UNIX System Administration Handbook - Evi Nemeth [228]
Designed and marketed as a 100 Mb/s token ring, FDDI once looked like it would be the easy solution to many organizations’ bandwidth needs.
Unfortunately, FDDI has been a disappointment in absolutely every way. In the early days of FDDI, the cost of FDDI interfaces often exceeded the cost of the workstations they were installed in (around $10,000 each) and performance was often worse than Ethernet (early DEC boards, for example). Interfaces are still expensive, but the performance is better. Modern interfaces yield around 80 Mb/s of throughput.
See page 268 for more information about maximum transmission units (MTUs).
For good performance, FDDI needs a much higher MTU than the default, which is tuned for Ethernet. An MTU value of 4,352 (set with ifconfig) is about right. Until the software used to move files around networks has been tuned for the speed and characteristics of FDDI, mere mortals will probably see performance numbers in the range of one-third to one-half the theoretical maximum.
The FDDI standard specifies a 100 Mb/s token-passing, dual ring, all-singing, all-dancing LAN using a fiber optic transmission medium, as shown in Exhibit D. The dual ring architecture provides for a primary ring that’s used for data transmission and a secondary ring that’s used as a backup in the event the ring is cut (either physically or electronically).
Exhibit D FDDI dual token ring
Hosts can either be connected to both rings (they are then referred to as class A or “dual attached” hosts) or just to the primary ring (class B or “single-attached” hosts). Most commonly, backbone routers and concentrators are dual attached, and workstations are single-attached, usually through a “concentrator,” a sort of fiber hub.
One advantage of token ring systems is that access to the network is controlled by a deterministic protocol. There are no collisions, so the performance of the network does not degrade under high load, as it does with Ethernet. Many token ring systems can operate at 90% to 95% of their rated capacity when serving multiple clients.
For physical media, the FDDI standard suggests two types of fiber: single-mode and multimode. “Modes” are essentially bundles of light rays that enter the fiber at a particular angle. Single-mode fiber allows exactly one frequency of light to travel its path and thus requires a laser as an emitting source.5
Multimode fiber allows for multiple paths and is usually driven by less expensive and less dangerous LEDs. Single-mode fiber can be used over much longer distances than multimode. In practice, 62.5 μm multimode fiber is most commonly used for FDDI.
Several fiber connector standards are used with FDDI, and they vary from vendor to vendor. Regardless of what connectors you use, keep in mind that a clean fiber connection is essential for reliable operation. Although self-service fiber termination kits are available, we suggest that wherever possible you have a professional wiring firm install the ends on fiber segments.
15.4 ATM: THE PROMISED (BUT SORELY DEFEATED) LAN
ATM stands for Asynchronous Transfer Mode, but some folks insist on Another Technical Mistake. One datacomm industry spokesman describes it as “an attempt by the phone company to turn your networking problem into something they know how to tariff.”
ATM is technically “special” because it promotes the philosophy that small, fixed-size packets (called “cells”) are the most efficient way to implement gigabit networks. ATM also promises capabilities that haven’t traditionally been promised by other media, including bandwidth reservation and quality-of-service guarantees.
ATM was widely marketed as an all-in-one switched network medium that could be used for LAN, WAN, and MAN needs. In modern times, ATM is mostly dead, preserved only in WAN environments where large telco corporations are still trying to leverage their misguided investments in ATM hardware.
On top of ATM’s 53-byte cells, five ATM Adaptation Layers (AALs) are described for cell transport. The purpose of each adaptation layer