Token Ring and FDDI Introduction
Introduction
As I stated in the introduction to this LAN portal, LANs based on token ring technologies are not exactly the rage these days. Token ring is an example of how something clever doesn't always capture the industry. There are many factors that influence why one technology has greater success over another, and, in the case of token ring, these forces were not in its favor. Nevertheless, I consider it valuable to learn about this type of technology simply because it is clever and the more that people are exposed to different ideas, the more they can expand upon those ideas. Who knows what new technology might be based on the concepts implemented in token ring?
I also figure it is useful to have some information on token ring because it is still taught in many communications courses (and there is still an installed base out there). I, myself, eliminated it from my syllabi a number of years ago, because given the limited amount of time for any university-level course, I cannot teach everything, and I prefer to focus on more current technologies, or historical technologies (like the bus-based Ethernet), such that the concepts are necessary to understand the current situation. Token ring is not like that - no current popular technology is an outgrowth of token ring. So, if someone does teach token ring, it should be made clear to the students that the days of "there is Ethernet and there is Token Ring" are long past (until further notice).
The token ring portion of the LAN portal will actually relate to two token ring LAN technologies: IEEE 802.5 Token Ring (which is, for all practical purposes, equivalent to the IBM Token Ring, on which the IEEE standard is based) and FDDI (Fiber Distributed Data Interface). (Unlike the Ethernet 802.3 specifications, which IEEE offers without charge, the 802.5 specifications - as rarely used as they are - are not freely available.) When I refer to the standardized IEEE (or IBM) Token Ring, I will capitalize the first letters of the name, as in Token Ring, but if I am referring to a general token ring type mechanism or algorithm, the entire words will remain in lower case letters.
Motivation
Throughout the Ethernet tutorial in this LAN I kept making references to Token Ring networks, in a sense, as an inspiration to improve Ethernet. The IBM Token Ring, introduced by IBM in 1984 and standardized by the IEEE, presented a real challenge to Ethernet, which I discussed in my presentation of the Ethernet hub architecture. You can go back and read what I wrote there, and also what I wrote when introducing the Ethernet switch, but I'll "recap" the two main challenges presented by Token Ring: a hub-based star-topology wiring scheme (Figure 30), which was marketed by IBM (the IEEE 802.5 did not specify this topology) and the deterministic access method (which means that the maximum amount of wait time to transmit could be determined, or calculated). These aspects of Token Ring addressed two limitations of Ethernet, limitations that were eventually addressed. As we discuss the Token Ring LAN, we'll see other neat features of the technology.
Figure 30: A LAN Hub
The IBM (and IEEE) Token Ring was addressing more or less the same market that Ethernet was. However, another token ring technology came on the scene several years later that pushed the concept of a LAN as a local network. FDDI (Fiber Distributed Data Interface) offered a network that could be as large as 100 kilometers in diameter and could connect thousands of stations (but 'only' 500 attached nodes - you'll find out how the others are connected). It was sometimes referred to by the term MAN (Metropolitan Area Network), and could clearly be used on large campuses and beyond. It also could serve as a backbone network to connect LANs. Developed by the ANSI X3T9.5 committee in the mid-1980s, it offered far greater transmission speed (100 Mbps) than was offered by either Ethernet or Token Ring at that time. Further, FDDI offered impressive architectural resiliency and reliability. The ANSI standard was later internationalized as ISO 9314.
In the following chapters, we'll look at each of these protocols.
