Specifications

RADring
Installation & Operation Manual Token Ring Desing Considerations
3/11/98 12:26 Fiber Optic Design Considerations 6-11
Fiber Network
Design
Considerations
In general, Token Ring networks with two different transmission media -
copper (STP or UTP) and fiber optic cable - can incorporate either TFC
modules or TFR modules. The major differences between the TFC and the
TFR are:
1. The TFR provides full repeating and jitter attenuation while the TFC
simply converts from copper to fiber.
2. The TFC is available with a two-link option.
3. The TFC is a more cost-effective solution than the TFR.
Note that while using TFC, its fiber-link distance is equal to the TFR's,
although the copper distance is shorter because of jitter considerations.
These recommendations are based on the fact that jitter is the major range
limiting factor. Since the TFC contains only conversion circuits, it does not
perform the functions of a full repeater, although the TJA jitter attenuator
module can be used in conjunction with the TFC to reduce jitter.
The design recommendations are:
•
The distance between the "previous" active station (workstation, server,
repeater, etc.) and the TFC must not exceed the following values:
At 4 Mbps, maximum 100 meters on Type 1 cable.
At 16 Mbps, maximum 50 meters on Type 1 cable.
•
The maximum allowable number of fiber optic TFC links in a ring
segment without repeaters is three.
Fiber Optic
Budget
Calculations
A separate analysis must be performed on fiber optic ring TFR and TFC
segments and TL-2/F lobe links, to ensure that their attenuation is not
excessive. The maximum allowable attenuation of a fiber optic link is
determined as a function of the optical power budget (described below).
Power Coupled in a Fiber
For short-distance links, the preferred light source is a LED. The LED radiates
a wide conical of light. The optical fiber has a much smaller diameter, and
the result is that a large part of LED light energy is lost. In fact, the larger the
fiber diameter, the more power is coupled into it.
The most common fiber diameters are: 50/125, 62.5/125 and 100/140
microns, where the first digits indicate core diameter (the active part), and
the second group of digits indicate clad diameter.
For example, let us compare the ratio of energy coupled into a 100/140
micron fiber, with respect to that coupled into a 50/125 micron fiber. Since
the LED is assumed to radiate uniformly within the angles of interest, the
power ratio is directly proportional to the area ratio, which is
(100/50)<M^>2<N>=<N>4.