LAN Protocol Overview

Network technologies can be classified as belonging to one of two basic groups.

  1. Local area network (LAN) technologies connect many devices that are relatively close to each other, many times within the same building. For instance, the computer data terminals in your favorite library have replaced card stacks and are used to display book information. These terminals would normally connect to each other, as well as the data base server, over a local area network.
  2. Wide area network (WAN) technologies connect a number of devices that can be many kilometers apart. For example, if there are two or more libraries in different areas of a large city, and they wanted to share their book catalog information, they would most likely make use of a WAN technology.

As you pursue your studies of the technologies that make up the CCNA core of subjects, you will note that quite a bit of time is dedicated to the subject of LANs. And, most of the study material teaches that there are three primary protocols used in LANs: Token Ring, FDDI, and Ethernet.

  • Token ringtechnology is a LAN protocol that resides at the Layer-2 data link layer of the OSI model. It uses a special three-byte frame called a token, which travels around a physical ring topology. Initially, in 1985, token ring operated at a speed of 4 Megabit/s. However, in 1989, IBM introduced the first 16 Megabit/s token ring products and the IEEE 802.5 standard was extended to support this new speed.With the development of switched and faster variants of Ethernet, token ring architectures lagged behind Ethernet. Token ring networks have since declined in usage and standards activity has ceased. Switched Ethernet has since dominated the layer 2 LAN networking market.
  • Fiber Distributed Data Interface (FDDI) provides a standard for data transmission in a local area network that can extend in range up to 200 kilometers, or 124 miles. Although the FDDI logical topology is basically a token ring network, it does not use the IEEE 802.5 token ring protocol as its basis. Instead, the FDDI protocol is derived from the IEEE 802.4 token bus timed-tokenprotocol. In addition to covering large geographical areas, FDDI local area networks can support thousands of users. FDDI uses a dual-attached, counter-rotating token ring topology.In a standard implementation, FDDI uses optical fiber, although it can be implemented over copper cable. In a copper network, FDDI is referred to as Copper Distributed Data Interface (CDDI).

    In the early to mid 1990s, FDDI was considered an attractive campus backbone technology. However, FDDI has since been made obsolete by Fast and Gigabit Ethernet due to their speed, lower cost, and common use.

  • The term Ethernet refers to a family of standards that define the physical and data link layers of the world’s most popular type of LAN. The different standards vary as to the speed supported, with speeds of 10 megabits per second (Mbps), 100 Mbs, and 1000 Mbs being common today. The standards also differ as far as the types of cabling and the allowed length of the cabling.For example, the most commonly used Ethernet standards allow the use of inexpensive Unshielded Twisted-Pair (UTP) cabling, whereas other standards call for the more expensive fiber-optic cabling. Fiber-optic cabling might be worth the cost in some cases because the cabling is more secure and allows for much longer distances between devices.

Although still in use in early network designs and implementations, Token Ring and FDDI are deemed to be legacy protocols and are no longer considered for new installations, except in rare and unique circumstances.

In my next post, we will examine Ethernet terminology and standards in much greater detail.

Author: David Stahl

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