchronous mode of FDDI is used for those applications whose bandwidth and response time limits are predictable in advance, permitting them to be pre-allocated by the FDDI Station Management Protocol. The asynchronous mode is used for those applications whose bandwidth requirements are less predictable or whose response time requirements are less critical. Asynchronous bandwidth is instantaneously allocated from a pool of remaining ring bandwidth that is unallocated, unused, or both.

ANSI is currently developing FDDI II, which is an extension of FDDI. FDDI II will have two modes: The Basic Mode (which is FDDI) and the Hybrid Mode that will incorporate the functionality of basic mode plus circuit switching. The addition of circuit switching will enable the support of isochronous traffic. Isochronous transmission is similar to synchronous, but with isochronous, a node is capable of sending data at specific times. Decreased source buffering and signal processing simplify the transmission of real-time information.

LAN Backbone

After selecting the medium access method(s), you need to decide the components of the LAN backbone, which include medium, hubs, switches, and access points. Part of this effort will also consist of showing how the components interface with each other. Therefore, you also need to specify the topology and configuration of the LAN.

The medium type fulfills the functionality of the physical layer of either the OSI or IEEE reference models. As described in Chapter 2, medium choices for wireless LANs are radio waves and infrared light. Recall that radio waves are capable of penetrating walls and can provide ranges of up to 1,000 feet, depending on the construction of the facility. However, radio waves are susceptible to interference from other systems. Infrared light will not penetrate walls, limiting the range to a single room or factory floor. Also, infrared light-based LANs do not support mobility (constant movement) very well. Most other systems, however, will not interfere with infrared light signals. Like all design elements, the choice of wireless medium depends on requirements. Radio waves will satisfy most requirements; however, there could be some unique requirements that point you toward the use of infrared light.

Choosing Wireless Media

Here are some suggestions on how to choose a wireless media:

Radio Waves

Consider the use of radio waves for the following situations:

  Users requiring mobility; that is, they need to move while accessing network resources.
  Users requiring access to network resources throughout the building.

Infrared Light

Consider the use of infrared light for the following situations:

  Radio wave interference is a potential problem.
  It is desirable to contain the communications signals within a closed area.
  Requirements identify the need to support high-speed, synchronous data transfers. (In this case use direct, token passing infrared.)
Choosing Physical Media

In addition to the wireless media, you might need to specify the use of physical media as well, especially if the network will include IEEE 802.3, IEEE 802.5, or FDDI wireline technologies. Here is an overview of these media types:

  Twisted-pair Wire. Twisted-pair wire uses metallic conductors, providing a path for current flow. The wire is twisted in pairs to minimize the electromagnetic interference resulting from adjacent wire pairs and external noise sources. A greater number of twists per foot increases noise immunity. Twisted-pair wiring is inexpensive to purchase and easy to install, and it is currently the industry standard for wiring LANs. IEEE 802.3, IEEE 802.5, and ANSI FDDI specify the use of unshielded twisted-pair (UTP) wiring. Consider the use of twisted-pair for wired connections inside buildings, unless you need a higher degree of noise immunity or information security.

The EIA 568 building wiring standard specifies the following five categories of unshielded twisted-pair wiring:

  Category 1. Old-style phone wire, which is not suitable for most data transmission. This includes most telephone wire installed before 1983, in addition to most current residential telephone wiring.
  Category 2. Certified for data rates up to 4 Mbps, which facilitates IEEE 802.5 Token-Ring networks (4 Mbps version).
  Category 3. Certified for data rates up to 10 Mbps, which facilitates IEEE 802.3 10baseT (ethernet) networks.
  Category 4. Certified for data rates up to 16 Mbps, which facilitates IEEE 802.5 Token-Ring networks (16 Mbps version).
  Category 5. Certified for data rates up to 100 Mbps, which facilitates ANSI FDDI Token-Ring networks.

The 10 Mbps version of IEEE 802.3, for example, requires at least Category 3 cable or higher, while the copper-based version of FDDI requires Category 5 cable. There is very little difference in price between Category 3 and 5 cable, and labor costs to install each are the same. Therefore, you should install Category 5 cable for all wired-network installations, regardless of whether you need the extra bandwidth or not. This will avoid expensive re-wiring if you require higher performance in the future.


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